EM Induction

Question 1

What is EM induction?

Answer 1

Process where an EMF is induced in a conductor through a change in magnetic flux.

Question 2

What are some examples of EMF induction?

Answer 2

Moving a wire so it cuts through a magnet’s flux lines
Moving a magnet into and out of a coil of wire
Moving a conductor into and out of a magnet’s field
Dropping a magnet through a coil of wire
Spinning a coil of wire in a mag. field
Placing a conductor in an electromagnet field and changing the current

Question 3

What is Lenz’s law?

Answer 3

That an induced EMF will oppose the change in flux producing it. Due to conservation of energy.

Question 4

What are some examples of Lenz’s law?

Answer 4

A magnet will fall slowly through a conductive tube.
A spinning conductor will spin slower through a mag. field
A non-mag conductor can move by moving a magnet over it

Question 5

What is Faraday’s law?

Answer 5

The magnitude of induced EMF is equal to the rate of change of flux. EMF = -N (delta flux / delta t)
EMF = change in flux linkage / time taken

Question 6

What is the EMF induced through a straight wire of length l moving at speed V through a field B?

Answer 6

Emf = Blv

Question 7

What shape does the emf in an a.c generator take?

Answer 7

A sine wave

Question 8

What is the flux linkage in a coil spinning, and what do the symbols represent?

Answer 8

N flux = BAN cos w t, N is turns in coil, A is area of coil, w is angular speed, B is mag field strength.

Question 9

What is the EMF induced from a coil spinning?

Answer 9

emf = BAN w sin (wt)

Question 10

What is the max induced EMF from an A.C generator?

Answer 10

BANw

Question 11

What is the peak-to-peak value of the EMF induced from an AC generator?

Answer 11

2BANw

Question 12

How do step-up transformers work?

Answer 12

More turns on 2nd coil than on 1st coil, so steps up voltage and steps down current.

Question 13

How do step-down transformers work?

Answer 13

More turns on 1st coil than on 2nd coil, so steps down voltage and steps up current.

Question 14

What is the equation for transformers?

Answer 14

Ns/Np = Vs/Vp, Ns is turns on secondary, Vp is voltage on primary. Then use V=IR.

Question 15

What are eddy currents?

Answer 15

Small currents created in the conductive iron core of a transformer by the changing mag. field

Question 16

Why do transformers have an iron core?

Answer 16

To maximise the flux linkage through the 2nd core by channelling the flux linkage from the 1st coil through it.

Question 17

Why do transformers lose power?

Answer 17

Eddy currents heating the iron core and reducing flux, heating in the wires that form the coils.

Question 18

How do transformers minimise power loss?

Answer 18

Low resistance wires to minimise the heating in wires.
A laminated iron core to increase resistance in the core and reduce the size of eddy currents.
Making the core out of soft iron that can be easily magnetised and demagnetised to reduce heating effect of magnetisation.

Question 19

What equation gives the power loss from transmitting power?

Answer 19

Combining I = P/V and P(loss) = I^2 R gives P(loss) = P^2 R / V^2, showing that transmitting power at high voltages and low currents reduces power loss due to heating.

Question 20

What is the peak voltage and peak-to-peak of AC?

Answer 20

Peak voltage is max voltage provided in either direction, peak-to-peak is 2x peak voltage.

Question 21

What is the rms current of an AC supply?

Answer 21

The value of direct current that would have the same heating effect. Essentially a conversion to DC.

Question 22

What is the power equations for a circuit supplied by an AC generator?

Answer 22

P = R (I(rms))^2, P = (V(rms))^2 / R