Cracking the SAT Physics Subject Test

THE MAGNETIC FORCE ON A CURRENT-CARRYING

WIRE

Since magnetic fields affect moving charges, they should also affect current-
carrying wires. After all, a wire that contains a current contains charges that move.

Let a wire of length ℓ be immersed in magnetic field B. If the wire carries a current
I, then the magnitude of the magnetic force it feels is

FB = IℓBsin θ

Current I is charge over

time (q/t) and the length

of a wire ℓ is distance

(d). Therefore, Iℓ = q(d/t),

which is a familiar formula:

FB = qvB sinθ = I ℓ Bsinθ

where θ is the angle between ℓ and B. Here, the direction of ℓ is the direction of
the current, I. The direction of FB is given by the right-hand rule as before,

remembering that the direction of the current is the direction that positive charges
would flow.

5. A U-shaped wire of mass m is lowered into a magnetic field B
   that points out of the plane of the page. What is the direction of the
   net magnetic force on the wire?