14. Photoelectric effect - description of experiment and explanation using work function. (4p.) Flashcards
Photoelectric effect
The photoelectric effect is an effect where electrons are emitted from certain materials when light of a certain minimum wavelength arrives at the material.
Photoelectric effect - description of experiment
It can be observed in a special circuit with a glass chamber, where a metal electrode, which will be illuminated, and another electrode, which will collect electrons are placed in vacuum. When the target electrode is illuminated, electrons appear in the space between the electrodes. When the other electrode is positive, the electrons are pulled to it. At some voltage all of them are collected. We call it saturation voltage. When the voltage sign is reversed, the current still flows in the circuit. Only when the voltage achieves some value, the current stops. It is called the stopping voltage.
Photoelectric effect - explanation using work function.
For the photoelectric effect to happen, the photon must have enough energy to overcome the work function of whatever atom the metal is made of, plus some extra energy. The work function is the amount of energy needed to break the electron free of the atom.
ℎ𝑣 = 𝐴 + 𝐸max
where 𝐸max represents the maximum kinetic energy that a photoelectron can have outside the surface, and A ( = 𝑒𝑉 ) is the work function.
The number of photoelectrons is proportional to the light intensity. The maximum kinetic energy of the electrons (stopping potential 𝑉 ) depends on the ! light frequency, but not on the intensity of radiation. There is a certain value of frequency 𝑣 ! , which corresponds to a stopping potential of zero. For light of a lower frequency than this, no photoelectrons are emitted.
Only one feature, the increase in the number of photoelectrons with the light intensity, can be understood in terms of the classical wave theory of light. But, according to this theory, also the kinetic energy of the photoelectrons should increase as the light beam is more intense, and the photoelectric effect should occur for any frequency of the light. These two features do not take place during the photoelectric phenomenon.
