Alternating Currents

Question 1

Describe the working principle of the transformer.

Answer 1

1. Primary coils causes a changing magnetic flux in the primary coil and the soft iron core.
2. Since the primary coil and the secondary coils are linked; this also causes a changing magnetic flux in secondary coil (in phase with the current)
3. From Faraday’s law, this induces and emf in the secondary coil.
4. Since this induced emf in the secondary coil is equal to the rate of change of magnetic flux in the secondary coil
5. The rate of change of magnetic flux in the secondary coil is out of phase with the magnetic flux in the secondary coil
   Hence, the induced emf is out of phase with the flux/ input current
6. From Lenz’s law, the emf induced in the secondary coil is out of phase to the primary coil to oppose the change caused by it.

Question 2

What is the ratio of the step up and the step down voltage?

Answer 2

Vs/Vp = Ns/Np

Question 3

What is an ideal transformer?

Answer 3

There is no energy loss during the step up and the step down/ power output = power input

Question 4

Describe the design of a transformer to increase the efficiency.

Answer 4

1. Both coils are insulated so that the coils can be wound tightly without shock.
2. Both coils are wound on top of each other to prevent the leakage of the magnetic flux.
3. Coils are made out of materials with low resistivity to decrease resistive heating in primary coils and secondary coils.
4. Soft iron core is used as it magnetises and demagnetises easily to decrease heat loss,
5. Soft iron core is laminated to decrease eddy currents, reducing heat energy loss.

Question 5

What are the advantages of a high voltage power transmission?

Answer 5

High voltage:
- For the same power, high voltage means smaller current, Hence less heating and thermal energy loss in cables and coils.

AC:

• Easy to vary output voltage efficiently
• Transformers only work with AC
• Generators already produce AC
• Easier to rectify AC -> DC then to invert DC