The magnetic field about a straight length of current-carrying wire is

a. similar to the shape of an electric field about a charged wire
b. circular in shape
c. aligned from north to south poles in the wire
d. in accord with the inverse-square law

b. circular in shape

Explanation:

The magnetic field around a current-carrying wire forms concentric circles around the axis of the wire. In particular, the direction of the field lines can be found by using the right hand rule:

- the thumb must be placed along the direction of the current in the wire

- the other fingers, wrapped around the wire, give the direction of the magnetic field lines

The strenght of the magnetic field around the wire decreases linearly with the distance from the wire, according to the equation:

where

is the vacuum permeability

I is the current in the wire

r is the distance from the wire

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