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 answer
Explanation
The correct answer is A, inversely on the distance from it. Outside a long straight wire, the field is B = μ₀I/(2πr), so doubling the distance halves the field.
This result follows from Ampère's circuital law, or from the Biot–Savart law applied to a long wire. The field lines are concentric circles around the wire, and their direction is given by the right-hand thumb rule: point the thumb along the current and the curled fingers show the field. The field grows in proportion to the current I and falls off as 1/r at every distance outside the wire.
B is wrong because the field becomes weaker, not stronger, as one moves away from the wire.
C is wrong because the 1/r relation holds at all distances outside the wire; it does not switch to a direct one.
D is wrong for the same reason; only inside a thick wire does the field rise in proportion to the distance from its axis.
Exam tip: straight wire: B ∝ I/r; centre of a circular loop: B = μ₀I/(2R); long solenoid: B = μ₀nI.
Practice Questions
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- B.biodiesel
- C.pulp for paper industry
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- C.It is used for disinfecting drinking water
- D.It is used for bleaching linen in textile industry
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