Magnetism and Electromagnetism: Key Takeaways for GCSE Revision

1. Permanent and Induced Magnets

  • Permanent Magnets:
    • Always have North and South poles.
    • Produce their own magnetic field (e.g., bar magnets).
    • Test: Can repel another permanent magnet.
  • Induced Magnets:
    • Become magnetic in a magnetic field (e.g., iron nails near a magnet).
    • Lose magnetism when removed from the field.
    • Always attracted to permanent magnets.

Key Rules:

  • Like poles repel (N-N, S-S).
  • Unlike poles attract (N-S).

Example:
Steel paper clips become induced magnets when near a magnet. The top of each clip becomes the opposite pole to the magnet, causing attraction.

Tip: Use the repulsion test to confirm if an object is a permanent magnet (only permanent magnets repel each other).

2. Magnetic Fields

  • Field Lines:
    • Start at North, end at South.
    • Closer lines = stronger field (strongest at poles).
  • Earth’s Magnetic Field:
    • Acts like a giant bar magnet.
    • Geographic North = Magnetic South (compass points to this).

Practical Tip: Use a plotting compass to trace field lines. Move the compass step-by-step, marking the needle direction.

Example:
In Figure 7.6, plotting compasses show the field direction around a bar magnet.

3. The Motor Effect

  • Force on a Current-Carrying Wire:
    • Formula: F=BILF=BIL
      • FF: Force (N), BB: Magnetic flux density (T), II: Current (A), LL: Length (m).
    • Left-Hand Rule:
      • Thumb = Motion, First finger = Field (N→S), Second finger = Current (+→-).

Example:
A wire with I=3.0 AI=3.0A, L=0.15 mL=0.15m, and B=0.2 TB=0.2T experiences a force:
F=0.2×3.0×0.15=0.09 NF=0.2×3.0×0.15=0.09N

Tip: If the wire is parallel to the field, no force acts (θ=0∘θ=0∘).

4. Electromagnets

  • Solenoids: Coils of wire with magnetic fields similar to bar magnets.
  • Strength Factors:
    • Increase current, number of turns, or add an iron core.
  • Applications:
    • Relays: Use a small current to control a larger one (e.g., car starter motors).

Example:
In Figure 7.18, a relay magnetises a solenoid, pulling contacts to connect the starter motor circuit.

Tip: Use insulated wire to prevent short circuits in coils.

5. Induced Potential (Generator Effect)

  • Induced PD: Created by:
    • Moving a conductor in a magnetic field.
    • Changing magnetic field around a conductor.
  • Lenz’s Law: Induced current opposes the change causing it.

Example:
A dynamo uses a rotating coil in a magnetic field to generate direct current (split-ring commutator). An alternator produces AC (slip rings).

Factors Increasing PD:

  • Faster motion, stronger field, more turns.

6. Transformers

  • Equation:
    VpVs=NpNsVs​Vp​​=Ns​Np​​
    • VpVp​, VsVs​: Primary/secondary PD.
    • NpNp​, NsNs​: Number of turns.
  • Power Conservation (Ideal):
    VpIp=VsIsVp​Ip​=Vs​Is​

Example:
A transformer steps 230V (primary) to 11.5V (secondary) with 5000:250 turns (step-down).

Tip: Transformers only work with AC (changing current creates a changing field).

7. The National Grid

  • High Voltage Transmission:
    • Reduces energy loss (Ploss=I2RPloss​=I2R).
    • Step-up transformers increase PD to 400,000 V for transmission.

Example:
Transmitting 25 MW at 250,000V (100A) loses 0.1 MW, vs 25,000V (1000A) losing 10 MW.

Tip: High PD = Low current = Less heating in cables.

Key Diagrams to Revise

  1. Magnetic Field Lines (bar magnet, solenoid, Earth).
  2. Motor Effect (left-hand rule application).
  3. AC Generator (slip rings) vs Dynamo (split-ring commutator).
  4. Transformer (core, primary/secondary coils).

