Question

Explain "Magnetic force never does any work on moving charges". 

Answer 1

1. Magnetic force is given by `vec"F"_"m" = "q"(vec"v" xx vec"B")`.
2. This makes direction of magnetic force `(vec"F"_"m")` perpendicular to direction of velocity of charged particles `(vec"v")`.
3. Thus, the magnetic force is in turn perpendicular to the displacement of charged particles.
4. According to properties of the dot product, `vec"F"_"m".vec"v" = 0,` for any magnetic field `vec"B"`.

Hence, magnetic force never does any work on moving charges.