Read the following and answer the questions below.
A science teacher sets up a demonstration: a straight copper wire is connected to a battery and a switch, and a compass needle is placed close to the wire. When the switch is closed, the needle deflects. The teacher then bends the wire into a circular loop and passes current through it. Iron filings on a cardboard placed through the loop reveal a field pattern. When the teacher winds the wire into a tight cylindrical coil of many turns and passes current through it, the field pattern changes dramatically and resembles that of a well-known permanent magnet.
(a) When the switch is closed in the straight-wire setup, what does the deflection of the compass needle indicate? [1]
(b) At the centre of the current-carrying circular loop, the magnetic field lines appear as parallel straight lines. Give a reason for this. [1]
(c) What shape of permanent magnet does the field of the tightly wound cylindrical coil resemble, and what are the corresponding 'poles' of this coil? [1]
(d) If the number of turns in the cylindrical coil is doubled while the current remains the same, how does the magnetic field inside the coil change? [1]
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
(a) The deflection of the compass needle indicates that the electric current flowing through the copper wire produces a magnetic field around it. This shows that electricity and magnetism are linked to each other.
(b) At the centre of a current-carrying circular loop, the magnetic field lines appear as parallel straight lines because the centre is far from all points on the loop's circumference. The curved field lines become nearly straight and parallel at this region, indicating a uniform magnetic field there.
(c) The field of the tightly wound cylindrical coil (solenoid) resembles that of a bar magnet. The end from which field lines emerge acts as the North pole, and the end where they enter acts as the South pole.
(d) If the number of turns is doubled (with current unchanged), the magnetic field inside the coil doubles, since the field of a solenoid is directly proportional to the number of turns.
Source: Chapter 12 – Magnetic Effects of Electric Current, Sections 12.1, 12.2 and Summary
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Explanation
- (a) The key phrase examiners expect is "magnetic effect of electric current" — Oersted's discovery.
- (b) CBSE expects the reasoning that the centre is equidistant/far from the circumference, making field lines straight and parallel (uniform field).
- (c) "Bar magnet" is the exact term from the textbook summary; name both poles clearly.
- (d) The proportionality B ∝ n is the core concept. State it directly: doubling turns → field doubles. No formula derivation is needed for 1 mark.