Homework Help: Magnetic Suspension

1. May 13, 2015

Angie K.

1. The problem statement, all variables and given/known data

A long horizontal wire carries a current of I1 = 48.1 A. A second wire, made of 2.5-mm-diameter copper wire and parallel to the first but a distance d = 14.9 cm below it, is held in suspension magnetically. What is the current I2 in the second wire? (Enter a positive value if I2 is to the right, negative if it's to the left.)

2. Relevant equations

B=mu0/2pi*(I)/r

3. The attempt at a solution

I am not sure how to calculate I (current) without knowing the B (magnetic field) .

Can I use the equation from above to figure out the magnetic fields with plugging in different r values? If so, then would I just set them equal to find the other current?

2. May 13, 2015

BvU

Hello there,

Sort of. You can use your equation to calculate the magnetic field at the position of the lower wire.
Once you have that, you will need another equation to deal with the suspension part.
I expect you can write a force balance for the two forces working on the lower wire ?

3. May 13, 2015

Angie K.

By force balance, do you mean F=I*L*B ?

4. May 13, 2015

BvU

That is one force, yes.

Now: you know B (I hope). And what about F ? I2 is to be found, so we must invent something for L. Any ideas ?

Last edited: May 13, 2015
5. May 13, 2015

Raghav Gupta

Hello BvU once again and Angie
That red equation is mu0i/2pi r or $\frac{\mu_0i}{2πr}$
I think you had a typo there and BvU was pointing it correct.

6. May 13, 2015

BvU

Hello Raghav !

RG: I did no such thing. And I don't think there was reason to do so.

AK: No reason for alarm: just written a bit awkward, so that a casual reader is easily wrong-footed.

I am sure you meant B = mu0/(2pi) * I / r

As you can see, RG's way of typesetting with LaTeX is much clearer and unambiguous. Worth looking into how to do that ! (see guidelines)