Chapter 2 of 8

All 8 chapters
  1. 1Magnetic fields, readunderstood
  2. 2Fields around wires and coils, readunderstood
  3. 3The motor effect, readunderstood
  4. 4Electric motors, readunderstood
  5. 5Electromagnetic induction, readunderstood
  6. 6Generators and dynamos, readunderstood
  7. 7Transformers, readunderstood
  8. 8Speakers and microphones, readunderstood

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

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • 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. Three pictures of a wire seen end on, with a dot in a small circle at the centre meaning the current comes out of the page towards you. Step 1: one circle round the wire, with arrows going anticlockwise. Step 2: two circles. Step 3: three circles. The circles get further apart as they get bigger. Every circle has arrows going anticlockwise.

    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. Three pictures of a wire seen end on, with a dot in a small circle at the centre meaning the current comes out of the page towards you. Step 1: one circle round the wire, with arrows going anticlockwise. Step 2: two circles. Step 3: three circles. The circles get further apart as they get bigger. Every circle has arrows going anticlockwise.

    2 Add a second circle further out. The field is weaker there, so there is a wider gap between the circles.

  3. Three pictures of a wire seen end on, with a dot in a small circle at the centre meaning the current comes out of the page towards you. Step 1: one circle round the wire, with arrows going anticlockwise. Step 2: two circles. Step 3: three circles. The circles get further apart as they get bigger. Every circle has arrows going anticlockwise.

    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 −.

A wire seen end on, with a cross in a small circle at the centre meaning the current goes into the page. Three circles round the wire, with arrows going clockwise. Label under the picture: current into the page.current into the page
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?

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

Answer: 4 µT

A graph with the axes field (µT) and distance from the wire (cm). Distance runs from 0 to 18 cm and field from 0 to 12 µT. A curve falls steeply from 12 µT at 3 cm and flattens out. A dot at 3 cm, 12 µT and a dot at 9 cm, 4 µT each have dashed lines to the axes.369121518246810120distance from the wire (cm)field (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.

A coil cut along its axis. A row of seven wires along the top, each a dot in a small circle for a current out of the page, and a row of seven wires along the bottom, each a cross in a small circle for a current into the page. A dashed line joins each top wire to the wire below it. Five field lines run through the coil from left to right, one along the axis and two on each side of it, nearly straight and evenly spaced. Outside the coil the lines curve away and leave the picture. The left end is labelled S and the right end is labelled N.SN
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.

The same coil cut along its axis, with the current reversed. The top row of seven wires now shows crosses and the bottom row shows dots. Five field lines run through the coil from right to left, evenly spaced. The left end is labelled N and the right end is labelled S.NS
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 iron nail with a coil of wire round it and a cell above, with the labels iron nail, coil and cells. Clips hang from the point of the nail, with the label paper clips.iron nailcoilcellspaper clips
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.

  1. 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.
  2. 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.
  3. Turn the cell round, so the current goes the other way. Touch the loop closed for one second again.
  4. 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 table with the columns Loop and Needle. Rows: open, stays north. closed, swings. reversed, other way.LoopNeedleopenstays northclosedswingsreversedother way

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.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A long straight wire carries a current into the page, seen as a cross. Looking at the page, which way does the field go round the wire?

    Tap your answer.

    1. Not this one. That is the field round a current coming out of the page, shown as a dot. For a cross, grip with the thumb pointing away from you and the fingers curl clockwise. Try another.

    2. Not this one. The field is never along the wire. It goes in circles round it, at right angles to the current. Try another.

    3. Not this one. Field lines of a wire do not point away from it. They go round it in closed circles. Try another.

    4. Yes, that is right. Grip the wire with the right hand, thumb along the current. The thumb points into the page, away from you, and the fingers curl clockwise as you look at the page.

    See the answer

    Clockwise.

    Grip the wire with the right hand, thumb along the current. The thumb points into the page, away from you, and the fingers curl clockwise as you look at the page.

    • If you chose A: That is the field round a current coming out of the page, shown as a dot. For a cross, grip with the thumb pointing away from you and the fingers curl clockwise.
    • If you chose B: The field is never along the wire. It goes in circles round it, at right angles to the current.
    • If you chose C: Field lines of a wire do not point away from it. They go round it in closed circles.

  2. Which hand and which rule gives the direction of the field round a wire with a current?

    Tap your answer.

    1. Yes, that is right. Use the right hand grip rule for a wire. The thumb points the way the conventional current flows. The fingers curl round the wire the way the field goes.

    2. Not this one. That is Fleming’s left hand rule, which gives the force on a wire in a field. It does not give the field of the wire. Try another.

    3. Not this one. That is the grip rule for a coil, not for a straight wire. For a wire, the thumb goes along the current. Try another.

    4. Not this one. The direction depends only on the direction of the current, not on the metal. Try another.

    See the answer

    The right hand grip rule: thumb along the current, fingers curling the way of the field.

    Use the right hand grip rule for a wire. The thumb points the way the conventional current flows. The fingers curl round the wire the way the field goes.

    • If you chose B: That is Fleming’s left hand rule, which gives the force on a wire in a field. It does not give the field of the wire.
    • If you chose C: That is the grip rule for a coil, not for a straight wire. For a wire, the thumb goes along the current.
    • If you chose D: The direction depends only on the direction of the current, not on the metal.

  3. You look at one end of a coil. The current goes clockwise round the turns at that end. What pole is that end?

    Tap your answer.

    1. Not this one. Clockwise, looking at an end, makes a S pole. Anticlockwise makes a N pole. Check with the grip rule: the fingers curl clockwise and the thumb points away, so the field leaves at the other end. Try another.

