Magnets, motors and generators. Attnom Academy

S4.13.1 Chapter 1

Magnetic fields

A magnet pulls a pin across the table without touching it. What is reaching out from the magnet, when there is nothing to see?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The magnet pushes and pulls only at its two ends.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The push and pull reach all round the magnet, strongest near the poles.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The push and pull stay inside the metal of the magnet.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Draw field lines around a bar magnet and between two poles
Explain the difference between permanent and induced magnets

A magnet can pull a pin across a table without touching it. The pull reaches out into the space round the magnet. That space is the magnetic field. In a magnetic field, a magnet or a magnetic material feels a force.

You cannot see a field, but a small magnet can feel it. A compass needle is a small magnet. It turns until it lines up with the field. The way its N end points is the direction of the field there: the way a N pole would be pushed.

We draw a field with lines. Each field line shows the direction of the field with an arrow, and many lines together show where the field is strong. Draw one line at a time, and let a compass show you the way it bends.

Draw it first

Build the field of a bar magnet with a compass

1 Put a small compass beside the magnet. Its needle lies along the field there. Draw a short line that follows the needle, with an arrow in the way the N end points.
2 Move the compass a little way along and draw again. Join the short lines into curves. Every curve leaves the N pole and bends round.
3 Add more lines. Outside the magnet each one runs into the S pole. The lines never cross, and they are closest together at the poles.

Here are the rules for drawing field lines. Outside a magnet, a line runs from the N pole to the S pole, and the arrow shows that way. Lines never cross, because the field has only one direction at each point. Lines are closer together where the field is stronger.

There is one more rule, and it is often missed. A field line has no start and no end. Outside the magnet it runs from N to S. Inside the magnet it carries on from S back to N. So every line is a closed loop.

Every line is a closed loop: N to S outside the magnet, S to N inside it. The two lines through the middle are so big that they leave the picture and come round far away. Only some lines are drawn. The field is also there between the lines.

Look at where the lines are close together. They bunch near the poles, where the field is strongest. Far from the magnet they spread out and the field is weak. The field gets weaker with distance, but it never stops suddenly.

Now put two magnets end to end. When a N pole faces a S pole, the lines join across the gap. The field is strong in the gap, and the magnets pull together.

Unlike poles facing: the lines join across the gap and the magnets attract.

When two N poles face each other, the lines do not join. They bend away from each other. Between the poles there is one point where the two fields cancel and the field is zero. It is called the neutral point.

Like poles facing: the lines push apart and the magnets repel. The field is zero at the neutral point.

Not every metal is magnetic. A magnetic material is pulled by a magnet. Of the common metals, only iron, nickel and cobalt are magnetic. Most steels are magnetic too, because steel is mostly iron. Some stainless steels, such as many kitchen spoons, are not. Copper, aluminium, gold, wood, plastic and glass are not magnetic.

Only a few metals are magnetic. Most metals are not. A kitchen spoon test is fair: many stainless steel spoons are not pulled.

There are two kinds of magnet. A permanent magnet keeps its magnetism all the time. A bar magnet of steel is one. An induced magnet is a piece of magnetic material that is a magnet only while it is in a magnetic field. An iron nail near a magnet is an induced magnet.

Why can iron become a magnet? Inside the metal are tiny regions called domains. Each domain is a very small magnet. In a piece of iron that is not magnetised, the domains point in all directions, and their effects cancel. In a magnet they are lined up.

See it three ways

As a picture

In words

In a magnet the domains line up. In unmagnetised iron they point in all directions and cancel. A field from outside can turn them to line up.

Bring a N pole of a magnet up to a soft iron nail. The field turns the domains in the nail, and the nail becomes a magnet. The end of the nail nearest the N pole becomes a S pole. The far end becomes a N pole. A S pole is nearest the N pole, so they attract.

The nail is an induced magnet while the bar magnet is there. Its S pole is nearest the magnet’s N pole.

An induced magnet is always attracted to the magnet that makes it. The pole nearest the magnet is always the opposite pole. Take the magnet away. Now the two materials differ. Soft iron loses almost all its magnetism at once, and the pins drop. Steel keeps its magnetism, so steel makes a permanent magnet. Most ordinary nails are mild steel and keep a little magnetism. Pure soft iron keeps almost none.

Soft iron: the magnet has gone, so the nail has almost no magnetism left and the pins have fallen.

Steel: the magnet has gone, but the nail kept its magnetism and the pins still hang.

Try it

Map a magnetic field with pins

You need: a bar magnet, if you can get one, a sheet of thin paper, a few small bits of steel wool or short iron pins, a small compass, if you have one.

Be safe: Steel wool and pins can cut or prick. Tear off tiny pieces with a cloth round your fingers, keep pins in a box, and keep both away from small children. Keep a strong magnet away from phones and bank cards.

Lay the magnet on a table and put the paper flat over it.
Sprinkle a few bits of steel wool, or lay pins, on the paper near the magnet.
Tap the paper gently. Look at the way the bits line up.
If you have a compass, move it round the magnet on the paper and watch its needle.

What you will see: The bits line up along curved lines that run from one pole to the other. They bunch near the poles, where the field is strong. Each bit becomes a tiny induced magnet, and it lines up with the field. The compass needle follows the same curves. A flat fridge magnet gives thin stripes instead. A speaker magnet gives rings. Their poles are placed differently.

If you cannot try it, this is what happens.

A wrong idea many people have

The magnetic field is only along the lines that are drawn.

Why it feels right: A picture of field lines shows empty gaps between the lines, and it is easy to think the field is missing there.

The test that breaks it: Put a compass between two of the lines. Its needle still lines up. A pin placed in a gap is still pulled. The field is there too.

What is true: The field is everywhere near the magnet. Field lines are a drawing. They show the direction of the field, and by how close they are, how strong it is.

A wrong idea many people have

If a magnet pulls something, that thing is a magnet too.

Why it feels right: Two magnets attract each other, so it seems that anything attracted must be one.

The test that breaks it: A magnet pulls a plain iron nail from either end. Turn the magnet round, and the nail is still pulled. A real magnet would push away from one of the two ends.

What is true: An induced magnet is always attracted, so attraction does not prove a magnet. Only repulsion proves it: a thing that is pushed away by a pole is a magnet.

A wrong idea many people have

Field lines start at the N pole and stop at the S pole, and they can cross.

Why it feels right: Drawings of the field outside a magnet show lines that seem to begin at one pole and end at the other.

The test that breaks it: Inside the magnet the field runs from S to N. The lines go on through the magnet and join up. If two lines crossed, the field would have two directions at one point, which cannot be.

What is true: Field lines are closed loops with no start and no end. Outside the magnet they run N to S, inside they run S to N, and they never cross.

Key words

magnetic field: the space round a magnet where it pushes or pulls on magnets and magnetic materials.
field line: a line that shows the direction of the field. Closer lines mean a stronger field.
neutral point: a place where two fields cancel and the field is zero.
permanent magnet: a magnet that keeps its magnetism, such as a steel bar magnet.
induced magnet: a magnetic material that is a magnet only while it is in a magnetic field.
magnetic material: a material that a magnet pulls: iron, nickel, cobalt, most steels and some alloys.
soft iron: iron that is easily magnetised and loses almost all its magnetism when the field is removed.

Back to your guess

The answer is B: The push and pull reach all round the magnet, strongest near the poles.

A magnet has a magnetic field all round it. The field is strongest near the poles and gets weaker with distance, but it is not zero anywhere near the magnet. A pin is pulled because it is inside the field. The field is not a thing you can see, so we draw it with lines.

Keep it

Copy this into your exercise book.

