Magnetism and electromagnetism
Magnet properties, fields from currents, the motor effect, Fleming’s left-hand rule and the turning effect on a coil.
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Before you begin
Magnets push and pull without touching, and an electric current turns out to be a magnet too. The lessons start with the properties of magnets and the sure test for one, then explain how a magnet induces magnetism in iron and steel and why each suits different jobs. They map magnetic fields with a compass, draw the fields made by currents in wires and solenoids, and finish with the force on a current in a magnetic field, the effect that makes a coil turn in a motor.
Be comfortable with: conventional current, because every hand rule in this topic uses it, and moments, because a coil turns when two forces act on opposite sides of its axis.
Learning goals
- Describe induced magnetism
- Distinguish temporary and permanent magnets
- Determine magnetic-field direction with a compass or bar magnet
- Interpret bar-magnet field patterns
- Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
- Interpret the field pattern around a straight current-carrying wire
- Draw the magnetic field pattern around a straight current-carrying wire
- Interpret the field pattern around a current-carrying solenoid
- Draw the magnetic field pattern around a current-carrying solenoid
- Relate current magnitude and direction to magnetic field
- Describe experiments showing the force on a current-carrying conductor in a magnetic field
- Predict force reversal when current or field reverses
- Use Fleming’s left-hand rule
- Explain the turning effect on a current-carrying coil
Syllabus statements covered
- state the properties of magnets
- describe induced magnetism caused by placing magnetic material close to a strong magnet or within a current-carrying solenoid
- distinguish between temporary magnets (e.g. iron) and permanent magnets (e.g. steel) in terms of their properties and uses
- describe how a bar magnet (e.g. a compass) can be used to determine the direction of a magnetic field
- draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets.
- draw the pattern of the magnetic field due to currents in straight wires and in solenoids and state the effect on the magnetic field due to change in the magnitude and/or direction of the current
- describe experiments to show the force on a current-carrying conductor in a magnetic field, including the effect of reversing (i) the current (ii) the direction of the field
- deduce the relative directions of force, field and current when any two of these quantities are at right angles to each other using Fleming’s left-hand rule
- explain how a current-carrying coil in a magnetic field (e.g. in a motor) experiences a turning effect (recall of structure of an electric motor is not required)
Lessons
Work through them in order.
- Testing magnetsPredict attraction and repulsion and use a known magnet to distinguish a magnet from unmagnetised iron.
- Induced magnetism and magnetic materialsExplain induced poles near a magnet or inside a solenoid, and choose a material from how easily it gains and keeps magnetism.
- Magnetic fields and field patternsTrace a field with a plotting compass and draw correct arrows, line shapes and neutral points for one or two bar magnets.
- Magnetic fields made by currentsDraw the fields of a straight wire and a solenoid and predict how current magnitude or direction changes them.
- The motor effect and coil turningUse the rail experiment and Fleming’s left-hand rule to predict force direction and explain why a coil turns.
Practise and check
Topic reference
Quick reference
| Idea | Rule | Note |
|---|---|---|
| forces between poles | like poles repel; unlike poles attract | only repulsion proves a bar is a magnet |
| field direction | the way the N end of a compass needle points | outside a magnet, from N to S |
| field round a straight wire | right-hand grip rule: thumb along the current, fingers give the field | circles, further apart further out |
| field of a solenoid | right-hand grip rule: fingers round the turns, thumb points to N | uniform inside; like a bar magnet outside |
| force on a current in a field | Fleming’s left-hand rule: First finger Field, seCond finger Current, thuMb force | current and field at right angles |
| temporary and permanent magnets | soft iron: easy to magnetise, loses it; hard steel: keeps it | iron for electromagnets, steel for permanent magnets |
Core knowledge to remember
- A magnet attracts magnetic materials such as iron, nickel, cobalt and steel. Like poles repel and unlike poles attract.
- A magnetic material placed close to a strong magnet, or inside a current-carrying solenoid, becomes magnetised: this is induced magnetism.
- Iron is a temporary magnet: it is easily magnetised and loses its magnetism easily. Steel is a permanent magnet: it is harder to magnetise but keeps its magnetism.
- A plotting compass shows the direction of a magnetic field.
- A larger current gives a stronger magnetic field; reversing the current reverses the field.
- A current-carrying conductor in a magnetic field experiences a force. Reversing the current or reversing the field reverses the force.
- A current-carrying coil in a magnetic field experiences a turning effect, because the forces on its two sides act in opposite directions.
Common mistakes
- Saying a bar must be a magnet because a magnet attracts it.
- Drawing field lines from S to N outside a magnet, or letting them cross.
- Using electron flow instead of conventional current in a hand rule.
- Using the right hand to find the force on a current-carrying conductor.
- Saying that reversing both the current and the field reverses the force.
- Saying the two opposite forces on a coil cancel out, so it cannot turn.