3The operating principle of electromagnetic devices

Most of the electricity we use is generated by electromagnetic induction. This process goes on in the generators at work in power stations, in wind turbines (Figure) and, on a much smaller scale, in bicycle dynamos. It is the process whereby a conductor and a magnetic field are moved relative to each other to induce, or generate, a current or electromotive force
(e.m.f.).

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To analyse the operating principle of electromagnetic devices (electromagnetic relay, generator and transformer);

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(G) Group work

GROUP 1: electromagnetic relay
GROUP 2: generator
GROUP 3: transformer

Application
Use Faraday’s law of electromagnetic induction to explain the operating principle of electromagnetic relay, generator and transformer;
Analysis
Explain the working principle of electromagnetic relay;
Explain the working principle generator;
Explain the working principle of transformer;

1. When a generator is used to produce electric current, the resulting electric energy originates from which source?
(a) The generator’s magnetic field.
(b)Whatever rotates the generator’s axle.
(c) The resistance of the generator’s coil.
(d) Back emf.
(e) Empty space.

2. Which of the following will not increase a generator’s voltage
output?
(a) Rotating the generator faster.
(b) Increasing the area of the coil.
(c) Rotating the magnetic field so that it is more closely
parallel to the generator’s rotation axis.
(d) Increasing the magnetic field through the coil.
(e) Increasing the number of turns in the coil.

3. Which of the following can a transformer accomplish?
(a) Changing voltage but not current.
(b) Changing current but not voltage.
(c) Changing power.
(d) Changing both current and voltage.

4. Which of the following statements about transformers is false?
(a) Transformers work using ac current or dc current.
(b) If the current in the secondary is higher, the voltage is
lower.
(c) If the voltage in the secondary is higher, the current is
lower.
(d) If no flux is lost, the product of the voltage and the
current is the same in the primary and secondary coils.

What has been learned
What remained unclear
What is necessary to work on