# Magnetic Fields and Current

1. Mar 7, 2008

### mer584

I put this in the intro forum about 3-4 hours ago and no one answered, can anyone here help?

1. The problem statement, all variables and given/known data
An experiment on the Earth's magnetic field is being carried out 1.00m from an electric cable. What is the maximum allowable current in the cable if the experiment is to be accurate to +/- 1.0%

2. Relevant equations
B= uI/2(pi)r
B is proportional to I/R where B is the magnetic field and pi=3.14
F=IBI where this is I one and I two.

3. The attempt at a solution
I'm not really sure to do with out any numbers. Is there a way to find B so you can then use the first formula I listed to find I using R=1m.

Is there a way to express B in terms of that percentage? The answer is 2.5A

2. Mar 7, 2008

### Staff: Mentor

I went ahead and deleted your post in Intro Physics, since you prefer the thread here. Keep in mind that it's Friday afternoon/evening for much of the world, so expecting replies in a couple hours may not be realistic.

Now, on to your problem. What is the magnitude and direction of the Earth's magnetic field (at least on the surface of the earth on the main continents, where presumably this experiment is being carried out)? If you want to be able to measure that to within 1%, then you want the magnetic field from the nearby wire to be about 0.01 of the Earth's magnetic field. That tells you (via the equation that you mention for the magnetic field surrounding a current-carrying wire) how much current you can tolerate in the nearby cable.

3. Mar 7, 2008

### mer584

O.K. I see it now. Thank you.