Your revision route
What you’ll learn
- All routes: distinguish permanent and induced magnets, and predict attraction or repulsion between poles.
- All routes: describe, draw and plot magnetic fields around bar magnets, wires and solenoids.
- All routes: explain how current, coil shape and an iron core create a useful electromagnet.
- Separate Physics: interpret diagrams of devices that use electromagnets.
- Higher Tier, both routes: apply Fleming's left-hand rule, F = BIl and the motor effect to rotating coils.
- Separate Physics Higher: explain loudspeakers, headphones, generators, microphones and transformers, including their input and output changes.
Build the big picture
Key ideas
All routes: poles are where the field grips hardest
Bring two permanent magnets together and the result can be attraction or repulsion; an induced magnet only joins the attraction case.
- Like poles repel and unlike poles attract. Both are non-contact forces between permanent magnets.
- Iron, steel, cobalt and nickel are magnetic materials. A magnet attracts them whichever pole approaches.
- An external field gives a magnetic material temporary magnetism, so it is attracted; after leaving the field, the effect rapidly fades.
The punchline: In a magnetic pole test, repulsion shows both objects behave as magnets; attraction alone does not.
All routes: a compass interrogates the field point by point
A magnetic field is invisible, but a tiny bar magnet in a compass turns until its north-seeking end reveals the local direction.
- Field direction at a point is the force direction on a north pole placed there; outside a bar magnet, lines run north to south.
- Closer field lines represent a stronger field. The field weakens with distance and is strongest near a magnet's poles.
- Plot with a compass: mark the needle direction, move the compass along that direction, repeat, then join the marks with an arrowed curve.
- Because a compass responds to Earth's field, the planet must contain a magnetic source in its core.
The punchline: Draw arrows north to south outside the magnet and vary line spacing with strength.
All routes: current writes circles into space
A straight current-carrying wire produces concentric field lines; wind the wire into a coil and those circles reinforce one another.
- Switch on only briefly: a nearby compass deflects, then reverses if current reverses. Use a low-voltage, current-limited supply; the wire can heat, so let it cool before touching.
- A solenoid produces a strong, nearly uniform field inside and a bar-magnet-like pattern outside. More current strengthens the field; it weakens with distance from a straight wire.
- Use the right-hand grip rule: point the right thumb along conventional current; curled fingers show the circular field direction.
- Insert an iron core and the field becomes stronger; the resulting device is an electromagnet.
The punchline: For a solenoid answer, connect many reinforcing turns to a strong interior field.
Three fields, three revealing patterns
Field lines map direction and relative strength. Their shape changes with the source; closer spacing always represents a stronger field.
Separate Physics: read electromagnetic devices as chains
A relay or electric bell is not a maze of parts; it is a sequence in which current creates magnetism and magnetism causes movement.
- Identify the coil and iron core, then trace when current flows and the core becomes magnetised.
- Follow the attraction of an armature or other magnetic part, and state what contact opens, closes or moves next.
- When current stops, the electromagnet's field collapses and a spring or gravity may return the mechanism.
The punchline: In a device diagram, narrate current → field → force → movement → circuit change.
Higher Tier, both routes: a current can feel a sideways force
Place a current-carrying conductor across a magnetic field and the two fields interact, pushing the conductor and magnet in opposite directions.
- This interaction is the motor effect. Force is greatest when conductor and field are at right angles and zero when they are parallel.
- Fleming's left-hand rule links first finger field, second finger conventional current and thumb force or motion.
- A stronger field, larger current or longer conductor in the field increases force; at 90°, F = BIl.
- Reverse current or magnetic field—not both—to reverse the force.
The punchline: State the three perpendicular directions before predicting the force.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Change the source
Explore what strengthens or reverses a field
Select a magnetic situation, then identify the field feature or induced behaviour that matters in that case.
Near the poles the field is strongest; its strength falls with distance. Outside the magnet, direction is north to south.
1 of 4 states explored
A field is a region of force. Distance, current and arrangement alter its strength; reversing a magnet or current reverses its direction.
Read the energy route
Match each device to its physical effect
Pair each device with the mechanism that links its input to its output. Route labels remain part of the answer.
Device diagrams become easier when you follow cause and effect through the coil, field and moving part rather than memorising a silhouette.
Map the dependencies
Separate Physics Higher: trace both generator routes
Explore the two alternative causes, the shared induced potential difference, and the condition for current.