    2. Yes, that is right. Curl the fingers of the right hand clockwise, as you look at the end. The thumb points away from you, along the field inside. The field leaves at the far end, which is N, so the near end is S.

    3. Not this one. A coil with a current has a N pole at one end and a S pole at the other end. Try another.

    4. Not this one. A neutral point is a place where the field is zero. The ends of a coil are where the field is strong. Try another.

    See the answer

    A S pole.

    Curl the fingers of the right hand clockwise, as you look at the end. The thumb points away from you, along the field inside. The field leaves at the far end, which is N, so the near end is S.

    • If you chose A: Clockwise, looking at an end, makes a S pole. Anticlockwise makes a N pole. Check with the grip rule: the fingers curl clockwise and the thumb points away, so the field leaves at the other end.
    • If you chose C: A coil with a current has a N pole at one end and a S pole at the other end.
    • If you chose D: A neutral point is a place where the field is zero. The ends of a coil are where the field is strong.

  4. Which change makes the field of a coil with a current stronger?

    Tap your answer.

    1. Yes, that is right. A soft iron core becomes an induced magnet. Its field adds to the field of the coil and can make it many times stronger. More turns or a bigger current also help.

    2. Not this one. Fewer turns means fewer fields to add up, so the field gets weaker. Try another.

    3. Not this one. Copper is not magnetic, so it adds no field of its own. A soft iron core is what makes the field stronger. Try another.

    4. Not this one. The field of a coil is in proportion to the current, so a smaller current gives a weaker field. Try another.

    See the answer

    Put a soft iron core inside the coil.

    A soft iron core becomes an induced magnet. Its field adds to the field of the coil and can make it many times stronger. More turns or a bigger current also help.

    • If you chose B: Fewer turns means fewer fields to add up, so the field gets weaker.
    • If you chose C: Copper is not magnetic, so it adds no field of its own. A soft iron core is what makes the field stronger.
    • If you chose D: The field of a coil is in proportion to the current, so a smaller current gives a weaker field.

  5. What is the field like inside a long coil with a current in it?

    Tap your answer.

    1. Not this one. It is outside the coil that the fields of the turns cancel more. Inside, they all point the same way and add up. Try another.

    2. Not this one. The field in the middle of a long coil is strong. It is nearly the same everywhere inside, not weaker in the middle. Try another.

    3. Not this one. Each turn does make circles close to the wire. Further in, the fields of all the turns join into one straight, even field. Try another.

    4. Yes, that is right. Inside a long coil the fields of all the turns point the same way and add up. The result is strong and nearly uniform: parallel field lines, evenly spaced.

    See the answer

    Strong, and almost the same all through, with straight lines evenly spaced.

    Inside a long coil the fields of all the turns point the same way and add up. The result is strong and nearly uniform: parallel field lines, evenly spaced.

    • If you chose A: It is outside the coil that the fields of the turns cancel more. Inside, they all point the same way and add up.
    • If you chose B: The field in the middle of a long coil is strong. It is nearly the same everywhere inside, not weaker in the middle.
    • If you chose C: Each turn does make circles close to the wire. Further in, the fields of all the turns join into one straight, even field.

  6. The current in a coil is reversed. What happens to its poles?

    Tap your answer.

    1. Not this one. The poles depend on the direction of the current. Reverse the current and the field reverses. Try another.

    2. Yes, that is right. By the grip rule, reversing the current reverses the way the fingers curl, so the thumb points the other way. The N and S ends swap.

    3. Not this one. A coil always has one N end and one S end. Reversing the current swaps them. Try another.

    See the answer

    They swap: the N pole becomes the S pole.

    By the grip rule, reversing the current reverses the way the fingers curl, so the thumb points the other way. The N and S ends swap.

    • If you chose A: The poles depend on the direction of the current. Reverse the current and the field reverses.
    • If you chose C: A coil always has one N end and one S end. Reversing the current swaps them.

  7. From an earlier chapter

    Which statement about the field lines of a bar magnet is true?

    Tap your answer.

    1. Yes, that is right. A field line has no start and no end. Outside the magnet it runs from N to S, and inside it runs from S to N, so the line closes up.

    2. Not this one. They carry on inside the magnet. Field lines have no start or end. Go to S4.13.1. Try another.

    3. Not this one. Field lines never cross. The strongest field is where the lines are closest. Go to S4.13.1. Try another.

    4. Not this one. The field is everywhere near the magnet. The lines are a drawing of it. Go to S4.13.1. Try another.

    See the answer

    They are closed loops: N to S outside, S to N inside.

    A field line has no start and no end. Outside the magnet it runs from N to S, and inside it runs from S to N, so the line closes up.

    • If you chose B: They carry on inside the magnet. Field lines have no start or end. Go to S4.13.1.
    • If you chose C: Field lines never cross. The strongest field is where the lines are closest. Go to S4.13.1.
    • If you chose D: The field is everywhere near the magnet. The lines are a drawing of it. Go to S4.13.1.

  8. A long straight wire makes a field of 20 µT at 2 cm. The current stays the same. What is the field at 8 cm?

    See the answer

    5 µT

    8 cm is 8 ÷ 2 = 4 times as far, so the field is 4 times smaller: 20 µT ÷ 4 = 5 µT.

  9. A current of 2 A in a long straight wire makes a field of 7 µT at 3 cm. The current is raised to 6 A. What is the field at the same place?

    See the answer

    21 µT

    6 A is 6 ÷ 2 = 3 times the current, so the field is 3 times bigger: 7 µT × 3 = 21 µT.

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.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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