A silver ball of radius 4.8 cm is suspended by a thread in a vacuum chamber. Ultraviolet light of wavelength 200 nm is incident on the ball for some time during which a total light energy of 1.0 × 10–7 J falls on the surface. Assuming that on the average one photon out of every ten thousand is able to eject a photoelectron, find the electric potential at the surface of the ball assuming zero potential at infinity. What is the potential at the centre of the ball?
Given, radius of the silver ball r=4.8cm
Wavelength of light λ=200nm
Energy falling on surface e=
Energy of one photon =hc/λ
Where h is Planck’s constant
C is speed of light
λ is the wavelength of light
∴ Energy =
=
=
=
It is given that one photon out of every ten thousand
is able to eject a photoelectron.
∴ No. of photoelectrons= 
=
It is given that potential at infinity is zero
Potential at the centre and at the surface of the ball are
Equal i.e; =Kq/r
Where K is the constant 
q is the charge on ball
r is the radius of ball
q =ne where n is the no.of photoelectrons
and e is charge on electron
∴ 
∴ required potential 
