A particle of charge $-16 \times 10^{-18}$ coulomb moving with velocity $10 \mathrm{~ms}^{-1}$ along the $\mathrm{x}$-axis enters a region where a magnetic field of induction $\mathrm{B}$ is along the $\mathrm{y}$-axis, and an electric field of magnitude $10^4 \mathrm{~V} / \mathrm{m}$ is along the negative z-axis. If the charged particle continues moving along the $\mathrm{x}$-axis, the magnitude of $\mathrm{B}$ is