Microsoft Word photoelectric effect

 
 
Figure 1: Electron in a potential well at a depth “
Φ
Φ
Φ
Φ” 
When a photon with frequency “ν” strikes the surface of a metal, it imparts all of its 
energy to a conduction electron near the surface of the metal. If the energy of the photon (hν) 
is greater than the work function (
Φ), the electron may be ejected from the metal. If the 
energy is less than the work function, the electron will simply acquire some kinetic energy 
that will dissipate almost immediately in subsequent collisions with other particles in the 
metal. By conservation of energy, the maximum kinetic energy with which the electron could 
be emitted from the metal surface T
max
, is related to the energy of the absorbed photon hν, 
and the work function 
Φ, by the relation, 
Φ
−
=
=
e
h
2
max
mv
2
1
max
T
ν
… (1) 
Now consider the case of electrons being emitted by a 
photocathode in a vacuum tube, as illustrated Fig.2. In 
this case, all emitted electrons are slowed down as they 
approach the anode, and some of their kinetic energy is 
converted into potential energy. There are three 
possibilities that could happen. 
i) 
First, if the potential is small then the potential 
energy at the anode is less than the kinetic energy of 
the electrons and there is a current through the tube. 
ii) The second is if the potential is large enough the 
potential energy at the anode is larger than the kinetic 
energy and the electrons are driven back to the 
cathode. In this case, there is no current.
iii) 
The third case is if the voltage just stops the 
electrons (with maximum kinetic energy T
max
) from 
E = -e 
Φ
Φ
Φ
Φ