AP Physics C: Electricity and Magnetism / Unit 12: Magnetic Fields and Electromagnetism
12.4 Ampère's Law
Notes
$$\oint\vec B\cdot d\vec l=\mu_0I_{enc}$$
- Use symmetric loops (circles around wires, rectangles in solenoids).
- Ideal solenoid: $B=\mu_0nI$ inside, about zero outside.
Practice
Field inside a solenoid with $n=1{,}000$ turns/m and $I=2$ A:
$\mu_0nI$.
Inside a long solid wire carrying uniform current, $B$ grows as:
$I_{enc}\propto r^2$, divided by $2\pi r$.
Ampère's law is most useful when:
$B$ constant along the loop.
Ready for more? Take the Unit 12 test.