‹‹ Previous | Page 1 | Next ››
J. J. Thomson's Experiment on Cathode Rays
J. J. Thomson performed a number of experiments to determine
the velocity of the cathode particles and
the ratio of change to the mass
of v: Electric and magnetic fields are applied
simultaneously and so arranged that the forces exerted on the cathode
particles by those fields are equal in magnitude and opposite in
Force on cathode particles (electrons) due to electric field
= Force on cathode particles (electrons) due to magnetic field.
e E = e B v
\ v =
= velocity of cathode rays.
= intensity of electric field.
= intensity of magnetic field.
V is the potential difference applied between the plates C and D (see
figure) and d is the
distance between the plates
:When only magnetic field is
applied, cathode particles move along a
circular path. The
centripetal force is provided by the magnetic force.
= velocity of the particle
= radius of the circular path
= intensity of magnetic field
= change to mass ratio of cathode ray (electrons)
The phenomenon of emission of electrons by a metal
surface when light is incident upon it is called photoelectric
effect. The electrons emitted in the process are called photoelectrons.
Einstein's Photoelectric Equation
Einstein suggested that,
Explanation of Photoelectric Effect: All
characteristics of photoelectric effect are explained by Einstein's photoelectric
equation, whereas wave theory can explain only one: the increase in
photoelectric current with intensity of incident radiation.
Photoelectric Cell and its Applications
A photoelectric cell works on the basis of the photoelectric
(b) Photoelectric cell
concave shaped cathode inside an evacuated glass bulb or tube emits
photoelectrons. These are collected by an anode which is maintained at a
positive potential with respect to cathode. When light is incident on the
cathode of a photoelectric cell, photoelectrons are emitted and current
flows. When light beam is cut off, current stops.
Applications of Photocell (Photoelectric Cell):