A generator in a power station produces an alternating current. State what is meant by alternating current and explain why alternating current, rather than direct current, cannot be used to operate an electromagnet that is required to repeatedly lift and release magnetic materials.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Alternating current is a current in which the electrons change direction of flow regularly and periodically.
An electromagnet requires a current in one direction to maintain a consistent magnetic field and polarity.
With a.c. the magnetic poles of the electromagnet reverse repeatedly, so the force on the magnetic material alternates between attraction and repulsion.
Therefore the electromagnet could not hold or lift the magnetic material reliably or consistently.
An electromagnet requires a current in one direction to maintain a consistent magnetic field and polarity.
With a.c. the magnetic poles of the electromagnet reverse repeatedly, so the force on the magnetic material alternates between attraction and repulsion.
Therefore the electromagnet could not hold or lift the magnetic material reliably or consistently.
Examiner tips
- Use the exact definition of alternating current – mention regular change of direction. Explain that a single polarity is needed for a steady magnetic field. Show the consequence: poles reverse, so the force alternates. Link the consequence to the inability to lift reliably.
Common mistakes
- Confusing alternating current with direct current in the definition. Failing to mention that the magnetic field reverses. Not linking the reversal of poles to the inability to lift the material.
Mark scheme (4 marks)
- Alternating current is a current in which electrons change direction of flow regularly / periodically
- An electromagnet requires direct current / a current in one direction to maintain a consistent magnetic field / polarity
- With a.c., the magnetic poles of the electromagnet reverse repeatedly, so the force on the magnetic material alternates between attraction and repulsion
- Therefore the electromagnet could not hold / lift the magnetic material reliably / consistently
Key terms in this question
alternating current · direct current · electromagnet · magnetic materials
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "State and Explain" question →
- Decode the mark scheme abbreviations →
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