A velocity selector consists of electric field $\overrightarrow{ E }= E \hat{ k }$ and magnetic field $\overrightarrow{ B }= B \hat{ j }$ with $B =12 mT$.

The value $E$ required for an electron of energy $728 eV$ moving along the positive $x$-axis to pass undeflected is:

(Given, , ass of electron $=9.1 \times 10^{-31} kg$ )

The mean intensity of radiation on the surface of the Sun is about $10^{8}\,W/m^2.$ The $rms$ value of the corresponding magnetic field is closet to

Two electrons are moving with same speed $v$. One electron enters a region of uniform electric field while the other enters a region of uniform magnetic field. Then after some time if the de-broglie wavelength of the two are ${\lambda _1}$ and ${\lambda _2}$  then

A plane electromagnetic wave of wavelength $\lambda $ has an intensity $I.$  It is propagating along the positive $Y-$  direction. The allowed expressions for the electric and magnetic fields are given by

The intensity of a light pulse travelling along a communication channel decreases exponentially with distance $x$ according to the relation $I = {I_0}{e^{ - \alpha x}}$ , where $I_0$ is the intensity at $x = 0$ and $\alpha $ is the attenuation constant. The attenuation in $dB/km$ for an optical fibre in which the intensity falls by $50$ percent over a distance of $50\ km$ is

The electric field of a plane electromagnetic wave varies with time of amplitude $2\, Vm^{-1}$ propagating along $z$ -axis. The average energy density of the magnetic field  (in $J\, m^{-3}$) is