How do magnets and electric currents interact, and how is this used in everyday devices?
Describe magnetic fields and the magnetic effect of a current, and explain electromagnets, the motor effect and electromagnetic induction
A focused answer to the O-Level Combined Science outcome on magnetism. Magnetic fields and poles, the magnetic effect of a current, electromagnets, the motor effect, and electromagnetic induction.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
SEAB wants you to describe magnetic fields and poles, the magnetic effect of an electric current, how electromagnets work, the motor effect (a force on a current-carrying wire in a magnetic field), and electromagnetic induction (a changing field inducing a current). The marks come from clear descriptions and from knowing how to strengthen or reverse each effect.
The answer
Magnetic fields and poles
A magnet has a north and a south pole. Like poles repel and unlike poles attract. A magnetic field is the region around a magnet where it exerts a force; field lines run from north to south and are closer together where the field is stronger. Iron, steel, nickel and cobalt are magnetic materials.
The magnetic effect of a current
When current flows through a wire it produces a magnetic field around the wire, in circles. Winding the wire into a coil (a solenoid) concentrates the field and produces a field like a bar magnet, with a north and a south end.
Electromagnets
An electromagnet is a solenoid with a soft iron core. It is magnetic only while current flows, so it can be switched on and off. It is made stronger by increasing the current, adding more turns to the coil, or using a soft iron core. Soft iron is used because it loses its magnetism when the current stops.
The motor effect
When a current-carrying wire sits in a magnetic field, it experiences a force. This is the motor effect and it is the basis of the electric motor. The force is larger with a bigger current or a stronger field, and its direction reverses if either the current or the field is reversed.
Electromagnetic induction
When a magnet moves near a coil, or a coil moves in a magnetic field, the changing magnetic field induces a voltage and, in a complete circuit, a current. There is only an induced current while the field through the coil is changing; a stationary magnet induces nothing. This is how a generator produces electricity.
Examples in context
Example 1. A scrapyard crane. A large electromagnet lifts cars and steel scrap by switching on a current, then drops the load by switching the current off. A permanent magnet could never release its load, so the ability to switch off is essential here.
Example 2. A bicycle dynamo. As the wheel turns, it spins a magnet near a coil. The constantly changing magnetic field induces a current that powers the bike's lights, an everyday use of electromagnetic induction, with brighter lights at higher speed.
Try this
Q1. State two ways to increase the strength of an electromagnet. [2 marks]
- Cue. Increase the current through the coil, and increase the number of turns on the coil (a soft iron core also increases the strength).
Q2. State what is meant by the motor effect. [2 marks]
- Cue. A current-carrying wire placed in a magnetic field experiences a force; this is the motor effect.
Q3. Explain why no current is induced when a magnet is held still inside a coil. [2 marks]
- Cue. A current is induced only when the magnetic field through the coil changes; a stationary magnet gives an unchanging field, so no voltage and no current are induced.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA student winds a coil of wire around an iron nail and connects it to a battery to make an electromagnet. State two ways to make the electromagnet stronger, and state one way to switch off its magnetism.Show worked answer →
Two ways to make it stronger: increase the current through the coil; increase the number of turns on the coil. (Using a soft iron core, as here, also increases the strength.)
To switch off the magnetism: open the switch so no current flows, and the soft iron core loses its magnetism.
Markers reward two correct strengthening methods (more current, more turns, iron core) and switching off the current to remove the magnetism, which is the key advantage of an electromagnet.
Original3 marksA bar magnet is pushed into a coil of wire connected to a sensitive meter. (a) State what is observed on the meter. (b) State what happens if the magnet is then held still inside the coil. (c) Name this effect.Show worked answer →
(a) The meter deflects, showing a current is induced as the magnet moves in.
(b) When the magnet is held still there is no change in the magnetic field through the coil, so no current is induced and the meter reads zero.
(c) This is electromagnetic induction.
Markers reward the deflection on movement, no current when stationary (because the field is not changing), and naming electromagnetic induction.
Related dot points
- Define current, potential difference and resistance, apply Ohm's law V = IR, and analyse series and parallel circuits and electrical power
A focused answer to the O-Level Combined Science outcome on electricity. Current, potential difference and resistance, Ohm's law, series and parallel circuit rules, and electrical power and energy.
- Describe transverse and longitudinal waves and the wave equation, and apply the laws of reflection and refraction of light
A focused answer to the O-Level Combined Science outcome on waves and light. Transverse and longitudinal waves, frequency, wavelength and the wave equation, reflection, and refraction of light.
- Distinguish temperature and thermal energy, describe conduction, convection and radiation, and explain melting and boiling using the particle model
A focused answer to the O-Level Combined Science outcome on thermal physics. Temperature versus thermal energy, the three methods of heat transfer, and melting and boiling explained with the particle model.
- State the principle of conservation of energy, describe energy stores and transfers, and apply the equations for work, kinetic and potential energy, power and efficiency
A focused answer to the O-Level Combined Science outcome on energy. Conservation of energy, energy stores and transfers, work done, kinetic and gravitational potential energy, power and efficiency.