Field lines run N to S outside a magnet and S to N inside: closed loops, never crossing. Close lines mean a strong field. Soft iron is an induced magnet. Steel is permanent.

Draw: A bar magnet with three loops of field line and arrows on them.

S4.13.2 Chapter 2

Fields around wires and coils

A plain copper wire is not a magnet. Yet a wire from a cell can make a compass needle turn. How?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Only a wire made of iron can move a compass needle.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The cell is a magnet, and its field runs along the wire.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Any wire that carries a current has a magnetic field round it.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use the right hand grip rule to find the direction of a field
Explain how a coil of wire makes a strong, even field

In 1820 a scientist saw a compass needle twitch when a current was switched on in a wire near it. That showed something new: a current has a magnetic field. Every moving electric charge has one. A wire with a current is a stream of moving charge.

Take a long straight wire. Its field is not along the wire. It goes in circles round it, with the wire at the centre. The field is strongest close to the wire, and it gets weaker with distance. So the circles are closer together near the wire.

Draw it first

The field round a straight wire

1 Draw the wire end on, with a dot. The dot means the current comes out of the page towards you. Draw a circle round it.
2 Add a second circle further out. The field is weaker there, so there is a wider gap between the circles.
3 Add a third circle. Put arrows on every circle. For a current out of the page they go anticlockwise.

We show a wire end on with a dot or a cross. A dot means the current comes out of the page towards you, like the tip of an arrow coming at you. A cross means the current goes into the page, like the feathers of an arrow flying away.

The right hand grip rule gives the direction of the field. Imagine gripping the wire with your right hand. Point your thumb the way the current flows. Your fingers curl the way the field goes round the wire. We always mean the conventional current, from the + of the cell round to the −.

Reverse the current and the field reverses. A current into the page has a clockwise field. The grip rule agrees: the thumb points away from you and the fingers curl clockwise.

How strong is the field? Near a long straight wire it depends on two things. A bigger current makes a stronger field, in proportion: twice the current, twice the field. A bigger distance makes it weaker: twice as far from the wire, half the field.

Worked example

A long straight wire makes a field of 12 µT at 3 cm from it. How strong is the field at 9 cm, with the same current?

The field falls in proportion to 1 over the distance.
The new distance is 9 ÷ 3 = 3 times the old one.
So the field is 3 times smaller: 12 µT ÷ 3 = 4 µT.

Answer: 4 µT

The field of a long straight wire falls quickly near the wire, then slowly. Nine centimetres out, a third of the field at 3 cm.

One wire makes a weak field. To make a strong one, wind the wire into a coil. A long coil is called a solenoid. Look at a slice cut along its axis. At the top the current comes out of the page, and at the bottom it goes into the page.

Inside the coil, the fields of all the turns point the same way, so they add up. Outside, the fields of the top and the bottom wires point against each other, so they cancel more. That gives a strong field inside, and the field lines inside are nearly straight and evenly spaced.

The field of a coil is like the field of a bar magnet. Inside it is strong and even. The lines leave the end labelled N. Lines that leave the picture come round again far away.

Which end of the coil is the N pole? Use the grip rule on the coil. Grip the coil with your right hand, with your fingers curling the way the current goes round the turns. Your thumb points along the field inside, towards the N pole.

Reverse the current and the poles swap. The same coil now has its N pole on the left.

Three things make the field of a coil stronger. Use more turns, so more fields add up. Use a bigger current. Put a soft iron core inside. The core becomes an induced magnet, and its own field adds to the field of the coil. A core can make the field many times stronger.

An electromagnet: a coil with a soft iron core. Switch the current off and the core loses almost all its magnetism, so the clips drop.

Try it

Make a compass needle turn with a current

You need: a torch cell, a torch bulb, about 2 m of thin insulated wire, cut into pieces with the ends bared, a small compass, tape.

Be safe: Use one torch cell only, and always keep the bulb in the loop. A wire joined straight across a cell gets hot and drains it. Close the loop for a second at a time. Never use mains electricity for any activity in this book.

Let the compass settle. Wind a piece of wire 5 to 10 times round the compass. Run it north to south on top and back underneath, along the needle.
Join the wire, the bulb and the cell in one loop. Touch the loop closed for one second. Watch the needle, then open the loop.
Turn the cell round, so the current goes the other way. Touch the loop closed for one second again.
Now wind the wire round a pencil into a coil of ten turns. Put the compass near one end of the coil and close the loop for a second.

What you will see: The needle swings to one side when the current flows, and goes back to north when it stops. The swing may be only 10 to 20 degrees. With the cell turned round it swings the other way. Near the end of the coil of ten turns it swings too: the coil acts like a bar magnet. The field of the current has to act against the field of the Earth, which holds the needle north.

If you cannot try it, this is what happens.

A wrong idea many people have

The field of a current runs along the wire, like water in a pipe.

Why it feels right: The current runs along the wire, so it seems natural that the field runs along it as well.

The test that breaks it: Lay a compass beside a wire with a current. The needle turns to point across the wire, not along it. Move the compass round the wire and the needle follows a circle.

What is true: The field goes in circles round the wire, at right angles to the current.

A wrong idea many people have

To find the field round a wire, use the left hand, as for the force on a wire.

Why it feels right: There are two hand rules in this book, and both start with a hand. It is easy to mix them up.

The test that breaks it: Put a compass near a wire with a known current and see which way its N end points. The right hand grip rule agrees with the compass. A left hand grip gives the opposite way.

What is true: The grip rule for the field of a current uses the right hand. Fleming’s left hand rule, in the next chapter, is for the force on a wire. They answer different questions.

A wrong idea many people have

An iron core makes a coil stronger by making the current bigger.

Why it feels right: The coil with the core is stronger, and a bigger current also makes a stronger coil.

The test that breaks it: Put an ammeter in the circuit. Push the core in and out. The reading does not change. Yet the nail picks up far more clips with the core in.

What is true: The core does not change the current. The core becomes an induced magnet, and its field adds to the field of the coil.

Key words

right hand grip rule: thumb along the current, fingers curled the way the field goes round it.
coil: a wire wound in many turns, so that the fields of the turns add up.
solenoid: a long coil. Its field is strong and even inside, like a bar magnet outside.
core: a piece of soft iron inside a coil that makes its field much stronger.
conventional current: the current drawn from the + of a cell round to the −.

Back to your guess

The answer is C: Any wire that carries a current has a magnetic field round it.

A current makes a magnetic field. The field of a straight wire goes in circles round the wire, so a compass beside the wire turns across it. Copper is not magnetic, and the cell is not a magnet. The current alone does it.

Keep it

Copy this into your exercise book.

A current has a field in circles round the wire. Right hand grip: thumb with the current, fingers with the field. A coil adds its turns: strong and even inside. More turns, more current, soft iron core.

Draw: A wire end on with two circles and arrows, and a coil with a N and a S end.

S4.13.3 Chapter 3

The motor effect

Hang a wire between the poles of a magnet and switch on a current. The wire jumps. Which way does it go, and how hard?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It is pulled towards the nearest pole of the magnet.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is pushed at right angles to both the field and the current.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It is pushed along the field, from the N pole towards the S pole.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use Fleming's left hand rule to find the force on a wire
Calculate force using F equals B times I times L

A wire with a current has a magnetic field round it. Put it in the field of a magnet, and the two fields act on each other. The result is a force on the wire. This is the motor effect. It is how every electric motor turns.

Here is why the force has the direction it has. We look at a wire end on, with a current out of the page. The magnet’s field points to the right. The wire’s own field goes anticlockwise round it.

