AQA GCSE Physics coverage

Magnetism and electromagnetism

Section 4.7
10 spec leafs

Notes and three levels of exam-style practice for each registered specification leaf in this section.

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4.7.1.1

Poles of a magnet

  • Magnetic forces are strongest at a magnet's north and south poles; like poles repel and unlike poles attract without touching.
  • A permanent magnet makes its own magnetic field, whereas an induced magnet becomes magnetic only while it is in another magnetic field.
  • For example, either pole of a permanent magnet attracts an unmagnetised iron nail because the nail becomes an induced magnet with the nearer end as the opposite pole.
  • A common error is to predict repulsion from an induced magnet: induced magnetism produces attraction, and most or all of it is lost quickly when the field is removed.

Tier 1 · Easy

2 marks
ORIGINAL

A north pole is moved towards the north pole of a second permanent magnet. State the interaction and name the type of force involved.

Tier 2 · Standard

3 marks
ORIGINAL

An unmagnetised iron pin is attracted to the south pole of a bar magnet. Explain why the pin is attracted and what happens after the magnet is taken far away.

Tier 3 · Hard

4 marks
ORIGINAL

Objects P and Q are each either an unmagnetised iron sample or a permanent magnet. P attracts the north pole of a known permanent magnet, while Q repels it. Decide which object must be a permanent magnet and explain why the evidence for P is inconclusive.

4.7.1.2

Magnetic fields

  • A magnetic field is the region where a magnet exerts a force on another magnet or on iron, steel, cobalt or nickel.
  • Plot a field by placing a compass at successive positions, marking the direction of its north-seeking end, and joining the marks with smooth directed lines.
  • Outside a bar magnet, field lines point from north to south and are closest at the poles; compass alignment with Earth's field is evidence that Earth's core must be magnetic.
  • A common error is to draw crossing field lines or arrows from south to north outside the magnet; each point has one field direction, defined by the force on a north pole.

Tier 1 · Easy

1 mark
ORIGINAL

State the direction of the magnetic field immediately outside a bar magnet near its north pole.

Tier 2 · Standard

4 marks
ORIGINAL

Describe how a student can map the direction of the magnetic field around a bar magnet using a small plotting compass.

Tier 3 · Hard

4 marks
ORIGINAL

Two field-pattern sketches are proposed for one bar magnet. Sketch A has widely spaced lines at the poles and crowded lines halfway between them; sketch B has crowded lines near the poles, no crossings, and arrows from north to south outside. Evaluate the sketches and relate a compass reading to your decision.

4.7.2.1

Electromagnetism

  • A current in a straight conductor creates concentric field lines; increasing current strengthens the field, increasing distance weakens it, and a nearby compass deflection demonstrates the effect.
  • Use the right-hand grip rule for direction: point the thumb in the conventional-current direction and the curled fingers show the field direction.
  • A solenoid's turns reinforce to make a strong, nearly uniform internal field and a bar-magnet-shaped external field; adding an iron core strengthens it and makes an electromagnet.
  • When interpreting an electromagnetic-device diagram, trace current to magnetic field and then to force or motion; a common error is to name the electromagnet without explaining that causal chain.

Tier 1 · Easy

2 marks
ORIGINAL

Give two changes that would increase the magnetic field strength at a fixed point inside a current-carrying solenoid.

Tier 2 · Standard

3 marks
ORIGINAL

A vertical wire carries conventional current upwards. Describe the field-line shape and how a student determines its direction when viewed from above.

Tier 3 · Hard

5 marks
ORIGINAL

Coil X has 80 closely spaced turns and no core. Coil Y has 80 identical turns carrying the same current but contains an iron core. Compare their fields, then explain why reversing the current through Y reverses its poles without removing its field-strength advantage.

4.7.2.2

Fleming's left-hand rule (HT only)

  • The motor effect is the force produced when a current-carrying conductor lies in a magnetic field; the conductor and field-producing magnet exert forces on each other.
  • For Fleming's left-hand rule, hold the thumb, first finger and second finger mutually perpendicular: thumb is force, first finger is field from north to south, and second finger is conventional current.
  • For a conductor perpendicular to the field, calculate force with F=BIlF=BIl; for example, B=0.30 TB=0.30\,\text{T}, I=2.0 AI=2.0\,\text{A} and l=0.40 ml=0.40\,\text{m} give F=0.24 NF=0.24\,\text{N}.
  • A common error is to use electron flow for the current finger or to apply F=BIlF=BIl unchanged when the conductor is not at right angles to the field.

