# Magnetostatics - Magnetic Flux + Force

## Homework Statement

See figure attached.

## The Attempt at a Solution

I can follow how he applies amperes law to obtain the magnetic field produce by the wire but I'm extremely confused how he writes the expression for the flux flowing through S.

What I understand to be the magnetic flux flowing through a surface S.

$$\int_{S}\vec{B}\cdot\vec{dS}$$

Since the wire is a cylinder I assume cylindrical coordinates.

$$B = \mu_{0}H$$

$$dS = rdrd\phi$$

My integral seems to be quite a bit different than his.

Can someone clarify the steps to getting his integral, and why he chooses to work in the yz plane?

Thanks again!

#### Attachments

• L3QZA2010.JPG
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