Topic 39 - Photoelectric effect

What is photoelectric effects?
It is the phenomenon of emission of electrons from the surface of metals when they are irradiated with electromagnetic radiation of sufficient frequency and suitable wavelength.

The emitted electrons are called photo electrons and the current so produced is called as the Photoelectric Current.The intensities of photo electrons vary with material.

In the setup below, when ultraviolet is allowed to fall metal plate P, the galvanometer shows deflection.The galvanometer does not deflect when the radiations are blocked from reaching the metal plate.

The electrons are dislodged from the metal surface when they absorbed ultraviolet radiation energy. The galvanometer deflects because of flow of current caused by attraction of electrons by plate C. The stream of electrons complete the circuit.

In the set up below, mercury lamp, zinc plate and both negatively and positively charged electroscope are used.

Causes divergence of the leaf in the uncharged electroscope. Radiations have no effects on the positively charge electroscope. Divergence of the negatively charged electroscope decreases.

For uncharged electroscope, photoelectrons leaves the plate with the net positive charge as they move away from the plate. Positive charges are repelled to the leaf and the plate causing divergence in the leaf.
Radiations have no effect on the positively charged electroscope since the photoelectrons are attracted back by positive charge hence no loss in electrons.
There leaf falls /decrease in divergence for negatively charged electroscope because of loss of charges. This because photoelectrons emitted are repelled by negative charges on the electroscope hence net loss of charge from the electroscope.

Work function (Wo )
Work function is the mminimum energy needed to disloge an electron from a metal surface.This energy is measured in electronvolt (eV) or Joules(J).

Work function can be expressed as

Where
 h = Planck's constant
fo = Threshold frequency.
Photoelectric effect will occur if the energy of incident radiation is greater than the work function.
Threshold frequency(f0 ) is defined as the minimum frequency of incident light required for the photoelectric emission to occur.
Threshold wavelength (λo ) is the maximum wavelength of radiations beyond which no photoemission will occur.
Quantum(quanta) is the small packets of energy propagated. Plural quanta.
Photon is the discrete amount of energy. This energy is given by:
                                               E = hf
Where h is the Planck's constant and f is the frequency of radiations.

Einstein's equation of photoelectric effect
Part of energy from incident radiation is used to dislodge an electron from a metal surface and the rest gives the electron kinetic energy.
                   Energy = (work function) + (maximum KE by the electron)
This can be expressed as.

Since work function (wo) = hfo

This equation is known as Einstein’s equation of photoelectric effect.

Some of the factors affecting photoelectric effect are:
a)Intensity of incident radiation.
b)Applied potential difference between plates.
c)Energy of the radiations.
d)The type of the metal.
For a given metal if the frequency and applied voltage are kept constant; increase in intensity increases photoelectric current. This means that the rate of emission of photoelectrons is directly proportional to the intensity of incident radiations.
Stopping potential is the potential difference applied to stop the electrons from being ejected from the surface when the light falls on it. A graph of stopping potential against frequency is as shown below.

The graph of Vs against f is straight line cutting the x – axis at fo.
The slope of this graph is h/e
The Vs intercept is -wo/e
From this graph both plank’s constant h and the work function Wo can be calculated.

Some of the applications include:
a)Photoemissive cells.
b)Photoconductive cells.
c)Photovoltaic cells.

Photoemissive cells