50 GCSE Questions on Magnetism and Electromagnetism

Section 1: Permanent and Induced Magnets

  1. What is the difference between a permanent magnet and an induced magnet?
  2. How can you test if an unmarked bar is a permanent magnet?
  3. Why do steel paper clips stick to a magnet in a chain (Figure 7.2)?
  4. Explain why induced magnets lose their magnetism when removed from a magnetic field.
  5. Which of these materials are magnetic: brass, steel, aluminium, iron?

Section 2: Magnetic Fields

  1. Describe how to plot the magnetic field of a bar magnet using a compass.
  2. Why do magnetic field lines never cross?
  3. What does the spacing between magnetic field lines indicate?
  4. Explain why a compass needle points north.
  5. How does the Earth’s magnetic field compare to that of a bar magnet?

Section 3: The Motor Effect

  1. State the equation for the force on a current-carrying wire in a magnetic field.
  2. A wire of length 0.5 m carries a current of 4 A in a magnetic field of 0.3 T. Calculate the force.
  3. Use the left-hand rule to determine the direction of motion for a wire in Figure 7.24.
  4. Why is there no force on a wire if it is parallel to the magnetic field?
  5. List three factors that affect the force on a wire in the motor effect.

Section 4: Electromagnets

  1. What is a solenoid?
  2. How can you increase the strength of an electromagnet?
  3. Explain the purpose of a relay in a car starter motor (Figure 7.18).
  4. Why is soft iron used for the core of an electromagnet?
  5. Why must the wire in an electromagnet be insulated?

Section 5: Induced Potential (Generator Effect)

  1. Define the generator effect.
  2. How does moving a wire faster through a magnetic field affect the induced PD?
  3. Explain Lenz’s Law with an example.
  4. What is the difference between an alternator and a dynamo?
  5. Why does a microphone produce an alternating PD (Figure 7.37)?

Section 6: Transformers

  1. State the transformer equation.
  2. A transformer has 200 primary turns and 50 secondary turns. If Vp=100 VVp​=100V, find VsVs​.
  3. Why do transformers only work with AC?
  4. Is a transformer with 1000 primary turns and 5000 secondary turns step-up or step-down?
  5. Calculate the secondary current if Vp=230 VVp​=230V, Vs=46 VVs​=46V, and Ip=2 AIp​=2A.

Section 7: The National Grid

  1. Why is electricity transmitted at high voltages in the National Grid?
  2. Calculate the power loss in a cable with I=50 AI=50A and R=2 ΩR=2Ω.
  3. What role do step-up transformers play in the National Grid?
  4. Explain why low current reduces energy loss in power lines.
  5. Why is AC used in the National Grid instead of DC?

Diagram-Based Questions

  1. In Figure 7.6, which direction does the compass point near the north pole of the bar magnet?
  2. In Figure 7.18, what happens to the solenoid when the ignition switch is turned?
  3. In Figure 7.34, why is the induced PD zero when the coil is vertical?
  4. Describe the magnetic field around the solenoid in Figure 7.16.
  5. In Figure 7.28, why does the loudspeaker coil move left when current reverses?

Application Questions

  1. Explain how an electromagnetic flow meter works (Figure 7.33).
  2. Why does a bicycle dynamo not light the lamp when stationary (Figure 7.46)?
  3. How does a moving-coil microphone convert sound to electrical signals?
  4. Why does a transformer melt a nail in Figure 7.47?
  5. Describe how a shake-up torch recharges its battery (Figure 7.44).

Extended Response

  1. Explain the operation of an electric motor, including the role of the split-ring commutator.
  2. Compare the magnetic fields of a bar magnet and a solenoid.
  3. Discuss the ethical implications of unequal global electricity access (Page 26).
  4. Analyse the trace on an oscilloscope for a generator rotating at double speed (Figure 7.56).
  5. Evaluate why high-voltage transmission is more efficient than low-voltage.

