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O-Level Electromagnetism

What the O-Level syllabus expects for Electromagnetism, and how to practise it.

What the syllabus expects

Draw the field patterns set up by a current in straight wires and by a current in solenoids, and state how changing the current's size and/or direction alters the field.
Describe how the current's magnetic effect is put to use in electromagnets, such as those in circuit breakers.
Describe experiments showing how a magnetic field exerts a force on a conductor carrying current and on a moving beam of charged particles, and what results from reversing either the current or the field's direction.
Using Fleming's left-hand rule, deduce how force, field and current are oriented relative to one another when two of them sit at right angles.
Explain why a coil that carries current experiences a turning effect when placed in a magnetic field (as in a motor), and how adding more turns or raising the current increases that effect.
Describe the job of the split-ring commutator in a single-coil, two-pole motor, and what winding the coil onto a soft-iron cylinder achieves.

How it's examined

Questions on this topic most often ask you to explain, state. About 3% of the past-paper style questions in Rae's bank for this subject sit in this topic.

Worked examples

Example 1 (2 marks)

(b) When a current flows through the coil, its adjacent turns are seen to draw closer together. Give the reason this happens.

Show the worked answer

Each turn of the coil carries current flowing in the same direction. Two parallel conductors carrying current in the same direction attract each other (their magnetic fields interact), so adjacent turns are pulled closer together.

Example 2 (2 marks)

A metal rod is placed inside a solenoid that carries a current, forming an electromagnet. (f) State which metal you would choose for the rod to make an electromagnet, and explain your choice.