# Finding B field within solenoid.

1. Apr 18, 2012

### Alouette

1.
A current balance is constructed to measure the force between a solenoid with n=10000turns per length and a straight wire segment of length l=0.05m inside the solenoid and perpendicular to the field. The current through both the solenoid and wire segment is I=14A. What is the B field inside the solenoid?

2.
B = μonI (solenoid)

B = (μoI)/(2∏r) (straight wire)

3.
I've gone over notes and the chapter, but seeing as I'm still struggling, would like some insight.

Drawing a diagram, on xy-plane I drew the solenoid on x-plane and since it says the wire is perpendicular to it, it is on the y-plane cutting across the solenoid.

My first attempt was to simply use the equations stated above and plug in the numbers:
μonI = (4∏ x 10^-7)*(10,000/0.05)*(14) = 3.52
Obviously wrong since it doesn't involve the straight wire segment.

Trying to use the equation for straight wire is of no use because of the r which I don't seem to have.

I go back to my drawing, RH rule suggests the wire has B field curling around it, while next to it, the solenoid's B field which loops around is there. Showing how little I know but my next thought was to add the two B fields, answer not different from first attempt. This is where I'm stuck.

Besides missing the concept, am I missing something obvious, or am I going to get the response of going to the book once again...

Last edited: Apr 18, 2012
2. Apr 19, 2012

### Alouette

Not even one tiny help? I'm not even looking for much.