Draw it first

Why the wire is pushed

1 The magnet makes a field from left to right. The wire carries a current out of the page. The wire is pushed up.
2 Add the two fields. Above the wire, the wire’s field points left and takes away from the magnet’s field, so the field is weaker. Below, they point the same way and the field is stronger.

The wire is pushed from the side where the field is strong to the side where it is weak. Here, that means up. Reverse the current, and the stronger side swaps to the top, so the wire is pushed down. Reverse the magnet, and the force reverses too.

You do not have to add up the fields every time. Fleming’s left hand rule gives the direction at once. Hold the thumb, the first finger and the second finger of your left hand at right angles to each other. Then each finger stands for one quantity.

First finger Field, seCond finger Current, thuMb Motion. The thumb shows the direction of the force, and so the way the wire moves.

Point the first finger the way the field points, from N to S. Point the second finger the way the conventional current flows. The thumb then points the way the wire is pushed. Turn your hand until the fingers match. The rule works when the wire is at right angles to the field.

Check it on the picture. The field points right, so the first finger points right. The current comes out of the page towards you, so the second finger points at you. Turn the hand until it fits: the thumb points up. That agrees with the force we found.

If the wire lies along the field, there is no force at all. Its current then runs the same way as the field, or the opposite way. The force is biggest when the wire is at right angles to the field. At other angles it is smaller.

How big is the force? For a wire at right angles to the field, F = B × I × L. F is the force in newtons, N. B is the magnetic flux density, the strength of the field, in tesla, T. I is the current in amperes, A. L is the length of wire in the field, in metres, m.

One tesla is the field that gives a force of 1 N on 1 m of wire carrying 1 A across it. So 1 T = 1 N per A per m. The tesla is a big unit. The table gives some typical fields.

Rough sizes. A magnet’s field is strongest at its poles, and the Earth’s field varies from place to place.

The force grows in proportion to each of B, I and L. Double the current and the force doubles. Use a magnet twice as strong and the force doubles. A wire twice as long in the field feels twice the force. Look at the graph for a fixed field and length.

A field of 0.2 T and 0.5 m of wire in it. Force against current is a straight line through the origin: each amp adds 0.1 N.

Rearrange the formula to find the other quantities. The current is I = F ÷ (B × L). The field is B = F ÷ (I × L). Always put the length in metres: change centimetres by dividing by 100.

Worked example

A wire carries 3 A across a field of 0.4 T. 0.15 m of the wire is in the field. Find the force on it.

Use F = B × I × L, with B = 0.4 T, I = 3 A and L = 0.15 m.
F = 0.4 × 3 × 0.15 = 0.18, so F = 0.18 N.

Answer: 0.18 N

The wire of the first worked example, with the field pointing left and the current into the page. The force is up.

Worked example

A force of 0.6 N acts on 0.2 m of wire in a field of 0.5 T. What current does the wire carry?

Rearrange F = B × I × L to give I = F ÷ (B × L).
B × L = 0.5 T × 0.2 m = 0.1.
I = 0.6 N ÷ 0.1 = 6 A.

Answer: 6 A

Worked example

A wire with 8 A crosses a field of 0.25 T. 12 cm of the wire is in the field. Find the force.

Change the length to metres: 12 cm ÷ 100 = 0.12 m.
Use F = B × I × L = 0.25 × 8 × 0.12.
0.25 × 8 = 2.
2 × 0.12 = 0.24, so F = 0.24 N.

Answer: 0.24 N

Try it

Practise Fleming’s left hand rule

You need: your left hand, a pencil and paper.

Draw a rectangle on the left of the paper and write N in it. Draw one on the right and write S in it. Draw an arrow between them from N to S: that is the field.
Draw a small circle with a dot in the middle of the gap. It is a wire with a current out of the page, towards you.
Hold your left hand flat on the page, then open the thumb, first finger and second finger at right angles. Point the first finger along the field arrow.
Turn your hand so that the second finger points up out of the page, towards you. Look at the thumb. Draw an arrow the way it points from the wire.
Now draw a cross instead of the dot, so the current goes into the page. Repeat the steps and see how the thumb changes.

What you will see: With the dot, the thumb points up the page. With the cross, it points down. Reversing the current reverses the force. Try again with the field pointing the other way, and see that it reverses the force too.

If you cannot try it, this is what happens.

A wrong idea many people have

The force on the wire points along the field, towards a pole.

Why it feels right: We think of a magnet as pulling things towards its poles, so a force towards a pole seems natural.

The test that breaks it: Use the left hand rule. The thumb is at right angles to the first finger, so the force is at right angles to the field. If you move a wire the way the thumb points, it never goes towards a pole.

What is true: The force is at right angles to both the field and the current. The wire is pushed sideways across the field.

A wrong idea many people have

L in F = B × I × L is the whole length of the wire.

Why it feels right: L is called the length of the wire, so it seems to mean all of it.

The test that breaks it: Take a long wire and put only 5 cm of it between the poles. The rest of the wire is outside the field and feels no force. The force is the same as for a wire that is only 5 cm long.

What is true: L is the length of wire that is in the field. Only that part feels the force.

A wrong idea many people have

A wire along the field feels the biggest force.

Why it feels right: The wire is lying in the strongest part of the field, so it seems it should feel the most.

The test that breaks it: Hold a wire so that the current runs the same way as the field. Your thumb and fingers cannot be at right angles to each other, because two of them would point the same way. There is no force.

What is true: The force is largest when the wire is at right angles to the field. Along the field it is zero.

Key words

motor effect: the force on a wire with a current in a magnetic field.
Fleming's left hand rule: first finger Field, seCond finger Current, thuMb Motion, at right angles.
tesla: the unit of magnetic flux density, T. One tesla is one newton per amp per metre.
magnetic flux density: the strength of a magnetic field, B, measured in tesla.

Back to your guess

The answer is B: It is pushed at right angles to both the field and the current.

The force on the wire is at right angles to the field and at right angles to the current. It does not point towards a pole or along the field. Fleming’s left hand rule gives its direction, and F = B × I × L gives its size.

Keep it

Copy this into your exercise book.

Left hand: First finger Field, seCond finger Current, thuMb Motion. F = B × I × L, with B in tesla, I in amps, L in metres of wire in the field. No force along the field.

Draw: A left hand with three labelled fingers, and the formula beside it.

S4.13.4 Chapter 4

Electric motors

A coil in a magnetic field is pushed at both sides. Why does it keep turning round and round, and not just flip over and stop?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A split ring reverses the current every half turn, so the force always turns it the same way.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The two forces on the coil are equal and opposite, so it spins freely with no force on it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The coil swings back and forth like a pendulum, and the magnet keeps it moving.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a split ring keeps a motor turning one way
Name ways to make a motor turn faster

A motor is a coil of wire that can turn between the poles of a magnet. A current flows round the coil. We look at it from the end of its axle. The long sides of the coil are wires at right angles to the field, so each feels a force.

The current goes one way along one side and the other way along the opposite side. By the left hand rule, the forces are opposite. One side is pushed up and the other is pushed down. Two equal and opposite forces in different places turn the coil.

The coil has turned 30 degrees clockwise past lying along the field. One side is pushed down, the other up, and the coil goes on turning clockwise.

The turning effect depends on the angle of the coil. When the coil lies along the field, each force is at right angles to the line from the axle. The turning effect is then the largest. As the coil turns, the effect gets smaller.

Draw it first

The turning effect as the coil turns

1 The coil lies along the field. The forces are at right angles to the coil, as far from the axle as they can be. The turning effect is largest.
2 The coil has turned 60 degrees clockwise. The forces are the same size, but they pull more along the coil, so the turning effect is smaller.
3 The coil has turned 90 degrees clockwise and stands at right angles to the field. The forces now act along the line through the axle, so they cannot turn it. The turning effect is zero.

