# Prove that the Force Acting on a Current-carrying Wire, Joining Two Fixed Points a and B in a Uniform Magnetic Field, is Independent of the Shape of the Wire. - Physics

Sum

Prove that the force acting on a current-carrying wire, joining two fixed points a and b in a uniform magnetic field, is independent of the shape of the wire.

#### Solution

Given:
Uniform magnetic field existing in the region of the wire = B
Let the electric current flowing through the wire be i.
Length of the wire between two points a and b = l
Magnetic force is given by

vecF = ivecl xx vecB

vecF = ilBsinθ

Let us consider two wires of length l, one straight and the other circular.
The circular wire is of radius a such that 2pia = l

Suppose the magnetic field is pointing along the z direction and both the wires are lying in the xy plane, so that the angle between the area vector and the magnetic field is 90°.
For the straight wire of length l lying in a uniform magnetic field of strength B :

Force , F = ilBsin(90^circ) = ilB

For the circular wire :

Length , l = 2pia

Angel between the area vector and magnetic field will again be 90°.
Force acting on the circular wire,

F = i(2pia)Bsin(90^circ)

= i2piaB = ilB

Both the forces are equal in magnitude. This implies that the magnetic force is independent of the shape of the wire and depends on the length and orientation of the wire.

Therefore, the magnetic force is independent of the shape of the wire.

Concept: Force on a Moving Charge in Uniform Magnetic and Electric Fields
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 12 Magnetic Field
Q 16 | Page 231