If a coil of 40 turns and area $4.0{\mathrm{cm}}^2$ is suddenly removed form a magnetic field, it is observed that a charge of $2.0\times {10}^{-4}\mathrm{C}$ flows into the coil. If the resistance of the coil is $80\mathrm{\Omega }$ , the magnetic flux density (in T ) is __________.