2.1 The Photoelectric Effect Flashcards
what is the photoelectric effect?
the emission of electrons from a metal surface when the surface is illuminated by light of a frequency greater than a minimum value known as the threshold frequency
what are photoelectrons?
electrons emitted in the photoelectric effect
what is the work function?
the minimum energy needed to release a photoelectron from the surface of a metal
what is the threshold frequency?
the minimum frequency of light required to cause the photoelectric effect
what causes the value of the maximum kinetic energy of the photoelectron to increase?
photoelectrons are emitted with a variety of kinetic energies from 0 to a maximum value
if the frequency of radiation increases the maximum value increases
what is the intensity of radiation in the photoelectric effect?
the amount of energy per second hitting an area of the metal
does the intensity of radiation affect the maximum kinetic energy of the photoelectrons?
it doesnt affect maximum kinetic energy
what is the relationship between number of photoelectrons emitted per second and the intensity of the radiation?
directly proportional
why doesnt wave theory explain the threshold frequency?
- if an EM wave were shone onto a metal, each free electron on the surface of the metal would gain a bit of energy from each incoming wavefront.
- gradually, each electron would gain enough energy to leave the metal.
- even if the EM wave had a lower frequency than the threshold frequency, the electrons would eventually gain enough energy and be emitted.
- however that does not occur
why doesnt wave theory explain the fact that kinetic energy depends only on the frequency?
- the higher the intensity of the wave, the more energy it should transfer to each electron
- so the kinetic energy of the electrons should increase with intensity
what is the relationship between energy of a photon, Planck’s constant and frequency?
E = hf
what is the relationship between energy of a photon, Planck’s constant, speed of light and the photons wavelength?
E = hc/lambda
which model can explain the photoelectric effect?
the photon model
what is the photon model?
- EM waves can only exist in discrete packets known as photons
- photons have a one-on-one, particle-like interaction with an electron in a metal surface
- each photon would transfer all its energy to one specific electron
how do you experimentally demonstrate the photoelectric effect?
- attach a zinc plate to the top of an electroscope (a box containing a piece of metal with a strip of gold leaf attached)
- the zinc plate is negatively charged and so is the metal in the box
- the negatively charged metal repels the gold leaf, causing it to rise up
- UV light is then shone onto the zinc plate, and electrons are lost due to the photoelectric effect
- as the zinc plate and metal loose their negative charge, the gold leaf falls back down as its no longer repelled
what does the work function value depend on?
the metal
what happens if the energy gained from the photon is greater than the work function?
the electron is emitted
what happens if the energy gained from the photon is lower than the work function?
the electron will not be emitted but instead will release the energy as another photon
what is the relationship between threshold frequency, work function and Planck’s constant?
f0 = phi/h
what is the energy transferred from EM radiation to an electron?
the energy of the photon absorbed (hf)
what is the kinetic energy the electron will be carrying when it leaves the metal equivalent to?
hf minus any other energy losses
why are electrons emitted from the metal with a range of kinetic energies?
the kinetic energy of the electron is equivalent to hf minus any other energy losses, which can vary
what is the relationship between Planck’s constant, frequency, the work function and the maximum kinetic energy of the electron?
hf = phi + Ek(max)
what is the relationship between the maximum kinetic energy of the electron, mass and velocity?
Ek(max) = 1/2m(vmax)^2
what is the relationship between the kinetic energy of the electrons and intensity of UV radiation?
they are independent of each other, as electrons can only absorb one photon at a time, no matter how many are around
what is stopping potential?
the potential difference needed to stop the fastest moving electrons travelling with kinetic energy Ek(max)
what is the relationship between charge on the electron, the stopping potential (in V) and the maximum kinetic energy (in J)?
eVs = Ek(max)
