magnetic-effect-of-electric-current MCQs for UPSC Prelims

25 practice questions on magnetic-effect-of-electric-current from the Electricity: Magnetic and Heating Effects section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 5 Hard

Sample questions

Q1
medium
An electromagnet can just lift ten steel pins. Its maker then doubles the number of turns of wire and leaves everything else unchanged. The number of pins it can lift will most likely
  1. A stay exactly the same
  2. B fall to zero
  3. C depend only on the colour of the wire's covering
  4. D increase
Show answer and explanation

Correct answer: D - increase

More turns give a stronger field for the same current, so the magnet holds more pins. Its strength cannot stay unchanged when the coil has been changed, the magnetism does not vanish, and the colour of the insulation has no effect whatsoever.
Q2
medium
Why must the wire wound around the core of an electromagnet be insulated?
  1. A So that the wire cannot be bent out of shape
  2. B So that the core stays cool when it is used in water
  3. C So that the current travels round the whole coil instead of crossing between touching turns
  4. D So that the coil looks neat and attractive
Show answer and explanation

Correct answer: C - So that the current travels round the whole coil instead of crossing between touching turns

Bare turns touching one another would let the current take a short path straight across them, so fewer turns would carry it and the magnet would be weak. Insulation is not for appearance, does not stop the wire bending, and is not there for cooling.
Q3
easy
A second cell is added to the circuit of an electromagnet, joined so that the two cells help each other. The electromagnet becomes
  1. A unchanged, because cells have no effect on magnetism
  2. B weaker, because the two cells share the work between them
  3. C completely dead, because two cells cancel each other
  4. D stronger, because a larger current now flows through the coil
Show answer and explanation

Correct answer: D - stronger, because a larger current now flows through the coil

Two cells joined so as to help each other drive a larger current through the coil, and a larger current gives a stronger magnetic field. Cells joined this way do not weaken the current or cancel it, and the current is exactly what produces the magnetism.
Q4
medium
In the compass-and-wire test, what happens to the needle when the switch is opened again?
  1. A It turns to point towards the cell
  2. B It stays in the deflected position for ever
  3. C It returns to its north-south position
  4. D It goes on spinning round and round
Show answer and explanation

Correct answer: C - It returns to its north-south position

With the switch open no current flows, so the magnetic field around the wire disappears and only the Earth's magnetic field acts on the needle, bringing it back to north-south. Nothing holds it in the deflected position, keeps it spinning, or makes it point at the cell.
Q5
medium
In which of these does an electromagnet play a part?
  1. A A glass thermometer
  2. B A paper hand fan
  3. C A candle lamp
  4. D An electric bell
Show answer and explanation

Correct answer: D - An electric bell

An electric bell contains a coil wound on iron that becomes a magnet and moves the hammer. A candle lamp, a paper hand fan and a glass thermometer contain no coil and carry no current, so no electromagnet is involved in any of them.
Q6
medium
Two compasses are placed near a straight wire carrying a steady current, one very close to the wire and one much further away. Which shows the larger deflection?
  1. A Both show exactly equal deflections
  2. B The one closer to the wire
  3. C The one further from the wire
  4. D Neither shows any deflection
Show answer and explanation

Correct answer: B - The one closer to the wire

The magnetic field of a current-carrying wire is strongest close to the wire and grows weaker with distance, so the nearer compass is turned further from north-south. Equal deflections would mean the field did not change with distance, and both compasses lie within the field, so both do respond.
Q7
medium
Which set of choices would make an electromagnet as strong as possible?
  1. A An iron core, bare wire, and a single turn
  2. B A plastic core, many turns, and no cell in the circuit
  3. C A wooden core, few turns, and a small current
  4. D A soft iron core, many turns of insulated wire, and a large current
Show answer and explanation

Correct answer: D - A soft iron core, many turns of insulated wire, and a large current

The strength of an electromagnet grows with the number of turns, with the current, and with an iron core that itself becomes magnetised. Wood and plastic cores add nothing, a coil with no cell carries no current at all, and a single bare turn gives an extremely weak field.
Q8
medium
A student wants to show her class that the magnetic field around a current-carrying wire has a direction at every point. The best plan is to
  1. A place several small compasses at different points around the wire and watch how their needles line up
  2. B hold a glowing bulb near the wire
  3. C touch the wire to feel which way it pulls
  4. D measure the total length of the wire
Show answer and explanation

Correct answer: A - place several small compasses at different points around the wire and watch how their needles line up

Small compasses set at various points around the wire line up along the field and show its direction at each place. A bulb responds to the current in its own circuit and not to a nearby field, touching a wire tells nothing about direction, and length is unrelated to the field's direction.

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