O-Level Physics · Secondary 3 and 4
Electromagnetism
Electromagnetism links electricity and magnetism. A current produces a magnetic field, a magnetic field exerts a force on a current-carrying conductor, and a changing magnetic field induces an e.m.f. in a conductor. These three ideas explain electromagnets, electric motors, generators and transformers, which together make modern power supplies possible. The topic builds directly on current electricity and on the idea of force from dynamics, and it is usually one of the last topics studied in upper secondary physics.
What the syllabus expects
- Draw the magnetic field patterns around a straight current-carrying wire and a solenoid, and use the right-hand grip rule
- Describe how the strength of an electromagnet can be increased and give some uses
- Use Fleming's left-hand rule to predict the direction of the force on a current-carrying conductor in a magnetic field
- Explain how a simple d.c. motor works, including the role of the split-ring commutator
- Describe electromagnetic induction and state the factors that affect the size of the induced e.m.f.
- Use Lenz's law to find the direction of an induced current
- Describe the structure and action of a simple a.c. generator
- Use Vs/Vp = Ns/Np and VpIp = VsIs for an ideal transformer
The magnetic effect of a current
A current in a straight wire produces a magnetic field made of concentric circles around the wire, strongest close to it. The right-hand grip rule gives the direction: grip the wire with your right hand with the thumb pointing along the conventional current, and your curled fingers show the direction of the field lines. A solenoid, a long coil of wire, produces a field like that of a bar magnet, with a strong, nearly uniform field inside. Using the grip rule on the coil, with fingers following the current, the thumb points to the north pole. An electromagnet is a solenoid wound on a soft-iron core. Its strength increases with more turns, a larger current and the soft-iron core. Soft iron is used because it is easily magnetised and demagnetised, so the magnet can be switched off. Uses include relays, circuit breakers and cranes for lifting scrap iron.
The force on a current-carrying conductor
A wire carrying a current across a magnetic field experiences a force, called the motor effect. Fleming's left-hand rule gives its direction. Hold the thumb, first finger and second finger of your left hand at right angles to each other. The first finger points along the field (from north to south), the second finger along the conventional current, and the thumb then shows the direction of the force, and so the motion. The force is larger with a larger current or a stronger field, and it is zero if the wire runs parallel to the field. Reversing either the current or the field reverses the force. Reversing both leaves the force in the same direction.
The d.c. motor
A simple d.c. motor is a rectangular coil that can rotate between the poles of a magnet. Current flows in opposite directions along the two sides of the coil, so by Fleming's left-hand rule one side is pushed up and the other down. These two forces produce a turning effect on the coil. A split-ring commutator, connected to the supply through carbon brushes, reverses the direction of the current in the coil every half turn, just as the coil passes the vertical position. This keeps the turning effect in the same direction, so the coil keeps rotating the same way. The turning effect can be increased by using more turns on the coil, a larger current or a stronger magnet, and by winding the coil on a soft-iron core.
Electromagnetic induction and the a.c. generator
An e.m.f. is induced in a conductor whenever the magnetic field linking it changes, for example when a magnet moves into a coil or a wire cuts across field lines. Faraday's law states that the size of the induced e.m.f. is proportional to the rate of change of the magnetic flux linkage. In practice, it is larger with faster motion, a stronger magnet and more turns on the coil. If the magnet is held still inside the coil there is no change, so no e.m.f. Lenz's law states that the direction of the induced current is such that it opposes the change producing it. When the north pole of a magnet is pushed into a coil, the end of the coil facing it becomes a north pole and repels it. In an a.c. generator a coil is rotated in a magnetic field, and slip rings with brushes take out an alternating e.m.f. Rotating the coil faster gives a larger peak e.m.f. and a higher frequency.