Tier 1 · Easy

2 marks
ORIGINAL

In Fleming's left-hand rule, state what the first finger and thumb represent.

Tier 2 · Standard

2 marks
ORIGINAL

A 0.18 m0.18\,\text{m} wire section is perpendicular to a 0.45 T0.45\,\text{T} magnetic field and carries 3.2 A3.2\,\text{A}. Calculate the force on the section.

Tier 3 · Hard

4 marks
ORIGINAL

A wire of active length 0.35 m0.35\,\text{m} experiences a 0.63 N0.63\,\text{N} downward force while carrying 4.0 A4.0\,\text{A} perpendicular to a uniform field. Determine the magnetic flux density. Then state the new force direction if only the current is reversed.

4.7.2.3

Electric motors (HT only)

  • In a motor, opposite sides of a current-carrying coil experience forces in opposite directions because their currents run oppositely through the magnetic field.
  • These forces form a turning effect; use Fleming's left-hand rule separately on each active side to predict the rotation direction.
  • A split-ring commutator reverses the current every half-turn, so the forces swap sides and the turning effect continues in the same rotational direction.
  • A common error is to say the coil turns because unlike poles attract; the required explanation is the motor-effect force on current-carrying conductors in a magnetic field.

Tier 1 · Easy

2 marks
ORIGINAL

State the energy transfer performed by an electric motor and name the effect that produces its turning force.

Tier 2 · Standard

4 marks
ORIGINAL

Explain why a rectangular current-carrying coil between magnetic poles begins to rotate rather than simply moving sideways.

Tier 3 · Hard

6 marks
ORIGINAL

A simple motor is turning slowly. Explain how increasing the current and adding a stronger magnet affect its motion, and explain why its split-ring commutator must reverse the current after each half-turn.

4.7.2.4

Loudspeakers (physics only) (HT only)

  • A moving-coil loudspeaker uses the motor effect to convert variations in electrical current into pressure variations in a sound wave.
  • The alternating current in the voice coil repeatedly reverses, so the motor-effect force reverses and the coil moves backwards and forwards in the permanent magnet's field.
  • The coil is attached to a cone: larger current variations produce larger cone displacements and a louder sound, while faster variations produce a higher-frequency sound.
  • A common error is to describe electromagnetic induction in a loudspeaker; induction is used by a microphone, whereas a loudspeaker requires force on a supplied current.

Tier 1 · Easy

2 marks
ORIGINAL

Name the effect used by a moving-coil loudspeaker and state what the cone transfers energy to.

Tier 2 · Standard

4 marks
ORIGINAL

Explain how an alternating electrical signal makes the cone of a moving-coil loudspeaker vibrate.

Tier 3 · Hard

4 marks
ORIGINAL

Signal A has twice the frequency of signal B but a smaller current amplitude. Compare the sounds produced when each signal drives the same ideal loudspeaker, and justify both comparisons using the coil's motion.

4.7.3.1

Induced potential (physics only) (HT only)

  • The generator effect induces a potential difference when a conductor moves relative to a magnetic field or when the magnetic field around it changes; a complete circuit then carries an induced current.
  • Increase the induced potential difference by increasing relative speed, field strength or coil turns; reversing either the relative motion or the magnetic-field direction reverses the induced potential difference and current.
  • The induced current creates its own magnetic field opposing the change that produced it, so pushing a magnet into a coil produces a magnetic effect that resists the push.
  • A common error is to say a stationary magnet permanently induces current in a stationary coil: induction requires relative motion or a changing magnetic field.

Tier 1 · Easy

2 marks
ORIGINAL

A bar magnet is held motionless inside a coil connected to a sensitive voltmeter. State the reading after the magnet has stopped moving and explain it.

Tier 2 · Standard

4 marks
ORIGINAL

A magnet is pushed into a 200-turn coil and then withdrawn at the same speed. Describe the two voltmeter pulses and state two changes that would increase their magnitudes.

