Topic 11: Electromagnetic induction

Question 1

State Faraday’s law of induction.

Answer 1

The size of the induced emf is proportional to the rate of change of flux linkage

Question 2

Explain, using Faraday’s law of induction, how a step-down transformer steps down a voltage

Answer 2

• There is a varying current/voltage in the primary coil, producing a varying magnetic field
• This produces a change in flux linkage in the secondary coil
• Therefore, a varying emf is produced in the secondary coil
• The voltage will be stepped down as there are more turns in the primary coil than in the secondary coil

Question 3

How are energy losses reduced in the core of a practical transformer?

Answer 3

• They have a laminated core which decreases eddy currents
• Therefore, less thermal energy is transferred to the surroundings

Question 4

What is an eddy current and how do they cause energy dissipation?

Answer 4

• A small current created in the iron core that come from the changing magnetic fields
• The current created moves free electrons within the core causing heating of the core and therefore energy dissipation

Question 5

State Lenz’s law.

Answer 5

An induced emf acts in the direction such that the current induced opposes the change which caused it.

Question 6

What is a transformer?

Answer 6

A device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa

Question 7

Step-up transformers are used in power stations to increase the voltage at which the electricity is transmitted. Explain why this is done.

Answer 7

• By increasing the voltage, the current would decrease
• High currents cause the wire to heat up and cause energy dissipation

Question 8

What is the photoelectric effect?

Answer 8

The phenomenon in which electrically charged particles are released from or within a metal when it absorbs electromagnetic radiation.

Question 9

State what is meant by the work function of a metal.

Answer 9

The minimum energy required to remove an electron from the metal’s surface.