Ch 14 · 4.5.4 Forces on a Current-Carrying Conductor · O Level Physics 5054

Topic 14 of 17 in Electricity and Magnetism — O Level Physics 5054 — 40 lessons.

Introduction

Put a wire carrying a current into a magnetic field and it gets a push. This force is what turns every electric motor, from the one in your phone's vibrator to the motors of an electric train. (Syllabus section 4.5.4.)

Key ideas — The motor effect

A conductor carrying a current in a magnetic field experiences a force, provided the current is not parallel to the field. The force is greatest when the current is at right angles to the field.

🧑‍🏫 Teacher's explanation

Imagine you are walking through a crowd. If everyone is walking in the same direction as you, it feels easy (weak resistance); if people are walking towards you, it feels much harder (strong resistance). In physics, a current-carrying wire has its own circular magnetic field around it. When you place this wire near a permanent magnet, the wire's field and the magnet's field 'interact'. On one side of the wire, the fields point in the same direction and add together, creating a very strong magnetic field. On the other side, they point in opposite directions and partially cancel out, creating a weak field. This imbalance of magnetic strength creates a physical push, known as the motor effect.\n\nTo prove this in a lab, you can hang a straight wire from a stand so it can swing freely. Connect the wire to a battery and place a strong magnet underneath it. If a force exists, the wire will move sideways. To prove it is the current causing the force, you can reverse the battery terminals (reversing the current) and observe the wire swing in the opposite direction. To prove it is the magnetic field, you can flip the magnet over (reversing the field) and observe the wire swing in the opposite direction again.

📖 Definitions

🔍 Worked example — Easy

A wire carrying a current is placed between the poles of a horseshoe magnet. If the current direction is changed from North to South, in what direction will the force on the wire change?

🔍 Worked example — Standard

Two parallel wires carry currents in the same direction. Describe the magnetic field pattern between them and the resulting force direction.

🔍 Worked example — Exam-style

A student performs an experiment to show the motor effect. They observe that a wire swings to the left. If the student flips the magnet so the North pole is now on the bottom instead of the top, what happens to the wire?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Fleming's left-hand rule

Hold the thumb, first finger and second finger of your left hand at right angles to each other:

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy box across a floor. If you push it forward, it moves forward. If you push it sideways, it moves sideways. However, in electromagnetism, the force doesn't act in the direction of the current or the field; it acts at a right angle to both. It is like a person standing on a rotating carousel: even though they are moving in a circle (the field/current), they feel a push pushing them outward (the force).\n\nWhen a current flows through a wire, the moving electrons create their own tiny magnetic field. When this wire is placed inside the magnetic field of a permanent magnet, the two fields interact. This interaction creates a physical push on the wire. This is known as the motor effect. To predict which way the wire will jump, we use Fleming's Left-Hand Rule, which acts like a 3D compass for forces.

📖 Definitions

🔍 Worked example — Easy

A horizontal wire carries a current from left to right. It is placed in a magnetic field directed from North to South (top to bottom). Using Fleming's Left-Hand Rule, determine the direction of the force acting on the wire.

🔍 Worked example — Standard

A wire is placed between the poles of a horseshoe magnet. The magnetic field lines run from the North pole to the South pole (vertically downwards). If the current direction is reversed from left-to-right to right-to-left, what happens to the direction of the force?

🔍 Worked example — Exam-style

In an experiment to demonstrate the motor effect, a wire is suspended between the poles of a horseshoe magnet. An ammeter shows a current flowing into the wire from the left. If the magnet is flipped so the North and South poles swap positions, describe the change in the force direction.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Two parallel wires

Two long parallel wires carrying currents each sit in the other's magnetic field, so each feels a force.

🧑‍🏫 Teacher's explanation

Imagine two people walking through a crowd. If everyone is walking in the same direction, the crowd moves smoothly. If people are walking towards each other, they bump into each other. In physics, a current-carrying wire creates a circular magnetic field around it. When a second wire is placed nearby, it sits inside this magnetic field. Because the second wire is also carrying a current, it experiences a force due to the magnetic field of the first wire.

📖 Definitions

🔍 Worked example — Easy

A horizontal wire carries a current I to the right. It is placed in a uniform magnetic field B directed into the page. Using Fleming's Left-Hand Rule, determine the direction of the force on the wire.

🔍 Worked example — Standard

Two parallel wires are placed vertically. Wire A carries a current upwards. Wire B carries a current upwards. Describe the interaction between the wires.

🔍 Worked example — Exam-style

An experiment is set up with two parallel wires. Wire 1 has a current flowing North. Wire 2 has a current flowing South. Describe the observed motion of the wires and explain it using magnetic field patterns.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State whether the wires will attract or repel each other. (b) Using Fleming's Left-Hand Rule and the concept of magnetic field patterns, explain your answer to part (a). [3 marks]

Practical notes

(Another version: a loose wire hanging between the poles of a magnet jumps out of the gap when current flows.)

Worked example

A horizontal wire runs east to west between magnet poles, with the field pointing north. Conventional current flows from east to west. Which way is the force?

🔍 Worked example — Easy

A straight wire carries a current from East to West. It is placed in a uniform magnetic field pointing North. Using Fleming's Left-Hand Rule, determine the direction of the force acting on the wire.

🔍 Worked example — Standard

A wire carries a current from East to West in a magnetic field pointing North. If the direction of the current is reversed to flow from West to East, what is the new direction of the force?

🔍 Worked example — Exam-style

Two parallel wires, A and B, carry currents in the same direction (both North to South). Wire A is to the West of Wire B. Describe the force between them.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) Reverse the current; reverse the magnetic field. (2) Increase the current; use a stronger magnet. (3) First finger: field; second finger: current; thumb: force (motion). (4) They repel: between the wires their fields are in the same direction and add up to a strong field, so each wire is pushed away from the other, towards the weaker field outside.

Ch 14 · 4.5.4 Forces on a Current-Carrying Conductor · O Level Physics 5054

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