Quantum Physics Flashcards
What is the photoelectric effect?
The emission of electrons from the surface of a metal when electromagnetic waves of frequencies larger than the threshold frequency are incident on the metal
What is the definition of a photon? Give the formula to calculate the energy of an emitted photoelectron
A discrete quantity of electromagnetic energy which has zero rest mass. They always move at the speed of light.
E=hf
E- energy / J
h- plancks constant 6.63x10^-34
F- frequency / Hz
Define the work function and give the formulas for finding the work function and threshold frequency
The work function is the minimum energy required to free an electron from the surface of a metal.
hf>W.F for electrons to be emitted
hf = W.F +KE
h = Planck’s constant, 6.63x10^-34
f = frequency, hertz/s^-1
W.F = work function, Joules
KE = Kinetic energy, Joules
f0 = W.F/h
f0 = threshold frequency
How do you convert joules to electronvolts and vice versa?
-Divide joules by 1.6x10^-19 to get eV
-Multiply eV by 1.6x10^-19 to get joules
What will increasing the frequency incident on the surface of a metal do?
Increase maximum kinetic energy.
REMEMBER: The energy of an emitted photon depends ONLY on its frequency
Name the 3 wave theories necessary to know
- Energy is evenly spread out over the wave front
- Energy carried is proportional to frequency
- Energy carried is proportional to intensity
What 3 things would you expect to find provided that electrons are waves?
- Each electron gains energy from each wavefront until it has enough to leave
- At lower frequencies electrons would be emitted, but it would take longer
- At higher intensities more electrons are emitted with higher kinetic energy
What 3 things do you actually find when testing out wave theory on electrons?
- Only certain frequencies of EM waves allowed for an electron to be emitted
- Below a certain frequency, no electrons are emitted
- Intensity has no effect on how many electrons are emitted or their kinetic energy
Why is there a range of the kinetic energies of an emitted electron?
-Electrons are arranged in different positions within the metal, some are closer to positive metal ions (closer to the nucleus)
-an electron on the surface only requires the minimum amount of energy to escape, but not all electrons are on the surface and require more work leave.
What is the de Broglie equation and what does it say about the relationship of wavelength to momentum?
Lambda= h/mv
- wavelength is inversely proportional to momentum
What properties do electrons have that suggest they are waves?
-Electrons can be diffracted by the gaps between atoms and produce a diffraction pattern
What conclusions can you draw from the Gold Leaf Electroscope practical?
- The emission of electrons depends ONLY on the frequency of the electromagnetic wave, NOT the intensity. Intensity only increases the number of electrons emitted
- Electron emission is almost instantaneous which demonstrates that electromagnetic waves have a particle nature
- There is a one to one interaction between photons and electrons
Explain how the gold leaf electroscope experiment demonstrates the photoelectric effect and provide evidence for the particulate nature of EM radiation (6 marks)
- Gold leaf falls due to emission of electrons. Photons above the work function cause electrons to be emitted, showing that there is a one to one interaction between photons and electrons
- An increase in intensity of electrons above the work function cause a higher incidence of electrons on the surface, causing it to fall quicker. However, electrons with energy below the work function will never cause electron emission, wave theory states that the emission would still happen but slower.
- There is an instantaneous emission of electrons from lights above the threshold frequency, wave theory states it will take longer
- If the gold leaf falls with a wave below the threshold frequency incident on it, it doesn’t fall due to the wave, rather due to a natural discharge of electrons over time
What experiments can be used to demonstrate:
1. Particulate nature of EM spectrum
2. Wave-like nature of EM spectrum
- Photoelectric effect
- Youngs double slit
What can electrons be diffracted through?
A thin graphite sheet
