The magnetic field produced by a current-carrying straight wire at a point outside the wire depends
- (a)inversely on the distance from it
- (b)directly on the distance from it
- (c)inversely at short distances and directly at large distances from it
- (d)directly on the distance (at short distances) and inversely on the distance (at long distances) from it
Correct — A, inversely on the distance from it. The magnetic field around a long straight current-carrying wire is B = μ₀I/(2πr). Because B is proportional to 1/r, the field falls off inversely with the perpendicular distance r from the wire.
- (b)directly on the distance from it — If the field rose with distance it would be strongest far away — the opposite of what is observed. B actually weakens as r increases.
- (c)inversely at short distances and directly at large distances from it — The 1/r dependence holds at all distances outside the wire; there is no switch-over from inverse to direct behaviour.
- (d)directly on the distance (at short distances) and inversely on the distance (at long distances) from it — This invented two-regime rule has no basis; the field is inversely proportional to r everywhere outside a long straight wire.
A steady current in a long straight wire sets up circular magnetic field lines around it — Oersted's 1820 discovery. The field magnitude is B = μ₀I/(2πr): it grows in proportion to the current I and falls inversely with the perpendicular distance r from the wire.
Field lines crowd together near the wire (strong field) and spread out farther away (weak field), which is the visual cue for the 1/r fall-off. The distractors invent direct or mixed dependences that do not exist for a straight wire.
- The field of a long straight wire is B = μ₀I/(2πr).
- B is inversely proportional to distance r and directly proportional to current I.
- The field lines are concentric circles around the wire; their direction follows the right-hand thumb rule.
- Oersted first showed in 1820 that a current-carrying wire deflects a nearby compass needle.
The magnetic field of a long straight wire weakens inversely with distance from it.
- Assuming the field increases with distance from the wire.
- Inventing different behaviour at short versus long distances.
A concept check on how B depends on distance, or a direction question solved with the right-hand thumb rule.
No directly related past PYQ was found.
- practice — not a real PYQ
If the distance from a long straight current-carrying wire is doubled, the magnetic field becomes
- (a)double
- (b)half
- (c)one-fourth
- (d)unchanged
Answer(b) half — B ∝ 1/r, so doubling r halves the field.
- practice — not a real PYQ
The magnetic field lines around a straight current-carrying conductor are
- (a)straight lines parallel to the wire
- (b)concentric circles around the wire
- (c)directed radially outward
- (d)elliptical
Answer(b) concentric circles around the wire — as described by the right-hand thumb rule.