A particle with a charge of $+10.0 \mu \mathrm{C}$ is moving at $300 \cdot \mathrm{m} / \mathrm{s}$ in the positive $z$ -direction.
a) Find the minimum magnetic field required to keep it moving in a straight line at constant speed if there is a uniform electric field of magnitude $100 . \mathrm{V} / \mathrm{m}$ pointing in the positive $y$ -direction.
b) Find the minimum magnetic field required to keep the particle moving in a straight line at constant speed if there is a uniform electric field of magnitude $100 . \mathrm{V} / \mathrm{m}$ pointing in the positive $z$ -direction.