Ch 3 · Electromagnetic Effects · IGCSE Physics 0625
Topic 3 of 3 in Electricity and Magnetism — IGCSE Physics 0625 — 60 lessons.
Introduction
Electricity and magnetism are two sides of one phenomenon. A current produces a magnetic field; a magnetic field exerts a force on a current (the basis of every electric motor and loudspeaker); and a changing magnetic field induces an e.m.f. (the basis of every generator and transformer). Together these effects generate, transmit and use almost all of the world's electrical energy.
Key ideas — The magnetic effect of a current
1. Straight wire. A current in a straight wire produces a magnetic field of concentric circles around the wire. The direction follows the right-hand grip rule: point your right thumb along the conventional current; your fingers curl in the direction of the field.
🧑🏫 Teacher's explanation
When electricity flows through a wire, it creates a 'magnetic disturbance' around it. Think of a current-carrying wire like a spinning fan: the air (the magnetic field) moves in circles around the blades. The faster the fan spins (more current), the stronger the wind; if you spin it the other way, the wind direction reverses.
📖 Definitions
🔍 Worked example — Easy
A straight wire carries a current of 5.0 A. Describe the direction of the magnetic field if the current is flowing vertically upwards.
🔍 Worked example — Standard
A solenoid has 200 turns of wire. If the current through the solenoid is increased from 2.0 A to 6.0 A, how does the strength of the magnetic field change?
🔍 Worked example — Exam-style
A relay is used to switch a high-power circuit. The electromagnet in the relay has 500 turns and carries a current of 0.020 A. If the number of turns is doubled and the current is tripled, calculate the factor by which the magnetic field strength increases.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Force on a current-carrying conductor (the motor effect)
4. The motor effect. A wire carrying a current at an angle to a magnetic field experiences a force. Demonstrate it with a loose wire (or a light aluminium rod on rails) between the poles of a magnet: when the current is switched on, the wire jumps. Reversing the current or reversing the field reverses the direction of the force; reversing both leaves it unchanged. There is no force if the wire is parallel to the field.
🧑🏫 Teacher's explanation
Imagine you are walking through a crowded hallway. If you walk straight through, you might bump into people, but if you walk sideways (perpendicularly) to the flow of people, you experience a much stronger push from the crowd. In physics, a magnetic field is like that crowd. When electrons flow through a wire, they are moving charges. As they move through a magnetic field, the field exerts a push on them. Because the electrons are confined within the wire, this push translates into a macroscopic force on the entire wire.
📖 Definitions
🔍 Worked example — Easy
A straight wire carries a current of 5.0 A through a uniform magnetic field. The wire is perpendicular to the field lines. If the force per unit length is 0.25 N/m, calculate the magnetic field strength B.
🔍 Worked example — Standard
A wire is placed in a magnetic field of 0.40 T. The wire has a length of 20 cm and carries a current of 2.5 A. Calculate the force acting on the wire if it is at an angle of 30^° to the magnetic field lines.
🔍 Worked example — Exam-style
An electron beam (charge e = 1.6 × 10^-19 C) enters a magnetic field of 0.50 T at 90^°. The velocity of the electrons is 2.0 × 10^6 m/s. Calculate the force on a single electron.
⚠️ Common errors
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🎯 Exam question
Key ideas — The d.c. motor
5. Turning effect on a coil. A current-carrying coil in a magnetic field experiences a turning effect, because the currents in the two sides of the coil are in opposite directions, so the forces on them are in opposite directions. The turning effect is increased by:
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy revolving door. If you push on one side, it turns. In a D.C. motor, we use electricity to create a 'push' (a force) on the sides of a wire coil. Because the current flows down one side and up the other, one side is pushed up while the other is pushed down, creating a turning effect called torque.
📖 Definitions
🔍 Worked example — Easy
A student notes that a motor's rotation is too weak. List three ways to increase the turning effect (torque) of the motor.
🔍 Worked example — Standard
A D.C. motor uses a coil with 20 turns. If the number of turns is doubled, by what factor does the turning effect increase?
🔍 Worked example — Exam-style
A motor coil is placed in a magnetic field. If the current through the coil is 2.5 A and the magnetic field strength is 0.40 T, and the wire length in the field is 0.15 m, calculate the force on one side of the coil (assume θ = 90^°).
