1) Four long, straight, parallel wires each carry current I. In plane perpendicular to the wires, the wires are at the corners of a square of side "a". Find the force per unit length on one of the wires if (a) all the currents are in the same direction, and (b) all the currents are in the opposite direction.(adsbygoogle = window.adsbygoogle || []).push({});

I got “0” for both parts (a) and (b) because when I use the right-hand rule, all B’s seem to cancel out, so F / l would invariably be 0, but this seems overly simple.

here's #2...

2) A solenoid carries “n” turns per unit length. Apply Ampere’s law to the rectangular curve shown to derive an expression for B assuming that it is uniform inside the solenoid and zero outside it.

I have no idea how to do this. I know that Ampere's Law is [tex]\oint B*dl = \mu*I[/tex], and the answer should be [tex]B = \frac{1}{2}\mu n I ( \frac{b}{\sqrt{b^2 + R^2}} + \frac{a}{\sqrt{a^2 + R^2}})[/tex]. I don't even understand the diagram. Can somebody get me started on this?

thx for any help in advance.

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