Move the conductor across field lines, or move the magnet past it, so linkage changes. Motion parallel to field lines in an unchanged uniform field need not induce a p.d.
1 of 4 states explored
Motion induces only when it changes field linkage; changing the surrounding field is the other route. A complete circuit is a condition for current, not induction.
Higher Tier, both routes: two forces make a coil turn
In a motor, current runs in opposite directions along opposite sides of a coil, so the magnetic field pushes those sides in opposite directions.
- The separated forces form a turning effect and rotate the coil.
- Increasing current or field strength increases the forces and usually the turning effect.
- Reversing current or field reverses the rotation direction; practical motors switch the coil current to keep rotation going.
The punchline: Explain rotation with opposite forces on opposite coil sides—not with attraction to one pole.
Separate Physics Higher: speakers and headphones turn current into sound
A varying current drives a coil in a permanent magnetic field; a speaker cone or headphone diaphragm transfers that motion to the air.
- Current in the coil experiences the motor effect; when current reverses, force reverses.
- The varying current makes the coil and attached speaker cone or headphone diaphragm vibrate with the signal.
- Those vibrations produce pressure variations that travel through the air as sound waves.
The punchline: Use the full chain: varying current → varying force → cone or diaphragm vibration → sound.
Separate Physics Higher: changing fields generate electricity
A conductor needs no battery: relative motion that changes its magnetic field linkage can induce a potential difference.
- Change a conductor's field linkage by cutting field lines, or change the field around it, to induce a potential difference; a complete circuit carries current.
- Change linkage faster by cutting field lines faster, use a stronger field or add coil turns to increase the induced potential difference.
- Reverse linkage-changing motion or field direction—but not both—to reverse the induced potential difference. The induced current's field opposes the original change.
- As a rotating coil's linkage changes, induced pd reverses each half-turn: slip rings give alternator ac; a split-ring commutator gives one-direction dynamo pulses.
The punchline: Name what changes, then state how size or direction of the induced output responds.
Motor and generator: reverse the question
Both involve conductors and magnetic fields. What enters the device and what emerges tell you which effect is operating.
- Higher Tier, both routes: motor effectCurrent in a magnetic field produces a force and can produce motion.
- Separate Physics Higher: generator effectRelative motion that changes field linkage—or a changing field—induces a potential difference; a complete circuit can carry current.
- Higher Tier, both routes: motor reversalReverse current or field—but not both—to reverse the motor force.
- Separate Physics Higher: generator reversalReverse the linkage-changing motion, or reverse the field—but not both—to reverse the induced potential difference.
Separate Physics Higher: alternator and dynamo output
On each plot, time is the horizontal axis and potential difference is the vertical axis. In a generator arrangement, a rotating coil changes field linkage and reverses induced p.d. every half-turn.
Separate Physics Higher: how one coil talks to another
A transformer needs change at every link. A steady direct current does not maintain the changing field required for continuous induction.
- Alternating currentAn alternating current flows in the primary coil.
- Changing fieldThe current produces a changing magnetic field in the easily magnetised iron core.
- Changing linkageThe changing field passes through the turns of the secondary coil.
- Induced outputThe core's changing field creates a potential difference between the secondary's ends; current then flows if that circuit is closed.
Separate Physics Higher: a microphone is a generator listening to air
Pressure variations move a diaphragm and coil; the motion changes the coil's field linkage and induces an electrical signal with the same pattern.
- Sound waves make the diaphragm and attached coil move back and forth.
- The moving coil's field linkage changes, so a changing potential difference is induced.
- In a complete circuit, the induced current varies with the sound's pressure pattern.
The punchline: Microphone: sound → movement that changes field linkage → induced electrical signal; loudspeaker runs the chain the other way.
Separate Physics Higher: transformers require an alternating input
Two insulated coils share no direct electrical connection, yet a changing field in their iron core transfers the pattern from one to the other.
- Alternating current in the primary creates a changing magnetic field in the easily magnetised iron core.
- That changing field passes through the secondary and induces an alternating potential difference; a complete secondary circuit carries current.
- Vp/Vs = np/ns. More secondary than primary turns steps potential difference up; fewer steps it down.
- Laminations and eddy currents are not required here; National Grid power and current calculations belong to Electricity.
The punchline: Use turns ratio for potential difference, and a changing field to explain induction.
Words worth knowing
Key definitions
- magnetic field
- The space where another magnet, or a piece of magnetic material, would experience a magnetic force.