Without anything else, the coil would swing past this upright place and then be pushed back, like a pendulum. It would rock to and fro and settle upright. To keep it turning one way, the current in the coil must reverse as it passes the upright place.

A split ring does this. It is a ring cut into two halves with a gap between them, one half joined to each end of the coil. Two brushes are fixed on either side and press on the ring. They carry the current from the cell into the ring. The ring turns with the coil.

The coil lies along the field. Each brush touches a half of the ring, so a current goes round the coil. The turning effect is largest.

When the coil stands across the field, the brushes sit on the gaps. For a moment no current flows, and there is no turning effect. The coil’s momentum carries it past this place. That is why a motor can start badly if it stops just there.

The coil stands across the field. Both brushes touch the gaps, so no current flows and the coil turns on by its momentum.

Once past the upright place, each half of the ring has moved to the other brush. The current now goes round the coil the other way. The side that was on the left is now on the right. It carries the current the other way, so it is pushed down. The coil goes on turning clockwise.

Half a turn later. The hatched half of the ring is now on the left, so each brush is on the other half. The right side still carries the current into the page and is pushed down, so the coil keeps turning clockwise.

The graph shows the turning effect against the angle of the coil from the field, in a model with a steady field. With the split ring it is always on the same side of zero. Without it, it changes sign after the upright place and pushes the coil back.

Curve A, solid, is a motor with a split ring. Curve B, dashed, is the same coil with no split ring. The ring turns the dips below zero into humps. The turning effect still falls to zero at 90 and 270 degrees. From 0 to 90 degrees and from 270 to 360 degrees the two curves are the same line.

A real motor has many coils at different angles and a ring with many segments. Then there is always some coil with a large turning effect, and the motor runs smoothly. The field is made radial, with curved poles, and a soft iron core strengthens it.

Each turn of a coil gives its own force. A coil of N turns has N wires on each side, each with the same current. So the force on one side is N × B × I × L. More turns means more force.

Worked example

A coil has 50 turns. Each side is 0.08 m long in a field of 0.2 T, and the current is 1.5 A. What is the force on one side?

Use N × B × I × L, with N = 50, B = 0.2 T, I = 1.5 A and L = 0.08 m.
50 × 0.2 × 1.5 × 0.08 = 1.2, so the force is 1.2 N.

Answer: 1.2 N

Worked example

The force on one side of a coil must be 2.4 N. Each side is 0.1 m long in a field of 0.4 T, and the current is 3 A. How many turns are needed?

Rearrange: N = F ÷ (B × I × L).
B × I × L = 0.4 × 3 × 0.1 = 0.12.
N = 2.4 ÷ 0.12 = 20 turns.

Answer: 20 turns

So what makes a motor turn faster? Anything that makes the force on the coil bigger. Use a stronger magnet, so B is bigger. Use more turns of wire. Use a bigger current, for example from more cells. The motor speeds up until friction and the load balance the turning effect.

To reverse a motor, reverse the direction of the force. Swap the connections to the cell, so the current reverses. Or turn the magnet round, so the field reverses. If you reverse both, the motor turns the same way as before.

Try it

Test a toy motor

You need: a small motor from an old toy, one or two torch cells, two short pieces of wire, tape, 1 m of thread and a paper clip, a clock that shows seconds.

Be safe: Keep hair, fingers and loose clothes away from the turning shaft and the thread. Use cells only, never mains. If the thread tangles or the motor stalls, take the wire off at once: a stalled motor gets hot.

Tape the wires to the two terminals of the motor. Tape one end of the thread to the motor’s shaft, and tie the paper clip to the other end.
Hold the motor up so the clip hangs. Touch the wires to one cell. The thread winds up on the shaft and lifts the clip. Time how long it takes to wind 1 m.
Swap the wires on the cell. The shaft turns the other way and lets the thread out.
Join two cells end to end. Wind 1 m again, and time it.

What you will see: Swapping the wires reverses the turn. Two cells give a bigger current, so the motor turns faster and winds up the thread in less time. The numbers in the table are one learner’s results. Yours will differ, but the pattern should be the same.

If you cannot try it, this is what happens.

A wrong idea many people have

The split ring changes the magnetic field.

Why it feels right: The ring is part of the magnet side of the picture, and it seems to switch something over each half turn.

The test that breaks it: The magnet is fixed and does not move. The ring is on the axle and carries the current from the brushes to the coil. Swap the cell connections and the coil reverses its turn. That shows it is the current that is changed.

What is true: The split ring reverses the current in the coil, not the field. The magnet stays the same.

A wrong idea many people have

The two forces push the whole coil sideways.

Why it feels right: A force pushes a thing in its own direction, so two forces seem to push the coil somewhere.

The test that breaks it: The forces are equal and opposite, so they cancel as pushes. The coil does not move sideways. They act on opposite sides of the axle, so together they turn the coil.

What is true: Equal and opposite forces at different places give a turning effect, and no overall push.

A wrong idea many people have

Without a split ring, the coil would still spin round, just slower.

Why it feels right: The forces still act on the coil, so it seems it would still go round.

The test that breaks it: Past the upright place, the same current would push the sides back the way they came. The coil would swing like a pendulum and stop upright.

What is true: With a steady current in one direction, the coil would rock and stop. The reversal of the current is what keeps it turning.

Key words

split ring: a ring in two halves that reverses the current in the coil every half turn. Also called a commutator.
brush: a fixed contact that presses on the ring and carries current to it.
turning effect: how strongly a force or forces try to turn something about an axle.

Back to your guess

The answer is A: A split ring reverses the current every half turn, so the force always turns it the same way.

The forces on the two sides are opposite, and that makes the coil turn. Each time the coil stands across the field, the turning effect falls to zero. The split ring reverses the current just there, so the next half turn is pushed the same way as the last.

Keep it

Copy this into your exercise book.

Opposite forces on the two sides turn the coil. The split ring reverses the current every half turn. Faster: stronger magnet, more turns, bigger current. Reverse the current or the magnet to reverse the turn.

Draw: A coil end on between N and S poles, with arrows on its sides and a split ring below.

S4.13.5 Chapter 5

Electromagnetic induction

A circuit has no cell in it, only a coil and a meter. Yet the needle moves when a magnet is pushed into the coil. Where does the current come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The magnet stores a current and pours it into the coil.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The magnet touches the wire and passes its magnetism along it as a current.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A changing magnetic field through a coil makes a voltage, and a current flows in a complete circuit.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why moving a magnet in a coil makes a current
Explain what makes the induced current bigger

In the motor effect, a current in a magnetic field makes a force. Now turn the idea round. Move a wire or a magnet, so that the wire is in a changing field. A voltage appears in the wire. This is electromagnetic induction, and the voltage is induced.

If the wire is part of a complete circuit, the induced voltage makes a current flow. It is an induced current. No cell is needed. The energy comes from whoever moves the magnet.

Join a coil to a meter with its zero in the middle. Push a magnet in, hold it still, and pull it out. Watch the needle each time.

Draw it first

Push a magnet into a coil, hold it, pull it out

1 Push the N pole into the coil. The needle swings to one side, say the right, while the magnet is moving.
2 Hold the magnet still. The needle goes back to zero, even though the magnet is next to the coil. A steady field makes no voltage.
3 Pull the magnet out. The needle swings the other way, to the left. The direction of the current has reversed.

Three facts come from this. The current flows only while the magnet moves, so it is the change that matters and not the magnet itself. Pulling out reverses the current. Turning the magnet round, so the S pole goes in, also reverses it.

