Motion, flux and induced fields

Science · Electricity & Magnetism · ages 19-21

Name ______________________ Date ____________

The flux rule and the magnetic-force picture give the same EMF for a rod on rails.
Circle one: True False
A rod of length l moves at v across uniform B, square on. What is the EMF?
- B l divided by v
- B l v
- B v divided by l
Why can an induced field push charges with no conductor present?
- It is made by the changing flux itself
- Air conducts whenever flux changes
- Static charges always leak outward
The rod speed doubles with B and l fixed. What happens to the EMF?
- It halves
- It stays the same
- It doubles
A 2.0 m rod moves at 3.0 m/s square to a 0.5 T field. Give the EMF in volts.
Answer: ______________
A copper sheet swings between magnet poles and slows down. Where did the motion go?
- Into heat from eddy currents
- Into extra magnetic charge
- Into faster electron drift
Rails with R of 2 ohms: B is 2 T, l is 1 m, v is 3 m/s. Give the current in A.
Answer: ______________
Induction needs a closed wire loop, since EMF cannot exist in empty space.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Changing area and pushes on charges describe one effect.
B l v · Flux grows by B l per metre moved, times v.
It is made by the changing flux itself · Changing flux creates a real field whether wire sits there or not.
It doubles · EMF is linear in v, so twice the speed means twice the EMF.
3 · B l v gives 0.5 times 2.0 times 3.0, which is 3.
Into heat from eddy currents · Eddy currents fight the motion and dump the energy as heat.
3 · Current is B l v over R, so 6 over 2 is 3.
False · The induced field exists around changing flux with no wire at all.