S4.13.3 For the adult
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?
What this chapter teaches
A wire with a current in a magnetic field feels a force at right angles to both. Fleming’s left hand rule gives the direction: first finger Field, second finger Current, thumb Motion. For a wire at right angles to the field, F = B × I × L, with B in tesla, I in amps and L in metres.
Three questions to ask
There is no one right wording. Listen for the idea.
- A current comes out of the page and the field points right. Which way does the wire move, and how did you find out?
- Why is there no force on a wire that lies along the field?
- A wire of 0.1 m in a field of 0.5 T carries 2 A. How do you find the force?
After the chapter, the learner can
- Use Fleming's left hand rule to find the force on a wire
- Calculate force using F equals B times I times L
What they copy into their notebook
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.