4.5.2 - The Photoelectric Effect Flashcards
Describe the photoelectric effect.
When electromagnetic radiation of a particular frequency is shone on the surface of a metal, electrons are emitted from its surface.
The electrons that are released are caused photoelectrons.
How can the photoelectric effect be demonstrated with a gold-leaf electroscope?
1) The gold leaf electroscope is composed of a brass stem to which a thin gold leaf is attached. There is a metal cap attached to the top of the stem and the metal (to be irradiated with electromagnetic radiation) is laid on the metal cap.
2) A metal plate (usually zinc) is placed on the metal and is then charged negatively by touching it with a negatively charged polythene rod, or by electrostatic induction.
3) When this is done, the metal stem and gold leaf will also become negatively charged, meaning that the stem and the leaf will repel each other.
(It is also possible to make the zinc plate, metal stem and the gold leaf positively charged).
What is Einstein’s photoelectric equation?
- Einstein suggested that each single photon could only eject one electron from the metal surface: either the photon energy was larger than the work function of the metal and the electron would be released with some kinetic energy, or the photon would not have enough energy and the emitted electron would stay on the metal surface.
Applying the principle of conservation of energy:
hf = φ + KE max
photon energy of an incident photon * Planck’s constant = the work function of the metal * the maximum kinetic energy of an electron once it has been ejected from the surface of the metal
What does Einstein’s photoelectric equation explain and show?
- Explains why there is a threshold frequency for emission of photoelectrons.
- Shows that light can behave like a stream of particles.
Why can the photoelectric equation not be explained using a wave model?
Since in the wave model, the energy of a wave is dependent on its amplitude (intensity), not its frequency.
What is the work function (of a metal)?
The minimum energy required to release an electron from its surface, overcoming the electrostatic attraction between the electron and the positive metal ions.
What is the threshold frequency?
The lowest frequency of radiation that will result in the emission of electrons from a particular metal surface.
(For most metals, this frequency occurs in the ultraviolet region of the electromagnetic spectrum).
What two main observations can be made from photoelectric effect experiments?
- Electrons will be emitted from the surface of a metal only if the incident radiation is above a minimum frequency called the threshold frequency. Below the threshold frequency, no electrons are emitted.
- Emission of electrons starts the instant the surface starts to be irradiated, provided that the incident radiation exceeds the threshold frequency.
What does the kinetic energy, and hence velocity of the emitted electrons not depend on?
The intensity of the incident light ray. Changing intensity releases more electrons but their kinetic energy does not increase.
(KE depends on the frequency of the radiation).
How can the maximum KE of photoelectrons be determined?
eV = 1/2 mv^2
(We can equate work done to accelerate or decelerate charged particles with the kinetic energy transferred to electrons).
What happens at the stopping potential?
All the emitted electrons have been brought to rest, so we obtain a value for the maximum kinetic energy by:
eV0 = KE max
charge on the electron * stopping potential (v0) = maximum kinetic energy
Describe the relationship between the rate of emission of photoelectrons and the intensity of the incident radiation.
The rate of emission of photoelectrons above the threshold frequency is directly proportional to the intensity of the incident radiation.
Describe Millikan’s photoelectric experiment.
- 1916, Robert Millikan
- Carried out a series of very detailed and accurate experiments that would completely verify Einstein’s explanations.
- Millikan irradiated the metals: sodium, potassium and lithium with monochromatic light.
- By applying a positive potential to the target metal, he could decelerate the electrons.
- He increased the size of the potential difference until the most energetic electrons were unable to reach the cathode, causing the current to fall to zero.
- This meant he could determine the stopping potential, Vs, of each metal, and show that this depends on the frequency of the radiation as predicted by Einstein.
(He obtained accurate values for the work functions of the metals and also an accurate value for Planck’s constant).