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State two ways to increase the turning effect of this motor. (b) If the current is doubled and the number of turns is doubled, calculate the factor by which the turning effect increases. [4 marks]
Key ideas — Electromagnetic induction
6. Inducing an e.m.f. An e.m.f. is induced in a conductor when it moves across (cuts) magnetic field lines, or when the magnetic field linking it changes. If the conductor is part of a complete circuit, a current flows. Demonstrate this by moving a magnet into and out of a coil connected to a sensitive ammeter (galvanometer): the needle deflects one way as the magnet goes in, the other way as it comes out, and not at all when the magnet is stationary.
🧑🏫 Teacher's explanation
Imagine a magnetic field as a series of invisible, stationary threads stretched across a room. If you hold a wire still, nothing happens. But if you move that wire quickly through the threads, you are "cutting" them. This action of cutting through magnetic field lines creates an electromotive force (e.m.f.), which is essentially the electrical pressure that pushes charges through the wire. Think of it like a person running through a dense crowd; the faster you move, the more "impact" you make with the people (the field lines) you encounter.
📖 Definitions
🔍 Worked example — Easy
A student performs an experiment to demonstrate electromagnetic induction. They move a bar magnet into a coil of wire connected to a galvanometer. Describe the observation and state the factors that would increase the deflection of the galvanometer needle.
🔍 Worked example — Standard
A simple a.c. generator consists of a coil rotating in a uniform magnetic field. When the plane of the coil is perpendicular to the magnetic field lines, the induced e.m.f. is 0 V. When the plane of the coil is parallel to the magnetic field lines, the induced e.m.f. is 12 V. Explain why the e.m.f. is zero when the coil is perpendicular to the field.
🔍 Worked example — Exam-style
A transformer has 500 turns on the primary coil and 50 turns on the secondary coil. The primary voltage V_p is 240 V. Calculate the secondary voltage V_s.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Transformers
9. Construction. A transformer has a primary coil and a secondary coil wound on the same soft-iron core. A step-up transformer has more turns on the secondary, giving a larger output voltage; a step-down transformer has fewer turns on the secondary.
🧑🏫 Teacher's explanation
Imagine a transformer as a bridge for magnetic energy. Instead of electricity flowing directly from one wire to another, the primary coil creates a 'pulsing' magnetic field because the current is alternating (AC). This pulsing field travels through the iron core and 'pushes' the electrons in the secondary coil, inducing a voltage.
📖 Definitions
🔍 Worked example — Easy
A transformer has 200 turns on the primary coil and 1000 turns on the secondary coil. If the input voltage V_p is 12 V, calculate the output voltage V_s.
🔍 Worked example — Standard
A step-down transformer converts 2400 V to 24 V. The primary coil has 4000 turns. Calculate the number of turns in the secondary coil N_s.
🔍 Worked example — Exam-style
An ideal transformer has a primary voltage of 220 V and a secondary voltage of 11 V. The secondary current is 2.0 A. Calculate the primary current I_p.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Calculate the number of turns in the secondary coil. [2] (b) If the secondary current is 5.0 A, calculate the primary current, assuming 100% efficiency. [2] [4 marks]
Practical notes
Worked example
Example 1 — step-down transformer. A transformer changes 230 V mains to 11.5 V for a lamp. The primary coil has 2000 turns. Calculate the number of turns on the secondary.
🔍 Worked example — Easy
A step-down transformer is used to convert 230 V mains electricity to 11.5 V for a lamp. The primary coil has 2000 turns. Calculate the number of turns on the secondary coil.
🔍 Worked example — Standard
The lamp in the previous example draws a current of 2.0 A. Assuming the transformer is 100% efficient, calculate the current in the primary coil.
🔍 Worked example — Exam-style
A power station transmits 500 kW of power through cables with a total resistance of 5.0 Omega. Calculate the power lost as heat if the transmission voltage is 10,000 V.
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) Move the magnet faster; use a stronger magnet; use a coil with more turns. (2) Concentric circles around the wire (direction given by the right-hand grip rule); reversing the current reverses the direction of the field. (3) More turns on the coil; larger current; stronger magnetic field. (4) N_s = 800 × 12/240 = 40 turns. (5) V_s = 230 × 8000/400 = 4600\ V. (6) A higher voltage means a smaller current for the same power, so less energy is wasted as heat in the cables, and thinner (cheaper, lighter) cables can be used. (7) I_s = 60/12 = 5.0\ A; I_p = 60/230 = 0.26\ A. (8) P = 20^2 × 8.0 = 3200\ W; at 2.0 A, P = 2.0^2 × 8.0 = 32\ W. (9) It reverses the current in the coil every half turn, so the forces on the sides of the coil keep turning it in the same direction. (10) An N pole. The induced current opposes the change causing it, so the coil repels the approaching N pole.