Magnetism and electromagnetism

A coil is wrapped around an iron nail and connected safely to a low-voltage supply. Which single change is most likely to make the electromagnet stronger?

1. A

   Reduce the number of turns on the nail

2. B

   Increase the current within the safe limit

3. C

   Replace the iron nail with a wooden stick

4. D

   Open the circuit

1. a

   Select and explain Select the best change and explain why it strengthens the field.

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   Correct choiceB

   1. 1

      Selects option B: increase the current through the coil while remaining within the safe limit.

   2. 2

      States that a larger current produces a stronger magnetic field around the coil.

   3. 3

      Recognises the safe-limit condition because excessive current can overheat the wire or supply.

An ideal transformer connects a 240 V alternating supply to a 12 V laboratory output. The primary coil has 800 turns. The output supplies 24 W to a device.

1. a

   Calculate Calculate the number of turns on the secondary coil.

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   1. 1

      Uses Vs ÷ Vp = Ns ÷ Np.

   2. 2

      Substitutes 12 ÷ 240 = Ns ÷ 800.

   3. 3

      Obtains Ns = 40 turns.

2. b

   Calculate Calculate the output current.

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   1. 1

      Uses P = VI and I = P ÷ V.

   2. 2

      Obtains I = 24 W ÷ 12 V = 2.0 A.

3. c

   State State why a transformer requires a changing current rather than a steady direct current.

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   1. 1

      A changing current produces changing magnetic flux and therefore an induced voltage in the secondary coil.

A small hydro generator is 4 km from a village. Engineers can transmit 60 kW at either 600 V or 6.0 kV through the same cables. Transformers would be needed at both ends of the higher-voltage option.

1. a

   Calculate Calculate the transmission current for each voltage, assuming the same 60 kW power.

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   1. 1

      Calculates 60 000 W ÷ 600 V = 100 A.

   2. 2

      Calculates 60 000 W ÷ 6000 V = 10 A.

2. b

   Explain Explain why the higher-voltage option reduces heating in the same cables.

   3
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   1. 1

      Uses cable heating power P = I²R or equivalent reasoning.

   2. 2

      States that the current is reduced by a factor of 10.

   3. 3

      Concludes that resistive heating is reduced by a factor of 100 for the same cable resistance.

3. c

   Evaluate Recommend an option, considering physics, infrastructure, safety, and community needs.

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   1. 1

      Uses lower transmission loss as evidence for the 6.0 kV option.

   2. 2

      Considers a relevant trade-off such as transformer cost, maintenance skills, insulation, or high-voltage safety.

   3. 3

      Gives a justified recommendation conditional on reliable protection, training, and lifecycle cost.