Magnetism and electromagnetism
Magnets and magnetic fields, electromagnets, the motor effect, electromagnetic induction, generators and transformers for G3 Pure and O-Level Physics.
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Before you begin
Magnets push and pull without touching, a current makes its own magnetic field, and a changing magnetic field makes a current. This topic follows that chain. It starts with magnets, magnetic materials and field patterns, moves on to electromagnets, the force on a current and the D.C. motor, and ends with electromagnetic induction, the A.C. generator, the transformer and how electricity reaches our homes. The magnetism and induction lab lets you test your predictions.
Be comfortable with: current electricity, including conventional current and P = VI, and forces, including moments.
Learning goals
- 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
- Describe induced magnetism
- Distinguish temporary and permanent 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 electromagnet applications
- Describe experiments showing the force on a current-carrying conductor in a magnetic field
- Describe magnetic force on a charged-particle beam
- Predict force reversal when current or field reverses
- Use Fleming’s left-hand rule
- Explain the turning effect on a current-carrying coil
- Explain how current and turns increase the turning effect
- Describe split-ring commutator action
- Describe the effect of winding a motor coil on a soft-iron cylinder
- Deduce that a changing magnetic field can induce an e.m.f.
- Deduce that induced e.m.f. opposes the change producing it
- Deduce factors affecting induced e.m.f. magnitude
- Describe a simple A.C. generator and slip rings
- Sketch a simple A.C. generator voltage–time graph
- Describe a simple iron-cored transformer
- Apply ideal-transformer equations
- Explain cable loss and high-voltage transmission
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 of changing the magnitude and/or direction of the current
- describe the application of the magnetic effect of a current in electromagnets (e.g. circuit breakers)
- describe experiments to show the force on a current-carrying conductor, and on a beam of charged particles, 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 and that the effect is increased by increasing (i) the number of turns on the coil (ii) the current
- describe the action of a split-ring commutator in a two-pole, single-coil motor and the effect of winding the coil on to a soft-iron cylinder.
- deduce from Faraday’s experiments on electromagnetic induction or other appropriate experiments: (i) that a changing magnetic field can induce an e.m.f. in a circuit (ii) that the direction of the induced e.m.f. opposes the change producing it (iii) the factors affecting the magnitude of the induced e.m.f.
- describe a simple a.c. generator (rotating coil or rotating magnet) and the use of slip rings (where needed)
- sketch a graph of voltage output against time for a simple a.c. generator
- describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations
- recall and apply the equations VP / VS = NP / NS and VPIP = VSIS to new situations or to solve related problems (for an ideal transformer)
- describe the energy loss in cables and deduce the advantages of high voltage transmission.
Lessons
Work through them in order.
Magnetism and fields
- Magnets and magnetic materialsPredict attraction or repulsion from the facing poles, and tell magnetic from non-magnetic materials.
- Properties of magnetsUse repulsion to test for a magnet, explain why a compass points north, and predict the poles of a cut magnet.
- Magnetic fields and field linesPlot field lines with a compass, and draw the field around a bar magnet and between two magnets.
- Induced magnetismPredict the poles induced near a magnet, and magnetise a steel rod with the current in a solenoid.
- Temporary and permanent magnetsCompare soft iron and steel, and choose the right one for an electromagnet or a permanent magnet.
Electromagnetism and the motor effect
- Magnetic field around a straight wireDraw the circular field around a straight wire and find its direction with the right-hand grip rule.
- Magnetic field of a solenoidDraw a solenoid's field, find its poles with the right-hand grip rule, and explain how to make an electromagnet stronger.
- Electric bellExplain how an electric bell's electromagnet and make-and-break contact keep the hammer striking the gong.
- Circuit breakerExplain how the electromagnet in a circuit breaker cuts off the current when it becomes too large.
- Force on a current-carrying conductorShow the force on a wire in a magnetic field, and find its direction with Fleming's left-hand rule.
- Magnetic forces on moving chargesPredict which way a magnetic field bends a beam of electrons or positive particles.
- The D.C. motorExplain how a D.C. motor keeps turning with a split-ring commutator, and how to increase its turning effect.
