Problem 3.359

In the inertial reference frame $K$ there is a uniform magnetic field with induction $B$. Find the electric field strength in the frame $K^{\prime}$ which moves relative to the frame $K$ with a non-relativistic velocity $\mathrm{v},$ with $\mathrm{v} \perp \mathrm{B}$. To solve this problem, consider the forces acting on an imaginary charge in both reference frames at the moment when the velocity of the charge in the frame $K^{\prime}$ is equal to zero.