# Magnetic field surrounding a long, straight wire.

1. Mar 6, 2009

### einsteinoid

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

In a lightning bolt, 13 C of charge flows in a time of 1.5 10-3 s. Assuming that the lightning bolt can be represented as a long, straight line of current, what is the magnitude of the magnetic field at a distance of 20 m from the bolt?

2. Relevant equations

B = mu(I)/2(pi)r

3. The attempt at a solution

Ok, the current is the change of charge over time, thus should be equal to 13/(1.5X10^-3), or 8666.66 A. The only other variable in in the equation would be "r", or radius, which should be the distance from the bolt (20 m).

So, by plugging 8666.66 into the "I" variable and plugging 20 into the "r" variable, my equation yields B to equal ".3421 T".

According to the answer key to this homework assignment, the value I should be getting for B is "8.67e-05 T". Am I using the incorrect equation?

Thanks,

-Will

2. Mar 6, 2009

### alphysicist

Hi einsteinoid,

What are you using for mu_0?

3. Mar 6, 2009

### einsteinoid

4(pi) X 10^-7

Btw, i accidently just reported your post haha. In the comment part i entered the value for the permeability of free space.... oops :?

4. Mar 6, 2009

### alphysicist

I think you are making a calculator error. You seem to be calculating:

$$\frac{\mu_0 I}{2} \pi r$$

I think if you put parenthesis around the 2 pi r you'll get the right answer.

5. Mar 6, 2009

### einsteinoid

Oh i see, wow thats an easy correction. Thanks!

6. Mar 6, 2009

### alphysicist

You're welcome!

And thanks for letting me know about the report, in case I get an inquiring email...