The explanation is in the field lines. As the magnet moves, the number of field lines that pass through the coil changes. A changing field through the coil, or a wire that cuts across field lines, induces a voltage. If nothing changes, there is none.

It does not matter whether the magnet moves or the coil does. What counts is that one moves relative to the other. A coil pushed over a still magnet gives the same effect. So does a change in the current of a nearby coil. You meet that again in the chapter on transformers.

What makes the induced voltage bigger? Move the magnet faster, so the field changes more quickly. Use a stronger magnet, so more field lines pass through the coil. Use more turns of wire, so each turn adds its own voltage. The needle swing shows each of these.

Push the magnet in fast and the needle swings further than in the slow push.

The first push of the strip again, at the same speed, but with twice as many turns of wire. The needle swings about twice as far.

The bars show a learner’s voltages for a magnet pushed into the same coil at three speeds. Twice the speed gives twice the voltage, and three times the speed gives three times. In an ideal coil, the induced voltage is also in proportion to the number of turns.

An example. Real results are close to this pattern, not exactly on it.

Worked example

A coil of 200 turns gets an induced voltage of 0.6 V when a magnet is pushed in. The magnet and the speed stay the same. What is the voltage with a coil of 500 turns?

The voltage is in proportion to the number of turns.
500 turns is 500 ÷ 200 = 2.5 times as many.
So the voltage is 0.6 V × 2.5 = 1.5 V.

Answer: 1.5 V

Worked example

A magnet pushed in at 0.2 m/s induces 0.15 V in a coil. What voltage does it induce at 0.6 m/s, in the same coil?

In an ideal case the voltage is in proportion to the speed.
0.6 ÷ 0.2 = 3, so the speed is 3 times as big.
The voltage is 3 times as big: 0.15 V × 3 = 0.45 V.

Answer: 0.45 V

Which way does the induced current flow? Lenz’s law says that the induced current is always in the direction whose magnetic field opposes the change that causes it. Push a N pole into the coil. The end of the coil nearest it becomes a N pole too, and so pushes back.

This must be so. If the coil pulled the magnet in, the magnet would speed up and make more current, and so on. That would make energy from nothing. Instead you must push against the force, and the work you do becomes the energy of the current.

For a straight wire moving across a field there is a quick rule, using the right hand. Fleming’s right hand rule: First finger Field, seCond finger Current, thuMb Motion. The thumb is the way the wire moves. The second finger then gives the way the induced current flows.

The right hand for induction, the left hand for the motor effect. The fingers stand for the same things in both.

Here is a way to remember the pair. The motor effect starts with a current and makes motion, so use the left hand. Induction starts with motion and makes a current, so use the right. Both hands have the same fingers: Field, Current and Motion.

Try it

Hear a current made by a magnet

You need: about 3 m of thin insulated wire, a tube a little wider than your magnet, such as a pill bottle or a roll of card, a small magnet, such as one from an old speaker, an old pair of earphones with the plug cut off and the wire ends bared, by an adult, sandpaper or a nail file.

Be safe: Ask an adult to cut off the plug and bare the ends. Clean the wire ends by rubbing them with sandpaper or a nail file, not a knife. Hold the earphone against your ear only gently. This makes only a tiny voltage, but never connect it to any other equipment. Keep magnets away from phones and bank cards, and watch your fingers when magnets snap together.

Wind the wire into a coil of about 100 turns round the tube. Leave a little wire free at each end. Rub the wire ends clean with sandpaper or a nail file.
Join the two ends of the coil to the two bare ends of one earphone wire. Hold the earphone gently against your ear.
Push the magnet quickly into the coil, then hold it still. Then pull it out quickly. If your magnet will not go in, move it quickly up to the end of the coil and away.
Try again with the magnet moved faster. Then try again with a coil of fewer turns.

What you will see: You may hear a faint click when the magnet moves in. You may hear another when it moves out. You hear nothing when it is still. A faster move gives a louder click, and a coil with more turns does too. If your earphone is silent, wind more turns, move the magnet faster, or use a stronger magnet.

If you cannot try it, this is what happens.

A wrong idea many people have

A magnet inside a coil makes a current, because of its magnetism.

Why it feels right: We know magnets and coils are linked, so a magnet near a coil seems to be enough.

The test that breaks it: Push a magnet into a coil joined to a meter, then leave it there. The needle swings as it moves and returns to zero when it stops. The magnet is still there and there is no current.

What is true: A current is induced only while the magnetic field through the coil is changing. A still magnet gives none.

A wrong idea many people have

Induction gives you more energy out than you put in.

Why it feels right: No cell is used, so the current seems to come from nowhere.

The test that breaks it: Push a magnet into a coil that is joined into a complete circuit. With a big coil and a strong magnet the push back is large enough to feel. You meet it on a bicycle too: with the lamp on, pedalling is harder. With the circuit broken, the push back disappears. The work you do against the push is the energy that appears in the current.

What is true: By Lenz’s law the induced current opposes the change. You must do work, and that work is the energy of the current.

A wrong idea many people have

Use the left hand rule for induction as well as for the motor effect.

Why it feels right: Both rules use Field, Current and Motion on three fingers, so one hand seems to do for both.

The test that breaks it: Use the left hand for a wire pushed up across a field to the right. You find a current out of the page. Now use the right hand for the same wire moved up. You find a current into the page. The two cases are opposite.

What is true: The left hand gives the force when a current is given. The right hand gives the current when a motion is given. The fingers stand for the same quantities, but the roles swap.

Key words

electromagnetic induction: making a voltage in a conductor by changing the magnetic field through it.
induced current: a current made by an induced voltage in a complete circuit.
Lenz’s law: the induced current is in the direction whose field opposes the change that makes it.
Fleming’s right hand rule: first finger Field, seCond finger Current, thuMb Motion, with the right hand. For a wire moving across a field.

Back to your guess

The answer is C: A changing magnetic field through a coil makes a voltage, and a current flows in a complete circuit.

A magnet stores no current. The voltage comes from the change. As the magnet moves, the magnetic field through the coil changes. That makes a voltage in the wire. This is electromagnetic induction. If the magnet stops, the voltage stops.

Keep it

Copy this into your exercise book.

A changing field through a coil induces a voltage. No change, no voltage. Bigger: faster, stronger magnet, more turns. Lenz: the current opposes the change. Right hand: Field, Current, Motion.

Draw: A coil, a meter and a magnet with an arrow, and a needle swung to one side.

S4.13.6 Chapter 6

Generators and dynamos

A bicycle lamp lights as you pedal, with no battery. A power station lights a whole city. How do they make electricity?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A They store electricity in a big magnet and let it out as a current.
You picked A. Read on. At the end, the chapter comes back to your guess.
B They turn a coil in a magnetic field, or a magnet in a coil, and induce a voltage.
You picked B. Read on. At the end, the chapter comes back to your guess.
C They burn fuel inside the coil to make electrons.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a bicycle dynamo or a power station generator works
Compare the output of an alternator with that of a dynamo

A generator is a machine that changes movement energy into electrical energy. It has the same parts as a motor: a coil and a magnet. But now something turns the coil, and a voltage is induced. A generator is a motor run backwards.

Look at a coil turned between the poles of a magnet. We look at it from the end of its axle, as in the chapter on motors. Each long side of the coil cuts across the field lines as it moves. A voltage is induced in each side.

A coil turned anticlockwise, as the curved arrows show, lying along the field. The sides move straight across the field lines, so the induced current is largest.

The induced voltage depends on how fast the sides cut the field lines. It is largest when the coil lies along the field, because then the sides move straight across the field lines. It is zero when the coil stands across the field, because then the sides move along the lines and cut none.

