# Magnetic field and wire current

#### dmsgo89

1. The problem statement, all variables and given/known data
A long, straight wire with linear mass density of 45g/m is suspended by threads, as shown in the figure . There is a uniform magnetic field pointing vertically downward. A 7.0A current in the wire experiences a horizontal magnetic force that deflects it to an equilibrium angle of 10 degree. What is the strength of the magnetic field B?

2. Relevant equations

B=(U_o I) / (2 *3.14 * r)

3. The attempt at a solution

I have U_0 given, I given. but can't find r.

How can find r to get B?

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#### kuruman

Science Advisor
Homework Helper
Gold Member
The equation that you have quoted gives the magnetic field produced by a very long wire. It is not relevant here. What equation do you know that gives the force on a current-carrying wire segment in a magnetic field?

F=ILB maybe?

#### Winzer

That's the right equation.
The next step is to draw a free body diagram and balance the forces.

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