Question

The photo cathode and collector plate are kept 10 cm apart and connected through a galvanometer without a battery. A magnetic field B exists parallel to the plates. The work function of the emitter is 2.39 $eV$ and the light incident on it has wavelength 400 to 600 $nm$. Find the minimum value of $B$ so that galvanometer shows null deflection.

Answer 1

$(KE{)}_{max}=hf-\varphi$ or
$(KE{)}_{max}=3.1-2.39=0.71$ $eV$
$r=\frac{mv}{qB}$
$B=\sqrt{\frac{2\left(KE\right)m}{qr}}$
$B=\sqrt{\frac{2(0.71times1.6\times {10}^{-19})\times 9\times {10}^{-31}}{1.6\times {10}^{-19}\times 1}}$
$=2.85\times {10}^{-5}T$