Detailed Answers

  1. Permanent magnets produce their own magnetic field and retain magnetism. Induced magnets become magnetic in a field but lose it when removed.
  2. Repulsion test: If the unmarked bar repels a known magnet, it is permanent (only permanent magnets repel).
  3. The magnet magnetises the clips. The top of each clip becomes an opposite pole, attracting the next clip.
  4. Induced magnets align domains temporarily; removal disrupts alignment, losing magnetism.
  5. Magnetic: Steel, iron. Non-magnetic: Brass, aluminium.
  6. Place compass near magnet; mark needle direction. Move compass to new position; repeat to trace field lines.
  7. Field lines show direction; crossing would imply two directions at one point, impossible.
  8. Closer lines = stronger field; wider spacing = weaker field.
  9. Compass needle aligns with Earth’s magnetic field (geographic north ≈ magnetic south).
  10. Earth acts like a bar magnet with magnetic south near geographic north.
  11. F=BILF=BIL
  12. F=0.3×4×0.5=0.6 NF=0.3×4×0.5=0.6N
  13. Thumb (motion), First finger (N→S), Second finger (current). Follow diagram direction.
  14. Force F=BILsin⁡θF=BILsinθ. If θ=0∘θ=0∘, sin⁡0∘=0sin0∘=0.
  15. Increase BB, II, or LL.
  16. A solenoid is a coil of wire producing a magnetic field similar to a bar magnet.
  17. Increase turns, current, use iron core, or reduce coil spacing.
  18. Relay uses a small current to magnetise a solenoid, closing contacts for a larger current.
  19. Soft iron magnetises and demagnetises quickly, ideal for temporary magnets.
  20. Insulation prevents short circuits between wire turns.
  21. Generator effect: PD induced when conductor cuts magnetic field lines.
  22. Faster motion → more field lines cut per second → higher PD.
  23. Induced current creates a field opposing the change (e.g., magnet moving into coil induces repulsion).
  24. Alternator uses slip rings for AC; dynamo uses split-ring commutator for DC.
  25. Diaphragm vibrations move coil in magnetic field, reversing PD direction → AC.
  26. VpVs=NpNsVs​Vp​​=Ns​Np​​
  27. 100Vs=20050⇒Vs=25 VVs​100​=50200​⇒Vs​=25V
  28. AC creates a changing magnetic field, inducing PD in secondary. DC has no change.
  29. Step-up (more secondary turns).
  30. VpIp=VsIs⇒230×2=46×Is⇒Is=10 AVp​Ip​=Vs​Is​⇒230×2=46×Is​⇒Is​=10A
  31. High voltage → low current → reduced I2RI2R power loss.
  32. Ploss=I2R=502×2=5000 WPloss​=I2R=502×2=5000W
  33. Step-up transformers increase voltage for efficient long-distance transmission.
  34. Power loss ∝I2∝I2. Lower II → exponentially less loss.
  35. AC allows easy voltage transformation using transformers; DC cannot.
  36. Compass points away from north pole (field lines exit N).
  37. Solenoid becomes magnetised, attracting iron contacts to complete the starter circuit.
  38. Coil sides move parallel to field → no cutting of field lines → zero PD.
  39. Uniform field inside, similar to bar magnet with N and S poles.
  40. Reversed current reverses force direction (left-hand rule).
  41. Turbine spins magnets past a coil, inducing PD proportional to oil flow rate.
  42. No motion → no cutting of field lines → no induced PD.
  43. Sound vibrates diaphragm → coil moves in magnetic field → induced AC.
  44. High secondary current (I=VRI=RV​) → large I2RI2R heating melts nail.
  45. Magnet moving in coil induces PD, recharging battery via diode (rectifies AC to DC).
  46. Motor: Current in coil creates force (motor effect). Commutator reverses current every half-rotation, maintaining continuous spin.
  47. Bar magnet: Fixed N/S poles. Solenoid: Field strength depends on current/turns; reversible polarity.
  48. Unequal access raises ethical issues (resource allocation, development disparities).
  49. Double speed: Double frequency and PD amplitude (Figure 7.35).
  50. High voltage reduces I2RI2R loss, making transmission more efficient.