Photoelectric Effect Flashcards
What is Planck’s quantisation theory?
That vibrational energies of atoms are quantized, meaning that they can’t possess just any value but only discrete values.
What is the photoelectric effect?
The ejection of electrons from a metal surface (cathode) when light of particular frequencies shines on it.
What is an electron volt?
The energy that is gained (or lost) when accelerating across a potential different (voltage) of 1V.
What effect does changing the intensity of incident light have on the electrons leaving the surface?
The rate of ejection of electrons from the surface of the metal plate. (Photocurrent)
What effect does changing the frequency (colour) of the incident light have on the electrons leaving the surface?
The maximum kinetic energy of ejected electrons.
What is photocurrent?
A measure of the rate of ejection of electrons.
What is the stopping voltage? When does it occur?
A measure of the maximum kinetic energy of the ejected electrons.
This occurs when no photoelectrons reach the anode side, meaning there’s no photocurrent observed.
(The smallest voltage for a minimum current)
If the energy of the photons (hf) is greater than the work function of the metal (Φ), then what happens?
Then electrons will be ejected from the metal plate.
If the energy of the photons (hf) is less than the work function of the metal (Φ), then what happens?
Then electrons will not be ejected from the metal plate.
What effect does changing the intensity of incident light have on stopping voltage?
It doesn’t affect the stopping voltage (Vo). There will be more/less electrons, but they will have the same speed/Ek since the energy is just being shared among them.
Ekmax = Vo
What is the threshold frequency?
The minimum frequency below which no photocurrent is measured.
Once it’s above the threshold frequency, the Ek will increase proportionally with the frequency.
What effect does the frequency have on the photocurrent?
It isn’t affected by the frequency, since there are still the same number of electrons leaving the metal plate. They are only gaining more kinetic energy, but there are still the same amount of electrons.
It’s like the electrons aren’t sharing the energy.
What effect does the frequency have on the stopping voltage?
If they leave will less kinetic energy, they have a lower stopping voltage and vice versa.
Why do some electrons not have the max kinetic energy?
Because the electrons that are in the inner part of the metal have to get to the surface. This means that they will collide with other particles, and in turn they will lose energy.
What does the wave model incorrectly predict about the photoelectric effect?
- If the intensity (amplitude) of light is increased, then the photoelectrons should have a greater kinetic energy. However, this isn’t the case with particles.
- Any frequency of light should be able to overcome the work function, so long as the intensity and time that the light shines on the metal are great enough.