Induction and the generator effect
- Electromagnetic inductionShow that a changing magnetic field induces an e.m.f., and find what makes the e.m.f. larger.
- Lenz's lawPredict the pole induced in a coil and the direction of the induced current, and explain why moving the magnet takes work.
- The A.C. generatorExplain how a rotating coil and slip rings produce an alternating voltage, and sketch its voltage–time graph.
- The transformerExplain how an iron-cored transformer changes an A.C. voltage, and use the turns-ratio and power equations.
- Power transmission at high voltageExplain why sending the same power at a higher voltage cuts the energy lost as heat in the cables.
Practise and check
Or choose
Topic reference
Quick reference
| Idea | What to remember |
|---|---|
| Forces between magnets | like poles repel; unlike poles attract |
| Test for a magnet | repulsion, testing both ends with the same known pole |
| Magnetic materials | iron, steel, nickel and cobalt |
| Soft iron and steel | soft iron: temporary magnet, for electromagnet cores; steel: permanent magnet |
| Field direction | the direction a compass’s north pole points; outside a magnet, from N to S |
| Field around a straight wire | concentric circles; direction from the right-hand grip rule (thumb along the current) |
| Solenoid | field like a bar magnet’s; fingers curl with the current, thumb points to N; stronger with more current, more turns per unit length or a soft-iron core |
| Force on a current | Fleming’s left-hand rule: First finger field, seCond finger current, thuMb force; reversing the current or the field reverses the force |
| D.C. motor | turning effect increased by more turns, a larger current, a stronger magnet; split-ring commutator reverses the current every half turn |
| Induced e.m.f. | only while the magnetic field through the circuit changes; larger for faster change, a stronger magnet or more turns |
| Lenz’s law | the induced e.m.f. opposes the change producing it |
| A.C. generator | rotating coil and slip rings; e.m.f. zero when the coil is at right angles to the field, maximum when parallel |
| Transformer | VP/VS = NP/NS; for an ideal transformer, VPIP = VSIS; needs A.C. |
| Transmission | for the same power, I = P/V; cable loss = I²R, so a high voltage means a small loss |
Core knowledge to remember
- Like poles repel and unlike poles attract. A magnet attracts unmagnetised magnetic materials by inducing magnetism in them.
- Soft iron is easily magnetised and loses its magnetism easily (a temporary magnet); steel is harder to magnetise but keeps its magnetism (a permanent magnet).
- The direction of a magnetic field at a point is the direction a compass’s north pole points there.
- A current produces a magnetic field. Increasing the current strengthens the field; reversing the current reverses it.
- A current-carrying conductor, or a beam of charged particles, in a magnetic field experiences a force. Reversing the current or the field reverses the force.
- A current-carrying coil in a magnetic field experiences a turning effect, which is increased by increasing the number of turns or the current.
- The split-ring commutator of a D.C. motor reverses the current in the coil every half turn, so the coil keeps turning in one direction. Winding the coil on a soft-iron cylinder increases the turning effect.
- A changing magnetic field induces an e.m.f. in a circuit. The direction of the induced e.m.f. opposes the change producing it.
- The induced e.m.f. is larger when the field changes faster, when the magnet is stronger and when the coil has more turns.
- A transformer needs an alternating current, so that the field in its iron core keeps changing and induces an e.m.f. in the secondary coil.
- Power is transmitted at high voltage because, for the same power, the current is smaller and so less energy is lost as heat in the cables.
Common mistakes
- Using attraction to prove that an object is a magnet. Only repulsion proves it.
- Drawing field lines from S to N outside a magnet, or drawing field lines that cross.
- Using the right hand for the force on a current, or the left hand for the field around a wire.
- Using electron flow instead of conventional current in the direction rules.
- Saying the commutator reverses the current every full turn. It reverses it every half turn.
- Mixing up the split-ring commutator (D.C. motor) and slip rings (A.C. generator).
- Saying a magnet held still inside a coil induces an e.m.f. Nothing is induced unless the field is changing.
- Saying a step-up transformer increases the power. It increases the voltage and decreases the current.
- Saying a high voltage reduces the cable loss without explaining that it reduces the current.