# Homework Help: Current carrying infinite wire

1. Jul 22, 2009

### zd1899

Q)An infinitely long current carrying wire , carrying current I moving towards positive Y is placed along Y-axis. In the X-Y plane , a circular loop of radius 'r' is placed at a distance 'r' from the wire carrying current 'i' in anticlockwise circulation. Find the force exerted by the infinite wire on the loop.

2. Relevant equations

Acc. to me the only equation required should be
dF=i {dlxB} where B is the magnetic field due to the infinitely long wire at the length element dl which is to be integrated .

3. The attempt at a solution

dF=i{dlxB}
dF= (i*U*I/2pi) * 1/r *dl (assuming dl to be a small length element on the loop.
where r is the distance of dl.

i replaced dl by rd$where$ is the small angle and now wished to integrate along $from 0 to 2pi but B has a dependence on 'r' only as it decreases along X axis. so i am stuck on how to integrate B along with d$.

2. Jul 22, 2009

### alphysicist

You are missing your unit vectors here for dl and B. It is important here to keep track of the direction or you will miss some important cancellations. (Notice that dl points in different directions as you move along the circle.)

That's true, but this r is the distance from the wire to the current element dl, and that distance will depend on your integration angle. (That is, replace the distance with an expression that involves theta.)

(By the way, you are using the symbol r for two different things in this problem: the given distances, and the distance from wire to current element. I would suggest changing one of them so there's no confusion.)

3. Jul 22, 2009

Thats the main trouble , i am not able to find a way to get the dependence of B on $because B is just dependent on distance from the wire. And i have already judged the directions , its the magnitude that bothers me. 4. Jul 22, 2009 ### alphysicist So we're talking about the same thing, look at this picture: http://img11.imageshack.us/img11/1123/wireandloop.jpg [Broken] So when the integration angle is theta, what is the distance between the wire and the current element dl (the bold arrow on the circle)? Last edited by a moderator: May 4, 2017 5. Jul 22, 2009 ### zd1899 r + r(1-cos$) ?

6. Jul 22, 2009

### rl.bhat

The force on a small element dl on the circular loop is given by
dF = μο/2π*I*i*1/x*dl.
dF is perpendicular to dl. Its vertical components dF*sinθ from upper and lower semicircular coils get canceled out. The horizontal components 2dFcosθ add up.
Substitute dl = r*dθ and x = 2r + r*cosθ.
To find F, take the integration form 0 to π.

Last edited: Jul 22, 2009
7. Jul 22, 2009

### alphysicist

If you have theta as in my picture, it would be r + r(1+cos theta) = r (2+cos theta)

If you are starting your integration on the left side (so that theta=0 corresponds to the leftmost point of the circle), then your expression would be correct.

8. Jul 23, 2009

### zd1899

@ rl.bhat why 0 to pi?? why not o to 2pi?

@alphysicist
yes i took it from the left actually and after pi/2 cos which change the sign as it will be pi-theta.

9. Jul 23, 2009

### zd1899

can some1 tell me the final answer so that i can confirm it? i do not have the answer.

10. Jul 23, 2009

### rl.bhat

Because I have taken two symmetric elements from upper and lower semicircles and I have resolved the forces acting on them. Horizontal components add up and vertical components cancel each other.
So F =2* μo/2π*I*i*Intg[ r*cosθ*dθ/r(2 + cosθ)] form 0 to π

Last edited: Jul 23, 2009