# Homework Help: Find the current in the wires

1. Apr 2, 2013

### Colts

1. The problem statement, all variables and given/known data
In the figure, two wires are hanging parallel to one another. Each wire hangs 7 degrees from the vertical on 5 cm long cords. If the wires weigh 35 grams per meter, how much current do they carry?
http://www.usi.edu/science/physics/pickett/206/15p2f1.jpg [Broken]
Also, the current is going opposite directions.

2. Relevant equations
Maybe,
F=i*L*B

3. The attempt at a solution
I don't know what equations work here. I know I need to find the force on each wire which pushes them apart, but how do I go about doing that?

Last edited by a moderator: May 6, 2017
2. Apr 2, 2013

### voko

I presume $i$ is the current. What are $L$ and $B$?

3. Apr 2, 2013

### Colts

L is the length of the wire and B is the magnetic field

4. Apr 2, 2013

### rude man

Draw a picture of the wires end-on and draw in the forces acting on the wires each of length L. Include the suspension wires and dimensions.

5. Apr 2, 2013

### voko

Very well, you have the basics right. Do as rude man suggests, and find out B felt by one as a result of the current in another wire, that will give you the force.

6. Apr 2, 2013

### Colts

I messed up. I read the problem as the wire is 5 cm long, but I think we treat this as the wire is infinite and the cords holding the wire is 5 cm long. So now I can use
B=(μ*i)/(2∏*D) where D is the distance from the wire and μ is the magnetic constant in equations.
Also, this is the equation for magnetic force from an infinite wire. We derived this in class.

Now how do I get B from this?

Can I still use the F=i*L*B and chose an arbitrary length of the wire and set that force equalt to the force f gravity acting on that piece?
Which will give me B and in turn i?

7. Apr 2, 2013

### voko

Yes, you can choose an arbitrary length of wire. Note that wire's density per length is given, so both the force and the mass will depend on this length, and it will cancel out.