The energies of electrons are commonly measured and expressed in terms of
a unit called an electron volt. An electron volt (ev) is
defined as the energy acquired by an electron when it is accelerated
through a potential difference of one volt.
Imagine an evacuated tube
which contains two parallel separate metal plates connected externally to
a battery supplying a voltage V. The cathode in this apparatus, the
negatively-charged plate, is assumed to be a photoelectric emitter.
Photons from an external light source with a frequency no
upon striking the cathode will supply the electrons with enough energy to
just free them from the surface of the cathode. Once free, the electrons
will be attracted by and accelerated towards the positively-charged anode.
The electrons, which initially have zero velocity at the cathode surface,
will be accelerated to some velocity u when they reach the anode. Thus the
electron acquires a kinetic energy equal to ½ mu2 in
falling through a potential of V volts. If the charge on the
electron is denoted by e this same energy change in ev is given by
the charge multiplied by the voltage V:
| (5) |
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For a given velocity u in
cm/sec, equation (5) provides a relationship between the energy
unit in the cgs (centimetre, gram, second) system, the erg, and the
electron volt. This relationship is:
The regular cgs system of
units is inconvenient to use on the atomic level as the sizes of the
standard units in this system are too large. Instead, a system of units
called atomic units, based on atomic values for energy,
length, etc., is employed.
Atomic units are defined in
terms of Planck's constant and the mass and charge of the electron:
Length.
Force.
Force has the dimensions of charge squared divided by distance squared or
Energy.
Energy is force acting through a distance or
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