- permanent magnet
- An object that produces its own persistent magnetic field.
- induced magnet
- A magnetic material given temporary magnetism by an external field; after the field is removed, most or all of that magnetism fades rapidly.
- solenoid
- A coil of wire whose current produces a strong, nearly uniform magnetic field inside.
- electromagnet
- A solenoid containing an iron core to strengthen its magnetic field.
- Higher Tier, both routes: motor effect
- A current-carrying conductor can experience a force in a magnetic field; the force is zero when current and field are parallel.
- Higher Tier, both routes: magnetic flux density
- A measure of magnetic field strength, represented by B and measured in tesla.
- Separate Physics Higher: generator effect
- A potential difference is induced by relative motion that changes a conductor's field linkage—for example, by cutting field lines—or by changing the field around it.
- Separate Physics Higher: alternator
- A generator that produces an alternating potential difference and alternating current.
- Separate Physics Higher: dynamo
- A generator arranged to produce a direct potential difference and direct current.
- Separate Physics Higher: transformer
- A device with primary and secondary coils on an iron core. Electromagnetic induction produces an alternating potential difference in the secondary; the turns ratio can step it up, down or leave it unchanged.
Calculate with confidence
Equations
Higher Tier, both routes: force on a conductor
F = B × I × l
With the wire crossing the field at 90°, calculate its force by multiplying flux density, current and the length inside the field.
| Symbol | Meaning | Unit |
|---|---|---|
| F | force | N |
| B | magnetic flux density | T |
| I | current | A |
| l | length of conductor in the field | m |
Exam tip: The Physics sheet supplies this equation. Use only the conductor length inside the field and only use F = BIl directly when the conductor is perpendicular to it.
Separate Physics Higher: transformer turns ratio
Vp / Vs = np / ns
The primary-to-secondary potential-difference ratio equals the primary-to-secondary turns ratio.
| Symbol | Meaning | Unit |
|---|---|---|
| Vp | primary potential difference | V |
| Vs | secondary potential difference | V |
| np | number of primary turns | no unit |
| ns | number of secondary turns | no unit |
Exam tip: The Physics sheet supplies this equation. Keep primary quantities together and check whether your answer predicts the stated step-up or step-down behaviour.
Follow it step by step
Processes to remember
Plot a bar magnet's field with a compass
- Place the bar magnet flat, keep it away from phones and bank cards, and do not let strong magnets snap together or trap fingers.
- Put a plotting compass near the magnet's north pole; identify and mark the positions of its north-seeking and south-seeking ends.
- Move the compass until its south-seeking end sits on the previous north-end mark, then mark the new north-end position.
- Repeat until the other pole is reached; join the marks smoothly and add an arrow from north to south.
- Restart near the north pole at several positions to reveal the full pattern and the closer spacing near the poles.
Exam tip: Use a small plotting compass and repeat enough starting positions to show how both direction and strength vary.
Higher Tier, both routes: apply Fleming's left-hand rule
- Point the first finger in the magnetic-field direction, from north to south.
- Point the second finger in the conventional-current direction, from positive to negative.
- Hold the fingers and thumb mutually perpendicular.
- Read the thumb as the force or motion direction, then repeat after any stated reversal.
Exam tip: Electron flow is opposite to conventional current; use the direction requested by the rule.
Read an electromagnetic device with route labels
- Separate Physics: for an electromagnet device, identify the current source, coil, iron core and moving magnetic part.
- Name the field produced or the changing field experienced.
- Higher Tier, both routes: apply the motor effect for force. Separate Physics Higher: apply the generator effect for induced potential difference.
- Follow the next moving or electrical part until the device's stated output is reached.
- Use only the size or reversal rule assigned to the stated route and tier.
Exam tip: Describe what each labelled part does; do not merely copy labels from the diagram.
See the thinking
Worked example
Worked example: calculate motor-effect force
Higher Tier, both routes: a 0.080 m wire carries 3.0 A at right angles to a uniform 0.50 T magnetic field. Calculate the force. State what happens to the force if the current alone reverses.
- Choose the equation for a perpendicular conductor: F = B × I × l.
- Substitute SI values: F = 0.50 × 3.0 × 0.080.
- Calculate the magnitude: F = 0.12 N.
- Use Fleming's left-hand rule: reversing current alone reverses the force direction.
Answer: The force is 0.12 N. Reversing only the current reverses the force direction but leaves its magnitude unchanged.
