• Support PF! Buy your school textbooks, materials and every day products Here!

Calculating angular acceleration of a wire

  • #1

Homework Statement


A rigid uniform horizontal wire PQ of mass M, pivoted at P, carries a constant current I. It rotates with a constant angular speed in a uniform vertical magnetic field B. If the current were switched off, the angular acceleration of the wire in terms of B, M and I would be
[It is not shown here, but in the diagram, PQ is the radius. P is the centre]

ans= ( 3BI ) / ( 2M )

Homework Equations


(angular acceleration) = (torque) / (Moment of Inertia)
(Moment of Inertia) = (MR^2) / 3 for a rod about one end
(Torque) = Force * Radius sin(angle between them)

The Attempt at a Solution



M of I = ( MR^2) / 3
Torque = Force * R
= BIR^2sin( angle between them)

My problem is here. Is the angle 45 degrees? If it is, shouldn't the answer be (3BI)/M*root2?

Also, if current is switched off, the only force acting is B. Even then the rod will be rotating for some time. But current will not be induced as the field is uniform. I am terribly confused.
 

Answers and Replies

  • #2
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
32,760
5,036
I believe it is rotating in a horizontal plane in a vertical field. All angles are right angles, no?
 

Related Threads on Calculating angular acceleration of a wire

Replies
1
Views
9K
Replies
3
Views
2K
Replies
1
Views
2K
  • Last Post
Replies
1
Views
1K
Replies
0
Views
6K
Replies
11
Views
1K
Replies
0
Views
1K
Replies
2
Views
2K
Top