alleryStart learning →What is the magnetic field strength at a shortest distance r from a long straight wire carrying current I?
AB = mu0 I / (2 r)
BB = 2 pi r / (mu0 I)
CB = mu0 I r / (2 pi)
DB = mu0 I / (2 pi r)
Answer & Solution
Correct answer: D. B = mu0 I / (2 pi r)
1. Experiment gives B = mu0 I / (2 pi r) for a long straight wire.
2. Here I is the current and r is the shortest distance from the wire to the point.
3. The constant mu0 is the permeability of free space.
4. Field falls off as 1/r, so doubling the distance halves the field.
5. Putting r on top would make the field grow with distance, which is the wrong behaviour.
_Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.9 Magnetic Fields Produced by Currents: Ampere's Law_
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