AP Physics C: Electricity and Magnetism / Unit 12: Magnetic Fields and Electromagnetism
12.2 Magnetism and Moving Charges
Notes
$$\vec F=q\vec v\times\vec B$$
- A charge moving perpendicular to $B$ moves in a circle: $\displaystyle r=\frac{mv}{qB}$.
- Velocity selector: $\displaystyle v=\frac{E}{B}$ when electric and magnetic forces balance.
Practice
Doubling a particle's speed in a uniform $B$ makes its circle's radius:
$r\propto v$.
In a velocity selector with $E=1{,}000$ V/m and $B=0.5$ T, particles pass undeflected at:
$\displaystyle \frac{E}{B}$.
The magnetic force changes a particle's:
$\vec F\perp\vec v$.
Ready for more? Take the Unit 12 test.