Transformers
A transformer has a primary coil and a secondary coil wound on a soft-iron core. An alternating current in the primary produces a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary. A transformer does not work on a steady direct current, because the field would not change. The voltages are in the ratio of the turns: Vs/Vp = Ns/Np. A step-up transformer has more turns on the secondary, a step-down transformer has fewer. An ideal transformer is 100% efficient, so the power in equals the power out: VpIp = VsIs. Electricity is transmitted over long distances at high voltage because, for the same power, a higher voltage means a smaller current, and the power lost as heat in the cables, I²R, becomes much smaller.
Worked example
An ideal transformer has 2000 turns on its primary coil, which is connected to a 240 V a.c. supply. Its secondary coil lights a lamp rated 12 V, 36 W at normal brightness. Calculate (a) the number of turns on the secondary coil, (b) the current in the lamp and (c) the current in the primary coil.
- Use the turns ratio: Ns/Np = Vs/Vp, so Ns = Np × Vs ÷ Vp = 2000 × 12 ÷ 240 = 100 turns.
- The lamp works normally, so it has 12 V across it and takes 36 W. Current in the lamp: Is = P ÷ V = 36 ÷ 12 = 3.0 A.
- For an ideal transformer, power in = power out, so VpIp = VsIs = 36 W.
- Primary current: Ip = 36 ÷ 240 = 0.15 A.
Answer: (a) 100 turns (b) 3.0 A (c) 0.15 A
Common mistakes
- Using the right hand for the motor effect. Fleming's left-hand rule is for the force on a current-carrying conductor. The right-hand grip rule is only for the field around a current.
- Pointing the first finger the wrong way along the field. Field lines run from the north pole to the south pole outside a magnet.
- Saying the commutator reverses the direction of rotation. It reverses the current in the coil every half turn so that the rotation continues in the same direction.
- Thinking a transformer works on a battery. A steady d.c. gives a steady field in the core, so no e.m.f. is induced in the secondary.
- Turning the turns ratio upside down. Write Vs/Vp = Ns/Np with the secondary on top on both sides, then check that a step-down transformer gives a smaller output voltage.
- Saying a current is induced while a magnet rests inside a coil. Induction needs a change in the magnetic field linking the coil, so there must be relative motion or a changing current.
Exam tips
- Before using Fleming's left-hand rule, mark the field direction and current direction on the diagram, then orient your hand. Write the resulting force direction in words as well as with an arrow.
- In Lenz's law answers, name the pole formed at the end of the coil nearest the magnet and say that it opposes the motion of the magnet.
- In transformer calculations, write the equation in symbols first and finish with a sense check, such as whether the voltage should go up or down.
Practice questions
1. Easy: A transformer has 500 turns on its primary coil and 25 turns on its secondary coil. The primary is connected to 230 V a.c. Find the output voltage and say whether it is a step-up or step-down transformer.
Show answer
Vs = Vp × Ns ÷ Np = 230 × 25 ÷ 500 = 11.5 V. The output voltage is lower, so it is a step-down transformer.
2. Medium: A coil is connected to a sensitive centre-zero galvanometer. Describe what the needle does when a magnet is (a) pushed into the coil, (b) held still inside the coil and (c) pulled out more quickly than it went in.
Show answer
(a) The needle deflects one way, because the changing field induces a current. (b) The needle returns to zero, because the field is no longer changing. (c) The needle deflects the opposite way, by a larger amount, because the change is reversed and happens faster.
3. Harder: A power station sends 50 kW through cables with a total resistance of 0.40 Ω. Compare the power lost in the cables if it is transmitted at 250 V and at 25 000 V.
Show answer
At 250 V: I = P ÷ V = 50 000 ÷ 250 = 200 A, so the loss is I²R = 200² × 0.40 = 16 000 W, about a third of the power sent. At 25 000 V: I = 50 000 ÷ 25 000 = 2.0 A, so the loss is 2.0² × 0.40 = 1.6 W. Raising the voltage 100 times cuts the current 100 times and the loss 10 000 times.
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