VISUAL PHYSICS ONLINE

6 ELECTROMAGNETISM

P60 010

(A)

A metal rod of length rotates with a constant angular speed in an anticlockwise sense in a uniform magnetic field . The current in the rod has a constant value .

What is the magnitude and direction of the force acting on the current carrying rod?

(B)

The rod is now aligned so that it rotated in the plane of the magnetic field.

What is the magnitude and direction of the force acting on the current carrying rod?

View solution below only after you have completed the answering the question.

Solution

How to approach the problem

Identify / Setup

Type: Force on a current element

Knowledge domain:

(A)

Straight conductor of length L carrying a current in a uniform B-field of strength B will experience a force F_B.

q is the angle between the conductor (current) and the direction of the B-field

The direction of the force is determined by the right hand palm rule

Execute/Evaluate

(1)

The angle between the current element and the magnetic field is

The magnitude of the force on the current element is

From the right-hand palm rule, the direction of the force is always at right angles to the current element and varies as the rod rotates. The direction of the force is always in the plane of the page (XY plane).

(B)

The angle between the current element continually changes as the rod rotates and the magnitude of the force on the current element is

magnitude varies with angle

From the right-hand palm rule, the direction of the force is always at right angles to the current element and therefore, always direction in the –z direction (into page).

Note:

This problem is based on a multiple choice question from HSC Physics 2001 (Q14). Even for multiple choice questions, you should use the how to approach the problem method.