Electric and magnetic forces: electromagnets, motors and generators
Week 9 · Lesson 3 Electric and Magnetic Forces: the magnetic effect of current, the electromagnet and its strength, force on a current-carrying wire, the electric motor and its torque, the electric generator · 3 lessons
3
A motor run backwards: the generator
Science Prep 3 Term 1, Unit 2: pp. 54–63
GoalExplain how a generator makes an emf by turning a coil in a magnetic field and compare AC and DC.
New words
generator · a device in which a coil turns in a magnetic field and produces an emf; it changes mechanical energy into electrical energyalternating current (AC) · a current whose direction changes with timedirect current (DC) · a current that flows in one direction only
Explanation
A motor turned by hand is a generator. When a coil turns in a magnetic field, it cuts the field lines, and this produces an emf, so a current flows in the circuit. A generator changes mechanical energy into electrical energy. The emf grows with the speed of turning: the lamp of a bicycle dynamo glows brighter as you pedal faster. A generator does not create energy; it changes it. Power stations use turbines turned by falling water, steam or wind to turn the generators. The stations produce alternating current, AC, the type used in homes, which can be sent over long distances with relatively low losses. In AC the direction changes with time; in DC it stays the same. A generator with a split ring gives DC. Emergency generators back up homes, hospitals and factories when the supply stops.
Worked examples
A student turns the shaft of a toy motor by hand with a small LED joined across its terminals. The LED lights, so the motor acts as a generator. If it does not light, swap the two connections: an LED works only one way round.
Pedalling
Lamp
stopped
off
slow
dim
fast
bright
A dynamo on a bicycle lights a lamp. At slow speed the lamp is dim; at fast speed it is bright, because the emf grows with the speed of turning. At rest, no lamp: no turning, no emf.
A generator turned by 50 J of work cannot give more than 50 J of electrical energy, because energy is not created. Some becomes heat in the wires and bearings, so the electrical output is a little less than 50 J.
Class activity
Motor as a generator, with a grown-up. Use a toy DC motor with a small LED joined across its terminals (an adult checks the connections). Spin the shaft by hand slowly, then quickly, and describe the LED. Swap the two connections if the LED does not light. Make a table: slow turning, fast turning, no turning. Then write one sentence on why this is called a generator, and one on where the energy comes from.
Practice
1
What two things are needed in a generator to produce an emf?
A coil and a magnetic field, with the coil turning so that it cuts the field lines.
2
What energy change does a generator make?
Mechanical energy into electrical energy.
3
How can the emf of a bicycle dynamo be increased?
Turn it faster (pedal faster).
4
What is the difference between AC and DC?
In AC the direction changes with time; in DC the current goes one way only.
5
Why does a generator not break the rule that energy is not created?
It only changes mechanical energy into electrical energy; the energy comes from the turning.