Draw it first

One half turn of a generator coil

1 The coil lies along the field. The sides cut across the field lines, so the current is largest. Side A, on the right, has the current into the page.
2 A quarter turn later the coil stands across the field. The sides move along the field lines and cut none, so the current is zero.
3 Half a turn from the start, the two sides have swapped places. Side A is on the left now, and its current is out of the page. In side A the current has reversed.

The current in the coil reverses twice in every turn. To take it out to a lamp, we need contacts that do not tear the wires. There are two ways to do it, and they give two different kinds of output.

With slip rings, each end of the coil is joined to its own full ring on the axle. A brush presses on each ring all the time. The lamp is joined to the brushes. The current that comes out reverses every half turn, so the output is alternating current, AC. A generator like this is an alternator.

Slip rings. Each ring keeps one brush on it all the time. The lamp stays joined to the same end of the coil, whatever the angle.

With a split ring, called a commutator, the brushes swap ends every half turn, as in the motor. A generator like this gives a current that always goes the same way. It is direct current, DC, but it comes in pulses. A generator like this is a dynamo.

The graph shows both outputs for a model coil with a peak of 6 V. The voltage is given against the angle of the coil from the field. The alternator, A, goes positive and negative. The dynamo, D, is always positive, in humps, and falls to zero twice a turn.

A is the alternator, with slip rings: AC. D is the dynamo, with a split ring: DC in pulses. One turn of the coil gives one cycle. From 0 to 90 degrees and from 270 to 360 degrees, curves A and D are the same line.

One full turn of this coil is one cycle of the alternating current. The frequency is the number of cycles in one second, in hertz, Hz. So a coil that turns 25 times a second gives 25 Hz. The period is the time for one cycle: period = 1 ÷ frequency.

Worked example

A simple coil turns 25 times a second. What is the frequency of the AC it makes, and the time for one cycle?

One turn is one cycle, so the frequency is 25 Hz.
The period is 1 ÷ frequency = 1 ÷ 25 = 0.04.
So one cycle takes 0.04 s.

Answer: 25 Hz and 0.04 s

Worked example

A generator makes AC at 25 Hz. How many cycles does it make in 8 seconds?

The frequency is 25 cycles every second, so 8 seconds hold 25 × 8 cycles.
25 × 8 = 200.
So in 8 seconds the generator makes 200 cycles.

Answer: 200 cycles

A bicycle lamp “dynamo” does it the other way round. A small magnet spins inside a fixed coil, turned by the wheel. The changing field induces a voltage in the coil. There is no ring to wear out. By our rule, it is really a small alternator, because its output is AC.

A magnet turning anticlockwise between fixed coils. As it turns, the field through each coil changes, and a voltage is induced.

A power station generator works in the same way. A turbine, turned by steam, water or wind, spins a big electromagnet inside fixed coils. The fixed coils carry the large current, so it needs no moving contact. Only the small current to the electromagnet goes through slip rings.

The turbine is turned by steam (from burning fuel or a nuclear reactor), by falling water, or by wind. The generator then makes AC.

A generator does not make energy. The energy comes from the fuel, the water or the wind that turns it. The generator changes some of that energy into electrical energy. Power station generators make AC at 50 Hz in most of the world and at 60 Hz in some places.

A split ring gives DC. Slip rings give AC.

To make a generator give more voltage, do what you do for induction. Turn it faster. Use a stronger magnet. Use more turns of wire. Turning faster also raises the frequency, because there are more cycles each second.

Try it

Make a motor into a generator

You need: a small motor from an old toy, a small LED from an old toy or torch, two short pieces of wire, a piece of string.

Be safe: Hold the motor firmly and keep fingers, hair and clothes away from the string and the shaft. Use only a motor and an LED. Never connect anything made this way to mains electricity.

Join the two wires from the motor to the two legs of the LED.
Wind the string round the motor’s shaft 15 to 20 times. Hold the motor firmly.
Pull the string sharply, so the shaft spins fast. Watch the LED.
If the LED stays dark, pull the string the other way, or swap its two wires: an LED lights only one way.
Try a slow pull, and then a fast one.

What you will see: The motor now works as a generator, and the LED may flash while the shaft spins fast. A faster spin gives a brighter flash, and a slow one gives little or none. A toy motor has a split ring, so as a generator it is a small dynamo and gives DC. That is why the LED lights for one way of turning only. The motor turned by hand makes the voltage, with no cell at all.

If you cannot try it, this is what happens.

A wrong idea many people have

A generator makes electrical energy from nothing.

Why it feels right: The lamp lights with no battery, so it seems the energy appears out of nowhere.

The test that breaks it: Pedal a bicycle with the lamp on, then switch it off. It becomes easier to pedal. With the lamp on, you do extra work against the push of the induced current. That work is the energy the lamp gets.

What is true: A generator changes energy from one form to another. The energy of movement becomes electrical energy, with some lost as heat.

A wrong idea many people have

Alternating current flows both ways, so the two ways cancel and it does no work.

Why it feels right: The current goes forward and then back, so it seems it gets nowhere.

The test that breaks it: Every lamp in a home runs on AC from the mains, and it glows. A wire heats up whichever way the current goes through it. Energy is transferred in every half cycle.

What is true: The current reverses, but energy still flows to the lamp in each half cycle. A lamp, a heater or a motor can work on AC.

A wrong idea many people have

A bicycle lamp has a battery inside that the dynamo charges.

Why it feels right: Most things that light up have a battery, and the lamp stays on for a moment when you slow down.

The test that breaks it: Lift the back wheel and spin it by hand. The lamp lights while the wheel spins and goes dark as it stops. There is nothing to store the energy.

What is true: A simple bicycle lamp is lit straight from the generator. It glows only while the wheel turns.

Key words

generator: a machine that changes movement energy into electrical energy by turning a coil and magnet relative to each other.
alternator: a generator with slip rings. Its output is alternating current.
dynamo: a generator with a split ring. Its output is direct current in pulses.
slip rings: two full rings on the axle, each with a brush, that carry the current out without swapping it.
frequency: the number of cycles each second, in hertz, Hz. The period is 1 ÷ frequency.

Back to your guess

The answer is B: They turn a coil in a magnetic field, or a magnet in a coil, and induce a voltage.

A generator is a motor run backwards. Something turns a coil in a magnetic field, or a magnet inside a coil, and the changing field induces a voltage. The generator changes the energy of movement into electrical energy. It does not make energy from nothing.

Keep it

Copy this into your exercise book.

A generator turns a coil in a field and induces a voltage. Slip rings: AC (alternator). Split ring: DC pulses (dynamo). One turn is one cycle. Period = 1 ÷ frequency. Faster gives more voltage.

Draw: A coil between N and S poles with two slip rings, and a wave beside it.

S4.13.7 Chapter 7

Transformers

A phone charger turns the supply of your home into a few volts. Power lines carry thousands of volts. What changes the voltage, and why do the power lines use so much?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Power lines use a high voltage so that the electricity travels faster and arrives sooner.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Power lines use a high voltage so that the current is small and little energy is lost as heat.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Power lines use a high voltage so that the wires carry more energy in total.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use the turns ratio to calculate output voltage
Explain why power lines carry electricity at very high voltages

A transformer changes the voltage of an alternating current. It has two coils of wire wound on a ring of soft iron, called the core. The coils are not joined to each other. The only link between them is the magnetic field in the core.

The coil joined to the supply is the primary coil. The coil that gives the output is the secondary coil. We write Np and Ns for their numbers of turns, and Vp and Vs for their voltages.