The conductor is at 90° to the field, so the equation applies directly. B, I and l keep the same magnitudes after reversal, while the left-hand-rule direction flips.
Protect the marks
Common mistakes
Watch out: Saying an induced magnet can repel the magnet that induced it.
Do this instead: Induced magnetism attracts. In a magnetic pole test, repulsion shows both objects behave as magnets with like poles facing; neither need be permanent.
Watch out: Drawing bar-magnet field arrows from south to north outside the magnet.
Do this instead: Outside the magnet, field lines point from the north pole to the south pole.
Watch out: Treating field lines as paths travelled by particles.
Do this instead: A field line shows the force direction on a north pole and relative field strength.
Watch out: Higher Tier, both routes: using Fleming's left-hand rule with electron-flow direction.
Do this instead: Higher Tier, both routes: the second finger uses conventional current, from positive to negative.
Watch out: Separate Physics Higher: calling the generator effect the motor effect.
Do this instead: Higher Tier, both routes: motor effect gives force from current. Separate Physics Higher: generator effect gives induced potential difference from change.
Watch out: Separate Physics Higher: saying a transformer works continuously with steady dc.
Do this instead: Separate Physics Higher: continuous induction needs a changing field, normally produced by alternating current.
Watch out: Separate Physics Higher: applying this content to every learner.
Do this instead: Loudspeakers, generators, microphones and transformer construction here are separate Physics Higher only.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
In a magnetic pole test, the tested ends of two objects repel. What can you conclude?
Answer: Both objects currently behave as magnets, and the tested ends are like poles.
An unmagnetised magnetic material or induced magnet gives magnetic attraction. This test does not reveal whether either magnet is permanent or an electromagnet.
Which way do magnetic field lines point outside a bar magnet?
Answer: From the north pole to the south pole.
That is the force direction on a north pole placed in the field.
Give two changes that strengthen a solenoid's magnetic field.
Answer: Increase the current and add an iron core.
A larger current strengthens the wire's field, while easily magnetised iron strengthens the solenoid's field.
Higher Tier, both routes: find F when B = 0.20 T, I = 4.0 A and l = 0.50 m at right angles.
Answer: 0.40 N
F = BIl = 0.20 × 4.0 × 0.50 = 0.40 N.
Separate Physics Higher: how can a generator produce a larger induced potential difference?
Answer: Change field linkage faster by cutting field lines faster, use a stronger magnetic field or use more turns on the coil.
Each change increases how rapidly or how strongly the magnetic field linkage changes.
Separate Physics Higher: which trace is the alternator and which is the dynamo—one crosses zero; one touches zero but stays positive?
Answer: The zero-crossing trace is the alternator's ac; the one-sided pulse trace is the dynamo's pulsating dc.
Slip rings preserve the reversal every half-turn. The split-ring commutator swaps external contacts, so the dynamo's output keeps one sign.
Separate Physics Higher: a transformer has twice as many secondary turns as primary turns. Is it step-up or step-down?
Answer: Step-up: the secondary potential difference is twice the primary potential difference.
Vp/Vs = np/ns, so doubling the secondary turns doubles Vs for the same Vp.
Good questions, clear answers
Frequently asked questions
Is magnetic attraction proof that both objects are magnets?
No. A magnet can attract unmagnetised magnetic material by inducing it. In a magnetic pole test, repulsion shows both objects behave as magnets, but not whether either is permanent or powered.
Why is a solenoid's field stronger than a straight wire's field?
Each turn produces a field, and inside the solenoid those fields point in the same direction and reinforce one another. An iron core strengthens the result further.
Separate Physics Higher: what is the difference between motor and generator effects?
Higher Tier, both routes: the motor effect uses current in a magnetic field to produce force. Separate Physics Higher: the generator effect uses motion that changes field linkage, or a changing field, to induce a potential difference.
Which parts are Higher Tier?
The motor effect, Fleming's left-hand rule, F = BIl and electric motors are Higher on both routes. Loudspeakers, generators, microphones and transformers in this topic are separate Physics Higher only.
Is there an AQA required practical for this topic?
No. Plotting fields and demonstrating electromagnets are valuable classroom investigations, but AQA does not designate a required practical within Magnetism and Electromagnetism.
Separate Physics Higher: why does a transformer need alternating current?
Separate Physics Higher: alternating current continually changes the primary coil's magnetic field. That changing field passes through the secondary and induces its potential difference.
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