Tier 3 · Hard

6 marks
ORIGINAL

A student feels a resisting force while pushing the north pole of a magnet into a short-circuited coil. Explain the resistance, predict what happens to the induced current direction when the magnet is pulled out, and state why pulling it out faster requires more work per second.

4.7.3.2

Uses of the generator effect (physics only) (HT only)

  • An alternator rotates a coil in a magnetic field and generates alternating potential difference, so its output reverses every half-turn.
  • A dynamo uses a split-ring commutator to exchange coil connections every half-turn, producing a direct potential difference of one polarity.
  • For steady rotation, the alternator graph crosses zero when the coil sides move parallel to the field and reaches maximum magnitude when they cut field lines at the greatest rate.
  • A common error is to draw a constant dc line for a dynamo: its output keeps one polarity but normally varies in magnitude as the coil rotates.

Tier 1 · Easy

2 marks
ORIGINAL

State which device generates ac, an alternator or a dynamo, and identify the output produced by the other device.

Tier 2 · Standard

4 marks
ORIGINAL

An alternator coil completes five rotations each second. Determine the frequency and period of its output potential difference, then state how the graph changes if the rotation rate doubles.

Tier 3 · Hard

5 marks
ORIGINAL

Two otherwise identical rotating-coil generators use the same field and speed. Generator P uses slip rings; generator Q uses a split-ring commutator. Compare their potential-difference-against-time graphs and explain the role of Q's commutator.

4.7.3.3

Microphones (physics only) (HT only)

  • A moving-coil microphone uses the generator effect to convert pressure variations in sound into variations in current in an electrical circuit.
  • Sound pressure makes a diaphragm vibrate; its attached coil moves relative to a permanent magnet's field, inducing a varying potential difference.
  • The induced signal follows the diaphragm: greater sound amplitude gives greater coil speed and signal amplitude, while sound frequency sets the signal frequency.
  • A common error is to supply a driving current to the microphone coil; the sound-driven motion induces the signal, whereas supplied current drives a loudspeaker.

Tier 1 · Easy

2 marks
ORIGINAL

State the input and output of a moving-coil microphone.

Tier 2 · Standard

4 marks
ORIGINAL

Explain the sequence by which a singer's sound produces an electrical signal in a moving-coil microphone.

Tier 3 · Hard

5 marks
ORIGINAL

Tones P and Q have the same pitch, but P is louder. Tones R and S have the same loudness, but R has the higher pitch. All four are recorded by one ideal moving-coil microphone. Compare the relevant electrical signals and explain the generator-effect link.

4.7.3.4

Transformers (physics only) (HT only)

  • A transformer has primary and secondary coils wound on an easily magnetised iron core; alternating current in the primary produces a changing core field that induces a potential difference in the secondary.
  • Use VpVs=npns\dfrac{V_{\mathrm{p}}}{V_{\mathrm{s}}}=\dfrac{n_{\mathrm{p}}}{n_{\mathrm{s}}}: more secondary turns than primary turns gives a step-up transformer, and fewer gives a step-down transformer.
  • For an ideal transformer, input and output powers are equal, so VpIp=VsIsV_{\mathrm{p}}I_{\mathrm{p}}=V_{\mathrm{s}}I_{\mathrm{s}}; raising transmission potential difference reduces current for the same power and therefore reduces heating losses.
  • A common error is to connect a transformer to steady dc: without a changing primary current there is no continuously changing core field and therefore no continuous secondary potential difference.

Tier 1 · Easy

2 marks
ORIGINAL

A transformer has 300 turns on its primary coil and 900 turns on its secondary coil. State whether it is step-up or step-down and give the potential-difference factor.

Tier 2 · Standard

3 marks
ORIGINAL

An ideal transformer has 12001200 primary turns and 8080 secondary turns. Its primary potential difference is 230 V230\,\text{V}. Calculate the secondary potential difference.

Tier 3 · Hard

6 marks
ORIGINAL

An ideal step-up transformer supplies 18.0 kW18.0\,\text{kW} at 12.0 kV12.0\,\text{kV} from a 240 V240\,\text{V} primary. Calculate the secondary current, the primary current and the secondary-to-primary turn ratio. Explain one transmission advantage of the high output potential difference.