Ch 15 · 4.5.5 The d.c. Motor · O Level Physics 5054

Topic 15 of 17 in Electricity and Magnetism — O Level Physics 5054 — 49 lessons.

Introduction

A fan, a drill, a washing machine and an electric car all turn electrical energy into spinning motion with an electric motor. Inside is a coil of wire, a magnet and a clever switch called a commutator that keeps the coil turning the same way. (Syllabus section 4.5.5.)

Key ideas — A coil in a magnetic field turns

Put a rectangular coil carrying a current in a magnetic field. The two sides of the coil that are at right angles to the field carry current in opposite directions, so by Fleming's left-hand rule they feel forces in opposite directions: one side is pushed up, the other down. Together these forces give a turning effect (a moment) about the coil's axis.

🧑‍🏫 Teacher's explanation

Imagine you are holding a heavy door. If you push the handle (far from the hinge) it turns easily, but if you push near the hinge, it is much harder. A d.c. motor works on a similar principle of 'leverage' called a moment. When electricity flows through a wire, that wire becomes a tiny magnet. If you put that wire inside a bigger magnetic field, the field pushes on the wire.

📖 Definitions

🔍 Worked example — Easy

A rectangular coil in a magnetic field experiences a force of 0.5 N on each of its two vertical sides. The distance from the center of the coil to each side is 0.10 m. Calculate the turning effect (moment) produced by these forces.

🔍 Worked example — Standard

A motor coil has 50 turns and carries a current of 2.0 A. If the force on each side of the coil is 0.40 N and the distance from the axis to the wire is 0.05 m, calculate the total moment.

🔍 Worked example — Exam-style

A student wants to double the turning effect of a d.c. motor. If the current is currently 3.0 A and the magnetic field strength is constant, what must the new current be to achieve exactly twice the original moment?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Why a commutator is needed

If the coil were connected directly to the supply, it would turn only until it was vertical, then the forces would push it back. To keep it turning the same way, the current in the coil must reverse every half turn.

🧑‍🏫 Teacher's explanation

Imagine you are pushing a child on a swing. If you only push when they are moving toward you, they will eventually stop and swing back. To keep them moving in a continuous circle or back and forth in one direction, you have to time your pushes perfectly so that every time they change direction, you change your push direction too.

📖 Definitions

🔍 Worked example — Easy

A coil in a motor has 50 turns. If the current is 2.0 A and the magnetic field strength is 0.50 T, how does the turning effect change if the number of turns is doubled to 100?

🔍 Worked example — Standard

A d.c. motor coil experiences a force F on each side of the coil. If the current is increased from 1.0 A to 3.0 A and the magnetic field strength is doubled, what is the factor by which the turning effect increases?

🔍 Worked example — Exam-style

A student observes a motor where the coil rotates but eventually stops and oscillates back and forth instead of spinning continuously. What is the most likely fault in the motor's construction?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State two ways in which the turning effect of the motor can be increased. [2] (b) Explain why a split-ring commutator is necessary for the motor to rotate continuously in one direction. [3] [5 marks]

Key ideas — The split-ring commutator and brushes

The coil's momentum carries it past the vertical position, where the gap in the ring briefly gives no current.

🧑‍🏫 Teacher's explanation

Imagine a person on a swing. If you only push them once, they swing back and forth. But if you time your pushes perfectly every time they reach the center, you keep them moving in one direction. A motor does something similar with electricity. A simple loop of wire in a magnetic field feels a force, but without a special trick, it would just flip back and forth like a pendulum. To make it spin continuously, we need to 'time our pushes' by reversing the electricity at exactly the right moment.

📖 Definitions

🔍 Worked example — Easy

A d.c. motor uses a coil with 50 turns. If the current is 2.0 A and the magnetic field strength is 0.50 T, describe how the turning effect could be increased.

🔍 Worked example — Standard

A motor coil rotates at 1200 revolutions per minute (rpm). Calculate how many times the split-ring commutator reverses the current direction in one minute.

🔍 Worked example — Exam-style

A student observes a d.c. motor. If the brushes are worn down and no longer make firm contact with the split-ring commutator, what is the most likely effect on the motor's performance? Explain using the concept of current.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — How a motor works, step by step

To reverse the direction of rotation: reverse the current (swap the supply leads) or reverse the magnetic field.

🧑‍🏫 Teacher's explanation

Imagine you are trying to push a heavy revolving door. If you push one side forward and the other side backward at the same time, the door spins. An electric motor does exactly this using magnetism. When electricity flows through a wire, that wire becomes a tiny magnet itself. If you place this 'wire-magnet' inside the field of a larger permanent magnet, the two magnetic fields fight each other, creating a physical push called a torque or turning effect.

📖 Definitions

🔍 Worked example — Easy

A student notes that a motor's rotation is too weak. List three ways the student could increase the turning effect (torque) of the motor.

🔍 Worked example — Standard

A motor coil has N = 50 turns. If the current is I = 2.0 A and the magnetic field strength is B = 0.50 T, calculate the force on a single side of the coil if the length of the side is L = 0.10 m. (Use F = BIL)

🔍 Worked example — Exam-style

A motor is rotating clockwise. If the technician reverses the polarity of the battery and keeps the magnets the same, what is the new direction of rotation? Explain why.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

A model motor turns slowly. Give three changes that would make it turn faster (with a greater turning effect).

🔍 Worked example — Easy

A student wants to increase the speed of a simple d.c. motor. List three physical changes to the motor's construction that would increase the turning effect.

🔍 Worked example — Standard

A motor coil has 50 turns and carries a current of 2.0 A. If the number of turns is doubled and the current is tripled, by what factor does the turning effect increase?

🔍 Worked example — Exam-style

A d.c. motor uses a split-ring commutator. Explain why the motor would stop spinning and simply oscillate back and forth if the split-ring commutator was replaced by a solid metal ring.

⚠️ Common errors

Common mistakes

Practice

Answers: (1) The two sides of the coil carry current in opposite directions, so they feel forces in opposite directions (one up, one down), which produce a turning effect. (2) More turns on the coil; a larger current; a stronger magnetic field. (3) It reverses the current in the coil every half turn, so the forces keep turning the coil in the same direction. (4) Reverse the current (swap the supply connections) or reverse the magnetic field.

Ch 15 · 4.5.5 The d.c. Motor · O Level Physics 5054

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