387
Dual Nature of Radiation
and Matter
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
8
m/s). The presently accepted value
of e/m is 1.76 × 10
11
C/kg. Further, the value of e/m was found to be
independent of the nature of the material/metal used as the cathode
(emitter), or the gas introduced in the discharge tube. This observation
suggested the universality of the cathode ray particles.
Around the same time, in 1887, it was found that certain metals, when
irradiated by ultraviolet light, emitted negatively charged particles having
small speeds. Also, certain metals when heated to a high temperature were
found to emit negatively charged particles. The value of e/m of these particles
was found to be the same as that for cathode ray particles. These
observations thus established that all these particles, although produced
under different conditions, were identical in nature. J. J. Thomson, in 1897,
named these particles as electrons, and suggested that they were
fundamental, universal constituents of matter. For his epoch-making
discovery of electron, through his theoretical and experimental
investigations on conduction of electricity by gasses, he was awarded the
Nobel Prize in Physics in 1906. In 1913, the American physicist R. A.
Millikan (1868-1953) performed the pioneering oil-drop experiment for
the precise measurement of the charge on an electron. He found that the
charge on an oil-droplet was always an integral multiple of an elementary
charge, 1.602 × 10
–19
C. Millikan’s experiment established that electric
charge is quantised. From the values of charge (e) and specific charge
(e/m), the mass (m) of the electron could be determined.
11.2 ELECTRON
EMISSION
We know that metals have free electrons (negatively charged particles) that
are responsible for their conductivity. However, the free electrons cannot
normally escape out of the metal surface. If an electron attempts to come
out of the metal, the metal surface acquires a positive charge and pulls the
electron back to the metal. The free electron is thus held inside the metal
surface by the attractive forces of the ions. Consequently, the electron can
come out of the metal surface only if it has got sufficient energy to overcome
the attractive pull. A certain minimum amount of energy is required to be
given to an electron to pull it out from the surface of the metal. This
minimum energy required by an electron to escape from the metal surface
is called the work function of the metal. It is generally denoted by
φ
0
and
measured in eV (electron volt). One electron volt is the energy gained by an
electron when it has been accelerated by a potential difference of 1 volt, so
that 1 eV = 1.602 ×10
–19
J.
This unit of energy is commonly used in atomic and nuclear physics.
The work function (
φ
0
) depends on the properties of the metal and the
nature of its surface. The values of work function of some metals are
given in Table 11.1. These values are approximate as they are very
sensitive to surface impurities.
Note from Table 11.1 that the work function of platinum is the highest
(
φ
0
= 5.65 eV) while it is the lowest (
φ
0
= 2.14 eV) for caesium.
The minimum energy required for the electron emission from the metal
surface can be supplied to the free electrons by any one of the following
physical processes: