The magnetic field strength of a current-carrying wire at a particular distance from the axis of the wire
- A.depends upon the current in the wire
- B.depends upon the radius of the wire
- C.depends upon the temperature of the surroundings
- D.None of the above
Correct answer
Explanation
The correct answer is A, depends upon the current in the wire. Outside a long straight wire the field is B = μ₀I/(2πr), so at a fixed distance r it is directly proportional to the current I.
Hans Christian Oersted found in 1820 that a current deflects a compass needle, showing that a current produces a magnetic field. Around a straight wire the field lines are concentric circles, and their direction is given by the right-hand thumb rule: the thumb along the current, the curled fingers along the field. Doubling the current doubles the field at the same point, while doubling the distance halves it.
B is wrong because outside the wire the field depends only on the current and the distance from the axis, not on how thick the wire is.
C is wrong because the temperature of the surroundings does not enter the formula; it can change the resistance, but the field depends on the current that flows.
D is wrong because option A is correct.
Exam tip: straight wire, B = μ₀I/2πr, so B ∝ I and B ∝ 1/r; direction by the right-hand thumb rule.
Practice Questions
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- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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