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Physics 12
ANSWERS: Motors and Generators Assignment
Part II: Comparing AC and DC
1. What is direct current?
Direct current (DC) is when the electric current flows in a constant direction. Direct current is
produced by batteries, solar cells, and commutator-type electric machines.
2. What alternating current?
The current changes polarity or direction, over time. The flow of electric charge periodically reverses
direction in AC current. The usual waveform of an AC power circuit is a sine wave
3. Compare and contrast AC and DC.
Cause of the
direction of flow of
Rotating magnet along the wire.
Steady magnetism along the wire.
The frequency of alternating current is
50Hz or 60Hz depending upon the
The frequency of direct current is zero.
It reverses its direction while flowing in
a circuit.
It flows in one direction in the circuit.
It is the current of magnitude varying
with time
It is the current of constant magnitude.
Obtained from:
A.C Generator
4. List some examples of when a DC electric motor might be used.
Many electrical appliances such as computers for example use DC current.
5. List some examples of when an AC electric motor might be used.
Power is supply to houses in AC current. Examples = washing machine, dryer, fans, refrigerator
Part III: Motors, Generators and Transformers
1. Compare and contrast motors and generators.
An electric motor converts electrical energy into mechanical energy. A generator converts
mechanical energy into electrical energy. Both use the principles of electromagnetism.
2. Where does the kinetic energy come from in the following types of generators?
a. Gas generators
Combustion of the gas
b. Windmill
The wind
c. Crank radio
The hand winding the spring inside
3. What is a transformer?
A transformer is an electrical device which converts alternating current from one voltage to another.
It has no moving parts and consists of two or more coils of insulated wire wound on a laminated
steel core.
4. Explain the difference between a step-up and a step-down transformer (a diagram may be
So a step up transformer increases the voltage and a step down transformer decreases the voltage.