X-rays are used to irradiate sodium and copper surfaces in two separate experiments and stopping potential are determined. The stopping potential is

Options

A. `V_("Na") gt V_("Cu")`
B. `V_("Na") = V_("Cu")`
C. `V_("Na") lt V_("Cu")`
D. `V_("Na") and V_("Cu")` are inversely proportional to v

Answer

A

Explanation

The work function `W_(0)=hv_(0)`
Since `W_(0)` for `Cu gt W_(0)` for Na, less energy is required to extract electrons from sodium than from copper.
`:.` For both of them the energy of incident X-rays is hv
As the K.E. of the ejected electrons is given by `E_("max")=hv-W_(0)`
`:.` The kinetic energy of the electrons ejected from sodium is more than that of copper.
But `E_("max")=eV_(0)` where `V_(0)` is the stopping potential
`:.` A greater stopping potential is required for sodium.
`:.V_(Na) gt V_(Cu)`