Question

A monochromatic ray of photons of energy $5eV$ are incident on cathode. Electrons reaching the anode have kinetic energies varying from $6eV$ to $8eV$ . Choose the current option.

• A. work function of the metal is $1eV$
• B. work function of the metal is $3 \, eV$
• C. current in the circuit is equal to saturation value
• D. current in the circuit is less than saturation value.

Answer 1

Answer: (D)

$\left(KE\right)_{max}=\left(\right.5-\phi\left.\right)eV$
when these electrons are accelerated through $5 \text{V}$ , they will reach the anode with maximum energy $=\left(\right.5 \, -\phi \, + \, 5\left.\right)eV$
$10 \, - \, \phi \, = \, 8 \, \phi \, = \, 2eV \, $
Current is less than saturation current because if slowest electron also reached the plate it would have $5 \, eV$ energy at the anode, but there it is given that the minimum energy is $6 \, eV$ .