Photoelectric Effect Flashcards
(23 cards)
Photoelectric Effect?
If an electron absorbs enough energy the bonds holding it to the metal break releasing the electron as a photoelectron
Threshold Frequency?
If the frequency of radiation has a frequency below a certain value then no photoelectrons are emitted from the surface of the metal
Intensity in photoelectric effect?
Intensity is the amount of energy per second hitting an area of metal, the maximum kinetic energy of the photoelectrons is unaffected by variations in intensity
Wave Theory Predictions?
The energy should be proportional to the intensity and be spread evenly over the wavefront. Each free electron would gain a small amount of energy from the incoming wave. If the EM wave had a low frequency eventually it would gain enough energy. However, electrons are never emitted unless the wave is above a threshold frequency which is why wave theory cannot explain the threshold frequency
Kinetic energy and wave theory?
The higher the intensity of a wave the more energy that is transferred to the electron meaning more KE as intensity increases according to wave theory. Wave theory cannot explain KE being dependant on frequency in the photoelectric effect
Photons?
The wave packets that are the only way EM waves can exist. Photons have a one to one interaction with electrons on metal surface as each photon transfers all its energy to one specific electron
Demonstrating Photoelectric Effect?
A zinc plate is attached to the top of an electroscope which is a box with a metal with a strip of gold leaf attached. The zinc is negatively charged so it repels the gold leaf causing it to rise up. UV light is shone onto the metal plate which causes electrons to be lost through the photoelectric effect and the gold leaf falls down as the plate is no longer negatively charged.
Work function?
When a metal surface is bombarded with photons before the electron can leave the surface it needs enough energy to break the bond holding it there and this energy needed to be overcome is the work function and is different for each metal. If the energy gained exceeds this then it means an electron can be emitted.
Energy less than work function?
If the energy is less than the work function the electron will just vibrate a bit more and heat the metal up and release the rest of the energy as another photon
KE and Intensity?
KE is independent to intensity as the surface electrons can only absorb one photon at a time. Increasing intensity just means more photons per second on an area, each photon has the same energy with a higher intensity
Stopping Potential?
Stopping Potential has the symbol Vs and is the potential difference needed to stop the fastest moving electrons travelling with a KE of Ek(max). The work done by this potential difference is equal to the energy they were carrying (eVS = Ek(max) where āeā is the charge of an electron
Measuring Maximum KE?
The maximum KE can be measured using stopping potential as the photoelectrons can lose their energy by doing work against an applied potential difference.
Electron Volt?
Otherwise eV is defined as the kinetic energy carried by an electron after it has been accelerated from rest through a potential difference of 1 volt. This energy gained is equal to the accelerating voltage
Discrete Energy Levels?
Electrons can only exist in certain well defined energy levels. The lowest energy level an electron can be in is called ground state and an electron is excited when its energy level is higher than ground state
Excitation?
The process of an electron moving to a higher energy level through absorbing the exact amount of energy difference between two energy levels
Ionisation Energy?
The amount of energy needed to remove an electron from the ground state of an atom
Removing Electrons?
When an electron is removed the atom has been ionised. The energy of each energy level identifies amount of energy needed to remove an electron from that level
Fluorescent Tubes?
High voltage applied across mercury vapour which accelerates free electrons that ionise mercury atoms from colliding with electrons in mercury atoms excites. When they de-excite UV photons are emitted and then absorbed by phosphorus coating. The phosphorus electrons get excited from this and when they de-excite they emit visible light.
Line emission spectrum?
Series of bright light on a black background. This is because atoms can only emit photons equal to the difference in energy levels so since only a few photon energies are allowed the only ones seen on the spectra are the corresponding wavelengths
Line absorption spectra?
Light from a continuous spectrum passes through a cool gas and certain photons when absorbed excite the electrons, these wavelengths are missing when the spectrum passes through the gas and create black lines on the continuous spectrum
Diffraction?
When a beam of light passes through a narrow gap and spreads out
Accelerating Electrons experimentally?
Smaller accelerating voltages give greater sized rings, higher electron speed means pattern gets narrower, particles of higher mass would produce a narrower pattern, diffraction only occurs if the particle interacting with object is about the same size as de broglie wavelength, smaller wavelengths result in less diffraction but also more detailed patterns
Electrons wave like properties?
Electrons accelerated through a graphite crystal when passing through spaces between atoms of crystal diffract and produced a pattern of rings
