Magnetism and Electromagnetism for Singapore O-Level Physics (6091): magnets and magnetic fields, the magnetic effect of a current, the force on a current-carrying conductor, and electromagnetic induction

What this module covers

Magnetism and Electromagnetism in O-Level Physics (SEAB 6091) reveals one of physics' deepest connections: electricity and magnetism are two sides of the same phenomenon. The module starts with permanent magnets and fields, shows that a current creates a magnetic field, develops the force on a current in a field (the motor effect), and finishes with the reverse process, electromagnetic induction, which underlies generators and transformers.

It builds on the current electricity of the previous module and explains the technology behind motors, generators and the power grid. Each dot point below has full worked answers and practice questions.

Magnets and magnetic fields

Magnets and magnetic fields introduces magnetic poles (like poles repel, unlike poles attract), magnetic materials, and the magnetic field as the region where a magnetic force acts. Field lines run from north to south outside a magnet, and the field is strongest where they are closest together.

A small plotting compass aligns with the field, which is how field direction is defined and mapped.

The magnetic effect of a current

The magnetic effect of a current shows that a current produces a magnetic field. A solenoid carrying current acts like a bar magnet, and its field is strengthened by more turns, a larger current or a soft-iron core. Switching the current off switches the field off, which is what makes an electromagnet useful.

The force on a current-carrying conductor

Force on a current-carrying conductor develops the motor effect: a current at right angles to a magnetic field experiences a force. Fleming's left-hand rule gives the direction (thumb force, first finger field, second finger current), and a current loop in a field turns, which is the principle of the electric motor.

Electromagnetic induction

Electromagnetic induction treats the reverse effect: a changing magnetic field through a coil induces an electromotive force. The induced voltage is larger for a faster change, a stronger field and more turns, and Lenz's law says it opposes the change that causes it. This is the basis of the generator and, with two coils on a core, the transformer that changes voltages for the power grid.

How this module is examined

• Map fields correctly. Field lines run north to south outside a magnet, closest together where the field is strongest.
• Use Fleming's left-hand rule. Match thumb to force, first finger to field, second finger to current for the motor effect.
• Explain induction by change. A voltage is induced only when the flux through the coil is changing; state what increases it.

Check your knowledge

Recall and reasoning questions across the module. Try them, then check the worked solutions.

1. State what happens when two like magnetic poles are brought together. (1 mark)
2. State three ways to increase the strength of an electromagnet. (3 marks)
3. Name the rule used to find the direction of the force on a current-carrying conductor in a magnetic field. (1 mark)
4. State two ways to increase the electromotive force induced when a magnet is moved into a coil. (2 marks)
5. Explain why a transformer does not work with a steady direct current. (2 marks)