Here is how it works. An alternating current in the primary makes a field that keeps changing. The iron core carries the field through the secondary. A changing field through a coil induces a voltage in it. So the secondary gives an alternating voltage.

A step-up transformer. The secondary has twice as many turns as the primary, so it gives twice the voltage.

Why does it need alternating current? A steady direct current makes a steady field. The field through the secondary does not change, so no voltage is induced. The lamp stays dark. Only when you switch the current on or off is there a brief change.

Each turn of the secondary gets the same voltage from the changing field. So the more turns, the more voltage. The voltages are in proportion to the turns. This is the rule for the turns ratio:

Vp ÷ Vs = Np ÷ Ns. Rearranged, Vs = Vp × Ns ÷ Np. Check it on the picture: 6 V × 10 ÷ 5 = 12 V. The secondary voltage is the primary voltage times the ratio of secondary turns to primary turns. Take care to put Ns on top.

A transformer with more turns on the secondary is a step-up transformer: it raises the voltage. One with fewer turns on the secondary is a step-down transformer: it lowers the voltage. Many phone chargers have a small step-down transformer inside.

Worked example

A transformer has 50 turns on the primary and 400 on the secondary. The primary voltage is 8 V AC. What is the secondary voltage?

Use Vs = Vp × Ns ÷ Np, with Vp = 8 V, Np = 50 and Ns = 400.
The ratio Ns ÷ Np is 400 ÷ 50 = 8.
So Vs = 8 V × 8 = 64 V.

Answer: 64 V

A step-down transformer. The secondary has half as many turns as the primary, so it gives half the voltage.

A transformer does not make power. For an ideal transformer, which wastes no energy, the power out equals the power in. Power is voltage times current, so Vp × Ip = Vs × Is. Raise the voltage and the current falls by the same ratio.

Real transformers waste a little energy as heat in the coils and the core. A very large power transformer can be more than 98 percent efficient, so for these sums we treat it as ideal. An ideal transformer is 100 percent efficient.

Now power lines. A power station needs to send a lot of power along wires over long distances. The wires have a resistance, so they get warm and lose energy as heat. The power lost in a wire is P = I² × R, with I the current and R the resistance of the line.

The power sent is P = V × I. To send the same power with a bigger voltage V, you need a smaller current I. Because the heat lost depends on I squared, a small current wastes very little. Make the voltage 10 times as big, and the current becomes a tenth and the loss a hundredth.

Worked example

A town needs 20 000 W sent along a line of resistance 2 Ω. Find the heat lost in the line at 2 000 V, and then at 20 000 V.

At 2 000 V the current is 20 000 ÷ 2 000 = 10 A.
The heat lost is I² × R = 10² × 2 = 200 W.
At 20 000 V the current is 20 000 ÷ 20 000 = 1 A.
The heat lost is I² × R = 1² × 2 = 2 W.

Answer: 200 W, then 2 W

The voltage went up 10 times and the loss went down 100 times. The table shows the same pattern for a bigger case. The power sent is 100 000 W, the line has a resistance of 5 Ω, and the voltage goes up by tens.

100 000 W sent along a line of 5 Ω. At the lowest voltage, half the power is lost. At the highest, almost none is.

So the grid works like this. A step-up transformer at the power station raises the voltage to many thousands of volts. The lines carry a small current. Near towns, step-down transformers lower the voltage in stages, down to the voltage homes use. That is still dangerous: never touch mains wiring. The voltages differ from place to place.

Example voltages. Real grids differ from place to place. High voltage lines are dangerous. Stay away from them and from fallen wires.

Try it

Hear why a transformer needs changing current

You need: a thick iron nail or bolt, thin insulated wire, a torch cell, an old earphone with the plug cut off and the wire ends bared, by an adult, sandpaper or a nail file, tape.

Be safe: Ask an adult to cut off the plug and bare the ends. Clean the wire ends by rubbing them with sandpaper or a nail file, not a knife. Use one torch cell only. Touch the wires to it for one second at a time, because the coil gets warm and drains the cell. Hold the earphone loosely near your ear, not pushed in. Never connect a transformer or coil to mains electricity.

Wind 50 turns of wire along the nail, for the primary. Leave the ends free. Wind 100 turns on the other half of the nail, apart from the primary, for the secondary.
Join the secondary to the earphone wires. Hold the earphone loosely near your ear.
Touch the primary wires to the cell for one second, then take them away.
Now try holding the primary joined to the cell for a moment and listen.

What you will see: You may hear a faint click when the primary is joined to the cell, and another when it is taken away. You hear nothing in between. A steady current makes a steady field, and the secondary needs a changing one. That is why transformers work on AC and not on DC.

If you cannot try it, this is what happens.

A wrong idea many people have

A transformer works on a battery just as well as on AC.

Why it feels right: A battery makes a current, and a current makes a field, so the secondary should get a voltage.

The test that breaks it: Join the primary to a cell and hold it on. After a click, the lamp or earphone on the secondary does nothing. A steady current makes a steady field, and a steady field induces no voltage.

What is true: A transformer needs a changing current in the primary. That is why it works on AC. A direct current gives only a brief change when it is switched on or off.

A wrong idea many people have

The turns ratio is Np ÷ Ns for the secondary voltage: Vs = Vp × Np ÷ Ns.

Why it feels right: The ratio Np ÷ Ns appears in the equation, and it is easy to put it the wrong way up.

The test that breaks it: A step-up transformer has more turns on the secondary. It must give a bigger voltage. Using Np ÷ Ns would give a smaller one for 50 turns and 400 turns: 8 V × 50 ÷ 400 = 1 V. That cannot be right.

What is true: The secondary voltage is bigger when the secondary has more turns. Put Ns on top: Vs = Vp × Ns ÷ Np.

A wrong idea many people have

A higher voltage in the power lines means more energy is lost.

Why it feels right: High voltage sounds dangerous and powerful, so it seems it should waste more.

The test that breaks it: Send 20 000 W at 2 000 V and the line loses 200 W. Send it at 20 000 V and it loses 2 W. The current is smaller, and the heat depends on the current squared.

What is true: For the same power, a higher voltage means a smaller current. A smaller current means much less heat in the wires. The energy lost is lower, not higher.

Key words

transformer: a device with two coils on an iron core that changes the voltage of an AC supply.
primary coil: the coil joined to the supply. Np is its number of turns.
secondary coil: the coil that gives the output. Ns is its number of turns.
step-up transformer: a transformer with more turns on the secondary, which raises the voltage.
step-down transformer: a transformer with fewer turns on the secondary, which lowers the voltage.
turns ratio: Vp ÷ Vs = Np ÷ Ns.

Back to your guess

The answer is B: Power lines use a high voltage so that the current is small and little energy is lost as heat.

The same power can be sent as a high voltage and a small current, or a low voltage and a large current. The heat lost in the wires depends on the current, squared. So a high voltage with a small current wastes much less energy. Transformers change the voltage at each end.

Keep it

Copy this into your exercise book.

A transformer needs AC. Vp ÷ Vs = Np ÷ Ns. Ideal: Vp × Ip = Vs × Is. Power lines: high voltage, small current, little heat (I² × R).

Draw: A transformer with two coils on a core, with turns and volts written beside it.

S4.13.8 Chapter 8

Speakers and microphones

A loudspeaker makes sound from a wire. A microphone makes a signal in a wire from sound. What is inside them, and how do they work?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A coil in the field of a magnet: a current moves the coil and the cone makes sound.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A small radio that makes the sound and sends it out of the speaker.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A tiny cell that stores the sounds until they are played.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a loudspeaker turns current into sound
Explain how a microphone turns sound into current
Find the magnet and coil inside an old broken speaker

A loudspeaker and a microphone are almost the same machine. Each has a magnet and a coil that can move. In the loudspeaker, a current makes the coil move. In the microphone, the coil is moved and a current is made.

