# Find the magnitude of the magnetic field 10 cm from the wire

## Homework Statement

A) Find the magnitude of the magnetic field 10 cm away from the wire at point P as shown in the picture. The current is directed along the x-axis.

B) Find its direction

Info: I = 5 amps. Point P lies on the Y-axis. Current on the X-axis.

Picture (I have drawn): http://twitpic.com/7e0l7e [Broken]

## Homework Equations

I THINK: B=u*I/2*pi*r

## The Attempt at a Solution

B = (4pi * 10^(-7)) * 5/2 *pi * .1m = 7.85 * 10^(-8) N

***I am not sure if this is correct. I have no means to check my work, so I am hoping someone can simply tell me whether or not I am correct, and if not, perhaps enlightening me as to which direction I should head to find the right answer. Much thanks.

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Doc Al
Mentor
I THINK: B=u*I/2*pi*r
Better to write it like this:
$$B = \frac{\mu_0 I}{2 \pi r}$$

Better to write it like this:
$$B = \frac{\mu_0 I}{2 \pi r}$$

Thank you, Doc Al. I redid my calculation in that format and got 7.96E-7. Does that seem correct?

Again, thank you.

Doc Al
Mentor
I redid my calculation in that format and got 7.96E-7. Does that seem correct?
Not exactly. When you plug in the value of μ0 = 4*pi*10^(-7), that pi will cancel with the pi in the denominator. So you should be able to do the entire calculation without touching a calculator.

Try it one more time. Not exactly. When you plug in the value of μ0 = 4*pi*10^(-7), that pi will cancel with the pi in the denominator. So you should be able to do the entire calculation without touching a calculator.

Try it one more time. Aaah, got it. That should've been evident from the beginning. Thank you, Doc Al. You have alleviated me some frustration. 