A stream of photons having energy 3 eV each impinges on a potassium surface. The work function of potassium is 2.3 eV. The emerging photo-electrons are slowed down by a copper plate placed 5 mm away. If the potential difference between the two metal plates is 1 V, the maximum distance the electrons can move away from the potassium surface before being turned back is–

Options

A. 3.5 mm
B. 1.5 mm
C. 2.5 mm
D. 5.0 mm

Answer

A

Explanation

Energy of incident beam = 3 eV
`phi =2.3eV`
`K_("max") = 3 - 2.3 = 0.7 eV`
At the point of returning back the entire KE is converted into electrostatic PE
Electric field `(E) =(DeltaV)/(Deltax) =1/5 =0.2 V-mm^(-1)`
`implies e xx 0.2 V xx "x" -0.7 eV =0.7eV`
`implies x = 3.5 mm`