CDS (II) 2021 General Knowledge · Question 2 of 120

Correct — D, parallel straight lines. A solenoid is a long coil of insulated wire wound closely in the shape of a cylinder. Each turn contributes a circular field of its own, and along the axis of the coil those contributions add while the sideways parts of neighbouring turns cancel. What survives in the interior is a field of one strength pointing one way, so the field lines drawn there come out evenly spaced and parallel to the axis. NCERT states it in as many words: the field lines inside the solenoid are in the form of parallel straight lines, which indicates that the magnetic field is the same at all points inside the solenoid, that is, the field is uniform inside the solenoid. Outside, the picture is different — the lines crowd out of one end, loop round and re-enter the other, exactly as they do for a bar magnet, which is why one end of a current-carrying solenoid behaves as a north pole and the other as a south pole. Slide a soft-iron core inside and the same arrangement becomes an electromagnet.

• (a)ellipse. — No field line inside the coil closes into an oval. Closed loops do appear, but they are the full circuits that run down the interior and back round the outside, not ellipses confined to the inside.
• (b)parabola. — A parabola is an open curve with a changing direction at every point. It would represent a field that changes direction as you move along the axis, which is the opposite of the uniform interior field a long solenoid produces.
• (c)hyperbola. — Same objection. Curved field lines signal a field whose direction varies from place to place; inside a long solenoid the direction is the same everywhere, so the lines cannot curve.

A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. Current through it sets up a magnetic field whose pattern outside is indistinguishable from that of a bar magnet, while inside the field is uniform — the same in magnitude and direction at every interior point of a long coil. Uniformity is exactly what a picture of equally spaced parallel straight lines conveys, since the spacing of field lines encodes strength and their tangent encodes direction.

The word doing the work in the stem is 'inside'. Outside the coil the lines certainly curve; a candidate who pictures the whole bar-magnet-like pattern and then answers from it can talk themselves into a curved option. Reading the field-line convention the other way round is the fastest check available: if the lines were parabolic or hyperbolic, the field direction would swing as you moved along the axis, and no long solenoid does that. The word 'long' matters too — near the ends of a short coil the interior field does begin to spread and weaken, so the uniform-parallel description is a statement about the central region of a long coil.

• NCERT's wording is that the field lines inside the solenoid are in the form of parallel straight lines, indicating that the magnetic field is the same at all points inside — the field is uniform.
• One end of a current-carrying solenoid behaves as a magnetic north pole and the other as a south pole, so the external field pattern matches that of a bar magnet.
• Field-line spacing encodes field strength: evenly spaced lines mean equal strength, and crowded lines mean a stronger field.
• A soft-iron bar placed inside a current-carrying solenoid is magnetised by the strong interior field; the combination is an electromagnet.
• For a long solenoid the interior field grows with the current and with the number of turns per unit length, and does not depend on the coil's diameter.

• Answering about the field outside the coil when the stem says inside.
• Forgetting that 'long' is part of the claim — the uniform interior field describes the central region of a long solenoid, not the region near the ends of a short one.
• Treating the north pole of a solenoid as fixed by its shape; reversing the current swaps the poles.

As a one-line description of the interior field, as a not-correct statement item mixing uniformity with the effect of current and of an iron core, or as a request for the direction of the field using the right-hand rule.

• practice — not a real PYQ

  The magnetic field produced outside a long current-carrying solenoid most closely resembles the field of

  • (a)a bar magnet
  • (b)a straight current-carrying wire
  • (c)a single circular loop of wire
  • (d)an isolated magnetic north pole

  Answer(a) a bar magnet — one end of the solenoid acts as a north pole and the other as a south pole.

• practice — not a real PYQ

  If the current through a long solenoid is doubled while everything else is unchanged, the magnetic field inside it

  • (a)becomes half
  • (b)is unchanged
  • (c)is doubled
  • (d)becomes four times

  Answer(c) is doubled — the interior field of a long solenoid is proportional to the current and to the number of turns per unit length.