Eddy Currents, AC Current and Transformers Flashcards

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1
Q

What are eddy currents?

A

Currents produced in a conductor by magnetic fields

(Lenz’ law in conductors)

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2
Q

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

A
  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
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3
Q

How are eddy currents created in this copper pipe?

A
  1. Flux linkage decreasing above -> current creates attracting field upwards
  2. Flux linkage increasing below -> current creates repelling field upwards
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4
Q

How does eddy current braking work?

A
  1. Part of disk leaving field -> current creates attraction to electromagnet
  2. Part of disk entering field -> current creates repulsion to electromagnet
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5
Q

When can eddy current braking not be used?

A

To hold a car stationary on a slope

(no change in flux linkage)

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6
Q

How does the oscilloscope trace look for an AC current?

(When the time base is switched on)

A
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7
Q

How does the oscilloscope trace look for an AC current?

(When the time base is switched off)

A
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8
Q

How does the oscilloscope trace look for an DC current?

(When the time base is switched on)

A
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9
Q

How does the oscilloscope trace look for an DC current?

(When the time base is switched off)

A
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10
Q

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

A

The equivalent DC voltage that would supply the same average power

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11
Q

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

A

The equivalent DC current that would supply the same average power

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12
Q

Label V0 and Vp→p on this AC oscilloscope trace

A
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13
Q

What is wrong with this calculation?

A

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

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14
Q

How does a step up transformer work?

A
  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)
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15
Q

How does a step down transformer work?

A
  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)
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16
Q

Why does a transformer only work with AC supply?

A

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

17
Q

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

A

This equation always works (no matter what efficiency)

18
Q

How do you calculate the efficiency of a transformer?

A

(The voltages and currents must be rms values)

19
Q

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

A
  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)
20
Q

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
A
  1. Laminate the core
  2. Use wire with low resistance
  3. Use soft iron core for magnet
  4. Wind primary coil over secondary coil
21
Q

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

A

Smaller currents = smaller energy losses to thermal

(P=I2R)