We cut a loudspeaker through the middle. A circular magnet makes a field across a narrow gap. The field runs outwards from the centre pole to the outer ring. A light coil of wire sits in the gap, wound on a short tube. The tube is fixed to a paper cone.

The coil sits in a field that points outwards, left on the left and right on the right. The current goes round the coil, so the forces on both sides push the same way.

Check the direction with the left hand rule. On the left side of the coil the field points left and the current goes into the page. On the right side the field points right and the current comes out. Both forces point up, so the coil and cone are pushed up.

The amplifier sends the speaker an alternating current. It reverses many times a second. When the current reverses, the force on the coil reverses, and the cone moves down. So the cone moves up and down, or in and out, with the same frequency as the current.

Draw it first

The cone moves in and out with the current

1 The current goes one way round the coil. The forces push the coil and the cone out. The cone pushes the air in front of it, and makes a region of higher pressure.
2 The current reverses. The forces reverse and pull the cone back. The air in front of it is thinned out, a region of lower pressure. The pattern runs out as a sound wave.

The moving cone pushes and pulls the air. That makes a sound wave, with the same frequency as the current. The pitch of the note is the frequency of the current. A current of 400 Hz makes a note one octave above a current of 200 Hz.

Current A makes a low note, 200 Hz. Current B makes a note twice as high, 400 Hz. Both are equally loud.

The loudness depends on the size of the current. A bigger current gives a bigger force, so the cone moves further. A bigger movement pushes more air and gives a louder sound. The pitch does not change.

The same note, 200 Hz. Current A is quiet. Current B is twice the size, so the cone moves twice as far and the sound is louder.

Worked example

A speaker coil has 2.5 m of wire in a field of 0.8 T. A current of 0.5 A flows. What is the force on the coil?

Use F = B × I × L, with B = 0.8 T, I = 0.5 A and L = 2.5 m.
F = 0.8 × 0.5 × 2.5 = 1, so F = 1 N.

Answer: 1 N

Worked example

A speaker makes a note of 250 Hz. How long does the cone take to move out and back once?

The cone makes one in and out movement for each cycle of the current.
The frequency is 250 Hz, so there are 250 cycles each second.
The time for one cycle is 1 ÷ 250 = 0.004 s.

Answer: 0.004 s

Now turn it round. In a microphone, a thin diaphragm moves when sound hits it. A coil is fixed to the diaphragm, in the field of a magnet. As the coil moves, the field through it changes, and a voltage is induced. This is electromagnetic induction.

Sound pushes the diaphragm down. The coil moves down in the field, and a current is induced in the coil.

The diaphragm moves back and forth in step with the sound wave, so the induced voltage alternates in step with it. A higher pitch gives a higher frequency of voltage. A louder sound moves the diaphragm further and gives a bigger voltage.

The two machines are linked. A speaker used backwards, with sound moving its cone, works as a poor microphone. Many microphones work this way. Others work in different ways, and the tiny ones in phones are built on a chip. All of them turn the sound wave into a changing voltage.

A loudspeaker turns a current into sound. A microphone turns sound into a current.

Try it

Find the magnet and the coil in an old speaker

You need: an old broken loudspeaker that is not joined to anything, a steel pin or paper clip, a torch cell, two short pieces of wire.

Be safe: Use only a speaker that is already out of its device, or from a battery toy with the batteries taken out. Never open anything that has been joined to mains electricity. The magnet is strong and can pinch fingers. Keep it away from phones and bank cards. The metal frame can have sharp edges. Do not cut the cone.

Look at the back of the speaker. You should see a round metal block. Hold a steel pin near it. It is pulled hard towards the block: that is the magnet.
Look at the front. Push very gently on the cone with a finger and let go. It springs back. In the middle of the cone is a small dome. The coil is under it, wound on a short tube that sits in the gap of the magnet.
Find the two thin wires that run from the cone to the terminals: they lead to the coil. If the cone is already torn, you may see the tube and its wire.
Use the two short wires to join the speaker’s two terminals to the torch cell for one second. The cone moves a little.
Swap the wires on the cell and touch again. The cone moves the other way.

What you will see: The pin is pulled by the magnet. The cone moves when a current goes round the coil, because the coil is in the field of the magnet. A current the other way moves it the other way. This is the motor effect at work. The same coil and magnet in a microphone would make a current when the cone is moved.

If you cannot try it, this is what happens.

A wrong idea many people have

A steady current in a loudspeaker makes a steady sound.

Why it feels right: A current makes the cone move, so a current that flows all the time seems to give a sound all the time.

The test that breaks it: Touch a cell to the coil of an old speaker. You hear one click, and the cone moves out or in and stays there. Nothing more happens while the current is steady. Take the cell away and you hear another click as the cone springs back.

What is true: A steady current pushes the cone out or in and holds it. Sound needs the cone to move back and forth. The current must keep changing, so the sound comes from an alternating current.

A wrong idea many people have

A louder sound has a higher pitch.

Why it feels right: Loud sounds seem sharper and more piercing than quiet ones, so people often mix up the two.

The test that breaks it: Turn the volume up on a radio, with the tuning the same. The sound gets louder, but the note stays at the same pitch. On the graphs, the louder note is a taller wave with the same number of cycles.

What is true: Loudness is the size of the vibration, the height of the wave. Pitch is the frequency, the number of cycles each second. They are different things.

A wrong idea many people have

A loudspeaker and a microphone are two quite different machines.

Why it feels right: One makes sound and the other picks it up, so they seem to do opposite jobs with different parts.

The test that breaks it: Look at the two pictures. Each has a magnet, a coil and a cone or a diaphragm. Connect a loudspeaker to a sensitive amplifier and speak into it. It works, badly, as a microphone.

What is true: They share the same parts. The loudspeaker uses the motor effect, and the microphone uses induction. They are the same idea run in opposite directions.

Key words

loudspeaker: a device that turns an alternating current into sound, by the force on a coil in a magnetic field.
microphone: a device that turns sound into an alternating voltage. A moving coil microphone uses induction.
cone: the paper part of a loudspeaker that moves with the coil and pushes the air.
diaphragm: the thin sheet of a microphone that sound moves.
pitch: how high or low a note is. It is the frequency of the sound.
loudness: how strong a sound seems. It depends on the size of the vibration.

Back to your guess

The answer is A: A coil in the field of a magnet: a current moves the coil and the cone makes sound.

A loudspeaker has a coil that can move in the field of a magnet. A current in the coil makes a force on it, by the motor effect. The coil moves a paper cone, and the cone pushes the air to make sound. A microphone is the same idea turned round.

Keep it

Copy this into your exercise book.

Loudspeaker: current in a coil in a field, force, cone moves, sound. Microphone: sound moves a coil in a field, voltage induced. Pitch is frequency. Loudness is the size of the current.

Draw: A speaker cut in half with a magnet, a coil and a cone, and arrows for the sound.

Build the field of a bar magnet with a compass

Map a magnetic field with pins

The field round a straight wire

Make a compass needle turn with a current

Why the wire is pushed

Practise Fleming’s left hand rule

The turning effect as the coil turns

Test a toy motor

Push a magnet into a coil, hold it, pull it out

Hear a current made by a magnet

One half turn of a generator coil

Make a motor into a generator

Hear why a transformer needs changing current

The cone moves in and out with the current

Find the magnet and the coil in an old speaker

Key words, A to Z