Electromagnetic Induction

Question 1

What factors affect induced voltage via the generator effect?

Answer 1

Number of turns of wire
Speed of relative movement
B-Field Strength

Question 2

State Faraday’s Law

Answer 2

The EMF induced is proportional to the rate of change of flux linkage

Question 3

What is the equation for magnetic flux?

Answer 3

Φ=BA

Question 4

What are the units of Φ?

Answer 4

Weber (pron. Way-ber), Wb

Question 5

State the equation for the Faraday-Lenz Law

Answer 5

ε=-Δ(NΦ)/Δt

Question 6

Explain why, on a trace of EMF against time for a magnet falling through a coil, the second peak is taller and thinner

Answer 6

The magnet accelerates due to gravity, meaning that, as it speeds up, there is a greater flux change in less time, so a greater EMF is induced

Question 7

For a single wire, length l, moving through a B-field of flux density B at velocity v, what is the equation for induced EMF?

Answer 7

ε=Blv

Question 8

List the 6 important points for Lenz’s Law

Answer 8

Movement of a conductor in a B-field
Change in flux linkage
EMF induced across conductor
EMF α Rate of change of flux linkage
Eddy currents are set up...
...To oppose the change that caused them

Question 9

Why can eddy currents only oppose the motion that caused them?

Answer 9

Because of the conservation of energy. Energy was spent to create the eddy currents, so it is impossible for the magnet to speed up

Question 10

How does a Traffic Sensor use EM induction?

Answer 10

A B-field is created by an alternating current carrying wire under the road. A car (a conductor) enters the B-field and cuts the lines of flux, causing a change in flux linkage, inducing an EMF across the car. The eddy currents created in the car cause a drop in energy from the main circuit, which can be detected by the system to indicate the presence of a vehicle

Question 11

What is a soft magnetic material?

Answer 11

One that is only magnetised when current flows

Question 12

In the jumping ring experiment, describe what happens when DC is used and explain why it happens

Answer 12

The ring jumps up then immediately falls back down
This is because there is only a change in flux linkage when the current is turned on and the field is created, after which the field is constant so no EMF is induced in the ring and it no longer repels the coil.