# Magnetic flux problem

1. Dec 7, 2004

### hd28cw

Find the magnetic flux crossing the portion of the plane
theta = x/4 defined by 0.01 m < r <0.05 m and 0 m < z < 2 m. A current of 2.50 A is flowing along z-axis along a very long wire.

in drawing the picture i know that there is a long thin wire with a current of 2.5 amps flowing positively on the z-axis and the plane is lying rotated at an angle of pi/4 with the magnetic field flowing in a counter clockwise direction.

How do I go about finding the magnetic flux.

2. Dec 8, 2004

### Galileo

Should that be theta=pi/4?

$$\Phi_B=\int_P \vec B \cdot d\vec a$$

Since the plane lies parallel with the z-axis, its normal points in the same direction as the magnetic field everywhere in the plane, so you can drop the dot in the integrand

$$\Phi_B=\int_P B da$$

Recall that the magnitude of the magnetic field of a long wire is:

$$B(r)=\frac{\mu_0 I}{2\pi r}$$
where r is the distance from the wire.