Ch 11 · 4.5.1 Electromagnetic Induction · O Level Physics 5054

Topic 11 of 17 in Electricity and Magnetism — O Level Physics 5054 — 40 lessons.

Introduction

Almost all the electricity we use is made by moving magnets near coils of wire, in power-station generators. Michael Faraday discovered in 1831 that a changing magnetic field produces a voltage in a wire. This is electromagnetic induction. (Syllabus section 4.5.1.)

Key ideas — Electromagnetic induction

An e.m.f. is induced in a conductor when:

🧑‍🏫 Teacher's explanation

Imagine you are walking through a heavy rainstorm. If you stand perfectly still, you don't get wet. If you walk through it very slowly, you get a little damp. But if you sprint through the rain, you get soaked instantly. In physics, the 'rain' is the magnetic field lines, and your 'speed' is how fast you cut through them.

📖 Definitions

🔍 Worked example — Easy

A student moves a bar magnet into a coil of wire containing 50 turns. If the rate of change of magnetic flux linkage is 2.0 × 10^-3 Wb/s, calculate the induced e.m.f. assuming the coil has 50 turns and the flux change per turn is constant.

🔍 Worked example — Standard

A coil with 200 turns is placed in a magnetic field. A magnet is moved towards the coil, increasing the magnetic field through the coil. If the induced e.m.f. is 4.0 V, what would the e.m.f. be if the number of turns is doubled and the speed of the magnet is also doubled?

🔍 Worked example — Exam-style

A magnet is moved towards a coil. The magnetic field strength B at the center of the coil increases from 0.20 T to 0.60 T in 0.10 s. The coil has 100 turns and an area of 0.01 m^2. Calculate the induced e.m.f. (Assume ΔPhi = Δ B × A).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the law that determines the direction of the induced current. [1] (b) Explain how the magnitude of the induced e.m.f. would change if the speed of the magnet is doubled. [2] [3 marks]

Key ideas — What makes the induced e.m.f. bigger

The size of the induced e.m.f. increases with:

🧑‍🏫 Teacher's explanation

Imagine you are running through a heavy rainstorm. If you walk slowly, you only get slightly damp. If you sprint through the rain, you get soaked instantly. In electromagnetism, the 'rain' is the magnetic field lines, and the 'speed' is how fast you move the magnet or the coil. The faster you 'cut' through those lines, the more electrical pressure (e.m.f.) you generate.

📖 Definitions

🔍 Worked example — Easy

A single loop of wire is moved through a magnetic field. If the rate of cutting magnetic field lines is doubled, what happens to the induced e.m.f.?

🔍 Worked example — Standard

A coil has 50 turns. When a magnet is moved through it, an e.m.f. of 2.5 V is induced. If the number of turns is increased to 150 turns while keeping the speed of the magnet constant, calculate the new e.m.f.

🔍 Worked example — Exam-style

A student moves a magnet into a coil with 20 turns at a constant speed, inducing 0.4 V. The student then uses a magnet with twice the magnetic field strength and moves it at twice the speed. Calculate the new e.m.f.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Direction: Lenz's law

Lenz's law: the induced current always flows in a direction that opposes the change that produced it.

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy door that has a spring-loaded mechanism. The harder you push, the harder the spring pushes back against you. Nature is remarkably similar. When you move a magnet near a coil, you are changing the magnetic environment of that coil. This change 'disturbs' the electrons in the wire.

📖 Definitions

🔍 Worked example — Easy

A student moves a bar magnet towards a single-turn coil. If the magnet's North pole is moving towards the coil, what is the magnetic polarity of the face of the coil closest to the magnet?

🔍 Worked example — Standard

A coil has 50 turns. A magnet is moved such that it cuts 20 magnetic field lines per second. If the magnetic field strength per line is 0.5 T, calculate the induced e.m.f. (Assume e.m.f. = N × ΔPhi / Δ t where ΔPhi is the change in flux per turn).

🔍 Worked example — Exam-style

A technician observes that moving a magnet into a coil produces an e.m.f. of 2.0 V. If the technician doubles the number of turns in the coil and moves the magnet twice as fast, what is the new induced e.m.f.?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the law that determines the direction of the induced current. [1] (b) Explain, in terms of magnetic poles, why the coil repels the magnet when the North pole of the magnet approaches the coil. [2] (c) If the speed of the magnet is doubled, state what happens to the magnitude of the induced e.m.f. [1] [4 marks]

Practical notes

(Alternatively, move a straight wire up and down between the poles of a U-shaped magnet; the meter deflects only while the wire is cutting the field lines.)

Worked example

A magnet is moved into a coil connected to a meter, and the needle moves 4 divisions to the right. Predict the reading when (a) the magnet is held still in the coil, (b) it is pulled out at the same speed, (c) it is pushed in twice as fast.

🔍 Worked example — Easy

A bar magnet is moved into a coil of wire connected to a sensitive galvanometer. When the magnet is moved into the coil at a constant speed, the galvanometer needle deflects 4 divisions to the right. What is the deflection if the magnet is held stationary inside the coil?

🔍 Worked example — Standard

A magnet is moved into a coil at a constant speed, causing a deflection of 4 divisions to the right. The magnet is then pulled out of the coil at the same constant speed. Predict the direction and magnitude of the deflection.

🔍 Worked example — Exam-style

A coil has 50 turns of wire. A magnet is moved into the coil, inducing an e.m.f. of 2.0 V. If the number of turns in the coil is increased to 100 turns and the magnet is moved in at the same speed, calculate the new induced e.m.f.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) Connect the coil to a sensitive centre-zero meter; push a magnet in and out; the needle deflects while the magnet moves, in opposite directions for in and out, and reads zero when the magnet is still. (2) Any two: move the magnet faster; use a stronger magnet; use more turns on the coil. (3) The current produced by an induced e.m.f. opposes the change producing it. (4) A S pole: it repels the incoming S pole, opposing the change (Lenz's law).

Ch 11 · 4.5.1 Electromagnetic Induction · O Level Physics 5054

All chapters in Electricity and Magnetism