# Wire Problem

#### nakatheking

I tried using the magnetic field for a wire equation * 2pir, but this didn't work. Please help!

#### topsquark

Forum Staff
You also have the contribution from the straight wire. Did you include this?

-Dan

#### nakatheking

This is all the information given... I believe they share the same current

#### nakatheking

okay so I calculated the magnetic field of the wire + magnetic field of loop * 2pir, but I am not getting what I want. Why?

#### topsquark

Forum Staff
I can't tell you where you went wrong if you don't tell me what you did. Please post what you have been able to do.

-Dan

#### nakatheking

So, we know that the formula for the magnetic field produced by a wire is given by $\frac{\mu I}{2 \pi r}$. I calculated the magnetic field at the center of the wire as as $B = B_{straight wire} + B_{loop} = \frac{\mu I}{2 \pi r} * 2 \pi r + \frac{\mu I}{2 \pi r}$, but this was wrong.

#### topsquark

Forum Staff
First note that $$\displaystyle \dfrac{ \mu _0 I}{2 \pi R} \cdot 2 \pi R$$ has the wrong units. It's always a good idea to check the units.

You've got some wrong formulas.
$$\displaystyle B_{wire} = \dfrac{ \mu _0 I}{2 \pi R}$$

$$\displaystyle B_{loop} = \dfrac{ \mu _0 I}{2R}$$

-Dan

#### nakatheking

First note that $$\displaystyle \dfrac{ \mu _0 I}{2 \pi R} \cdot 2 \pi R$$ has the wrong units. It's always a good idea to check the units.

You've got some wrong formulas.
$$\displaystyle B_{wire} = \dfrac{ \mu _0 I}{2 \pi R}$$

$$\displaystyle B_{loop} = \dfrac{ \mu _0 I}{2R}$$

-Dan

Could you please explain how you got the formula for $B_{loop}$?

#### topsquark

Forum Staff
Could you please explain how you got the formula for $B_{loop}$?
This should help.

-Dan