# In Thomson's experiment to determine $\large\frac{e}{m}$ of an electron , it is found that an electron beam having a kinetic energy of $45.5\;eV$ remains undeflected, when subjected to crossed electric and magnetic fields. If $E=1 \times 10^{3} Vm^{-1}$, the value of $'B'$ is (mass of the electron is $9.1 \times 10^{-31}\;kg$) :

$\begin {array} {1 1} (1)\;2.5 \times 10^{-3} \;Wb m^{-2} & \quad (2)\;5.0 \times 10^{-4} \;Wb m^{-2} \\ (3)\;2.5 \times 10^{-4} \;Wb m^{-2} & \quad (4)\;1.0 \times 10^{-4} \;Wb m^{-2} \end {array}$