Magnetism and Electromagnetism for Singapore N(A)-Level Science (Physics) 5105/5106: magnets, magnetic materials and magnetic fields, the magnetic effect of a current with electromagnets and the motor effect, and electromagnetic induction in generators and transformers

What this module covers

Magnetism and Electromagnetism explains the close link between electricity and magnetism in N(A)-Level Science (Physics) 5105/5106. It begins with permanent magnets and the invisible fields around them, then shows how an electric current makes its own magnetism, which is used in electromagnets and motors, and finishes with the reverse idea, electromagnetic induction, where movement and magnetism together make electricity.

These ideas power much of modern life, from doorbells and cranes to generators and the transformers on the grid. Each dot point below has full worked answers and practice questions.

Magnets and magnetic fields

Magnets and magnetic fields sets out the properties of magnets: they have a north and a south pole, like poles repel and unlike poles attract, and they attract magnetic materials such as iron and steel. A magnetic field is the region around a magnet where it exerts a force, drawn as field lines that run from north to south and are closest together where the field is strongest.

The magnetic effect of a current

The magnetic effect of a current shows that a current in a wire creates a magnetic field around it. Coiling the wire and adding a soft iron core makes an electromagnet, whose strength rises with more turns, a larger current or a soft iron core, and which can be switched off. When a current-carrying wire sits in a magnetic field it feels a force, the motor effect, which drives electric motors.

Electromagnetic induction

Electromagnetic induction is the reverse process: moving a magnet near a coil (or a coil near a magnet) induces a voltage, and a current if the circuit is complete. A faster movement, a stronger magnet or more turns gives a larger induced voltage. Induction is the principle of the generator, which makes electricity, and the transformer, which changes the size of an alternating voltage for transmission and use.

How this module is examined

• Apply the pole rule. Like poles repel, unlike poles attract, just like electric charges.
• Strengthen electromagnets correctly. More turns, larger current, soft iron core; and they can be switched off.
• Increase induction. Faster relative movement, stronger magnet or more turns gives a larger induced voltage.

Check your knowledge

Recall and explanation questions across the module. Attempt them, then check the worked solutions.

1. State what happens when two north poles are brought close together. (1 mark)
2. Name two magnetic materials. (2 marks)
3. State two ways to make an electromagnet stronger. (2 marks)
4. Name the effect that produces a force on a current-carrying wire in a magnetic field. (1 mark)
5. State one device that uses electromagnetic induction. (1 mark)