Photons and Quantum Physics Flashcards
Energy equation
Planck constant (H) x frequency
Speed equation
c=F x Lambda
=frequency X wavelength
Work done
Q x V
q= charge of an electron
Electron volt
The unit of energy equal to the work done when an electron is moved through a potential difference of 1 volt
J->eV
/1.6x10^-19
eV->J
x1.6x10^-19
Equation of charge when electron free to move
qV=1/2mv^2
Equation of electron when electron free to move
eV=1/2mv^2
The photoelectric effect
Electrons can escape from the surface of certain metals if the surface of the metal is illuminated by radiation of sufficient frequency
Describe what happens in the photoelectric demonstration
The zinc is negatively charged and so the gold leaf is repelled by the metal plate as it is also negatively charged.
The negatively charged gold leaf is attracted to the positively charged UV rays meaning it returns back to normal
This is due to the electrons escaping the zinc when UV light is shone on it
The Laws of photoelectric emission
The number of electrons released from the surface is proportional to the intensity of the incident radiation, photo electrons per second are directly proportional to the intensity of radiation
Emission occurs with no observable time lag
The kinetic energy of the photo electrons depends only on the frequency of the light and NOT on the intensity of the light, KEmax proportional to frequency
There exists a threshold frequency f0 of emission when f<f0 no photo electrons are emitted. f0 depends upon the type of metal.
What is the name for the certain amount of energy electrons need to be released from the potential energy well?
Work function
Work function equation
=hf0
Explain the photocell experiment
Light is allowed to hit a photo-emissive surface. The photo electrons electrons cross the evacuated tube and go to eatch via the ammeter. The current therefore measures the amount of photo electrons emitted by the photo cathode.
The potential of the collector plate can be made more positive or negative relative to the emitting surface
the collector plate is made more negative in order to repel the electrons thus creating a potential hill
KE max equation in photocell
KEmax=eVs
Vs= kinetic energy of the electrons
Ke/frequency graph
Gradient=planks constant
X-intercept=f0-frequency threshold-frequency when KE=0
Y intercept=work function
different metal for KE/frequency graph
gradient stays the same but work function would change and so would the threshold frequency
different light intensity for KE/frequency graph
wouldn’t affect graph
Significance of the photocell experiment
proves that em spectrum waves are quantifed into particles of energy, proves the particle nature of lights + validates planks constant
Electron diffraction experiment
Electrons evaporated off a heated cathode are accelerated towards a thin carbon target
The electrons are abler to pass through the carbon and hit a screen to give a pattern. The spacing between the carbon atoms behaves like a diffraction grating.
The existence of electron diffraction provides evidence that electrons behave like waves.
Radiation pressure equation
Momentum change per second/area of beam
Absorption spectra
Obtained by passing continous spectra through a cold gas, electrons jump up energy level
Emission spectra
Obtained by heating a gas so that it emits light,electron jumps down energy level and give off a photon, the electrons need less energy as they are closer to the nucleus
Photon energy equation
hf=E1-E2
