Eddy Currents, AC Current and Transformers

Question 1

What are eddy currents?

Answer 1

Currents produced in a conductor by magnetic fields

(Lenz’ law in conductors)

Question 2

Why does the magnet take longest to fall through the full copper pipe?

Answer 2

1. Magnet in freefall
2. Plastic not a conductor so no eddy currents (still in freefall)
3. Copper pipe incomplete so can’t create eddy currents (still in freefall)
4. Eddy current reduce acceleration

Question 3

How are eddy currents created in this copper pipe?

Answer 3

1. Flux linkage decreasing above -> current creates attracting field upwards
2. Flux linkage increasing below -> current creates repelling field upwards

Question 4

How does eddy current braking work?

Answer 4

1. Part of disk leaving field -> current creates attraction to electromagnet
2. Part of disk entering field -> current creates repulsion to electromagnet

Question 5

When can eddy current braking not be used?

Answer 5

To hold a car stationary on a slope

(no change in flux linkage)

Question 6

How does the oscilloscope trace look for an AC current?

(When the time base is switched on)

Answer 6

Question 7

How does the oscilloscope trace look for an AC current?

(When the time base is switched off)

Answer 7

Question 8

How does the oscilloscope trace look for an DC current?

(When the time base is switched on)

Answer 8

Question 9

How does the oscilloscope trace look for an DC current?

(When the time base is switched off)

Answer 9

Question 10

For AC supply what is V_rms and how is it calculated?

Answer 10

The equivalent DC voltage that would supply the same average power

Question 11

For AC supply what is I_rms and how is it calculated?

Answer 11

The equivalent DC current that would supply the same average power

Question 12

Label V₀ and V_p→p on this AC oscilloscope trace

Answer 12

Question 13

What is wrong with this calculation?

Answer 13

When using electricity formulas must use rms values for voltage (and current)

Question 14

How does a step up transformer work?

Answer 14

1. AC current flows through primary coil
2. Magnetic field flows through secondary coil
3. Changing flux linkage in secondary coil larger
4. Greater emf induced (so bigger voltage)

Question 15

How does a step down transformer work?

Answer 15

1. AC current flows through primary coil
2. Magnetic field flows through secondary coil
3. Changing flux linkage in secondary coil smaller
4. Smaller emf induced (so smaller voltage)

Question 16

Why does a transformer only work with AC supply?

Answer 16

If supply is DC
Flux linkage in secondary coil doesn’t change
So emf isn’t induced (Faraday’s law)

Question 17

How do you calculate the voltage (rms) in the secondary coil?

Answer 17

This equation always works (no matter what efficiency)

Question 18

How do you calculate the efficiency of a transformer?

Answer 18

(The voltages and currents must be rms values)

Question 19

What are the main causes of thermal loss in a transformer?

Answer 19

1. Large Eddy currents in magnet (P=I²R)
2. Large currents in primary and secondary coils
3. Hysteresis losses (magnet’s resistance to change in flux linkage)
4. Flux losses (not all flux passing through secondary coil)

Question 20

How are the main causes of thermal losses in a transformer reduced?

1. Large Eddy currents in magnet
2. Large currents in primary and secondary coils
3. Hysteresis losses
4. Flux losses

Answer 20

1. Laminate the core
2. Use wire with low resistance
3. Use soft iron core for magnet
4. Wind primary coil over secondary coil

Question 21

Why are step up transformers used to transport electricity over long distances?

Answer 21

Smaller currents = smaller energy losses to thermal

(P=I²R)