In the interial reference frame `K` there is a unifrom magentic field the induction `B`. Find the electric feidl strength in the frame `K'` which moves relative to the frame `K` with a non-relatistic veclocity `v`, with `v _|_ B`. To solve this problem, consider the forces acting an an imaginary charge in the frame `K'` is equal to zero.

Explanation

Consider a particle with charge `e`, moving with velocity `vec(v)`, in frame `K` it experiences a force `vec(F) = e vec(v) xx vec(E)`
In the frame `K'`, moving with velocity `vec(v)`, relative to `K`, the particle is at rest. The means that there must be an electric field `vec(E)` in `K'`, so that the particle experinces a force,
`vec(F) = e vec(E') = vec(F) = e vec(v) xx vec(B)`
Thus, `vec(E') = vec(v) xx vec(B)`