A system of physical chemistry - Index of

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A system of physical chemistry - Index of

CHAPTER VI.

(Systems not in equilibrium) — Quantum theory and the photo-electric effect—

Photochemical reactions— Einstein's Law of the photochemical equivalent—

Thermal reactions— Reaction velocity from the standpoint of the quantum

theory.

The Photo-electric Effect.

Hallwachs in 1888 was the first to show that a body carrying a charge

of negative electricity loses its charge on being exposed to ultra-violet

light ; on the other hand a positively charged body is not discharged.

Later, Hallwachs and Righi showed, independently, that an insulated

metal exposed to ultra-violet light acquires a positive charge. That is

to say, negatively charged particles, electrons, are emitted from the

metal under the action of ultra-violet light. This phenomenon is

known as the photo-electric effect. Certain metals, such as the alkali

metals and zinc and aluminium, exhibit this effect even under the in-

fluence of ordinary " visible" light, the effect being greatest in the case

of the alkali metals. It is essential in all these cases that the surface

should be fresh and clean ;

if a layer of oxide forms, the effect is greatly

weakened. This is known as photo-electric fatigue.

This emission of electrons is bound up with an absorption of the

light. The periodic electric force in the radiation sets the electrons on

the surface into more violent vibration, so that some of them are ejected

from the

possessed

metal into the neighbouring space. If v is the velocity

by an ejected electron, its kinetic is energy ^mv^, where m

is the mass of the electron. If the surface be charged positively to a

potential of V volts, which potential is just able to prevent the escape

of the electron, it follows that Ye, where e is the charge on the electron,

is just equal to ^mv'^, i.e. the kinetic energy which the electron would

have had if no potential had existed.

Experiment has shown that the photo-electric effect really comprises

two phenomena, apparently distinct from one another. These are

known as the normal photo-electric effect and the selective photo-electric

effect respectively. We shall consider the normal effect in the first

place.

The photo-electric current, produced by the moving electrons which

have been set free from the surface, depends upon two factors, (i) the

number of electrons emitted in unit time, and, (2) the speed of these

electrons. If the intensity of the light be increased, without altering

the colour or quality {i.e. without altering the vibration frequency), it

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