What is a photon?
- A small quantized bundle of energy.
- The energy of a beam of light is a product of hf (the energy of one photon) and N (the number of photons)
What is the photoelectric effect? Describe the apparatus used to test it.
- A phenomenon where electromagnetic radiation incident on a metallic surface causes electrons to be emitted from the surface.
- An evacuated tube: light passes through an opening and falls on the photo-surface, which emits electrons. These arrive at a collecting plate. Wires connecting the two allow a current to flow.
How can you experimentally and calculatory work out the maximum kinetic energy of electrons emitted as part of the photoelectric effect?
Place a power supply in the circuit so that the collecting plate repels electrons. Only electrons with the most energy reach the plate. The voltage is increased, and the magnitude of the voltage at which the current becomes zero is called the stopping voltage, Vs.
Emax = eVs
since the work done is the change in kinetic energy.
How does the intensity of light affect the current produced by the photoelectric effect?
- It does not affect the stopping voltage.
- Less intense light will produce a lower current.
How does the frequency of light affect the current produced by the photoelectric effect?
- The higher the frequency, the higher the current.
- The higher the frequency, the higher the magnitude of the stopping voltage.
- The critical or threshold frequency fc is the frequency below which no electrons are emitted at all, even if the light is very intense.
What happens when you plot maximum kinetic energy against frequency for different photo-surfaces?
- A straight line with no negative y (Ek) values.
- The x axis intercept is the critical frequency.
- Different photo-surfaces are the parallel lines.
- A y intercept at the work function.
What are the four surprising observations of the photoelectric effect that cannot be understood in terms of light as a wave and why?
- Intensity of light does not affect the energy of emitted electrons: If light is a wave, a more intense beam should carry more energy and so electrons should have more energy.
- Electron energy depends on frequency of incident light: the formula for energy of light as a wave does not include frequency, so it should not affect it.
- There is a minimum frequency below which no electrons are emitted. “
- Electrons are emitted with no time delay: In the wave model, electrons would have to wait to accumulate enough energy from a low intensity beam to escape the metal, causing a delay.
How does Einstein’s explanation of the photoelectric effect work?
- A single photon of frequency f is absorbed by a single electron in the photo-surface, increasing the energy of the electron by hf.
What is the work function?
- Φ The minimum energy required for an electron to escape a photo-surface.
- The rest of the energy transferred by the photon is converted to kinetic energy:
Ek = hf - Φ
What is the de Broglie hypothesis?
- The concept that particles can have wave-like properties; ie. they obey basic phenomena such as diffraction and interference and have an assigned wavelength.
- Called the duality of matter
- For a particle of momentum p, λ = h/p
How can you derive an equation for the wavelength of an electron accelerated through a potential difference V?
Ek =p²/2m eV = p²/2m p = √(2meV) λ = h/√(2meV) λ proportional to 1/√V
Pair annihilation and pair production
Pages 490-491
What is the Bohr condition for the quantization of angular momentum?
The angular momentum of an electron is quantised in integral values of h/2π :
angular momentum = mvr = nh/2π
where n is the quantum (orbital) number (integer)
r is the orbital radius
What does the Bohr model of hydrogen predict about allowed orbits of electrons?
Allowed orbits are ones for which an integral number of electron wavelengths fit on the circumference of the orbit, so that they form a standing wave.
Pages 492-4
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What is the wave function, Ψ?
The probability of finding an electron at a distance r from its source is
P(r) = IΨI²ΔV where V is the volume considered.
- Where Ψ² is the highest, you are most likely to find the electron.
What is the Heisenberg uncertainty principle for position and momentum?
- It is not possible to measure simultaneously the position and momentum of a particle with indefinite precision. (a fundamental property of nature)
ΔxΔp ≥ h/4π
Δx and Δp are uncertainty in position and momentum
h is Planck’s constant
You can also replace the inequality with approximation.
What is an equation for kinetic energy in terms of momentum?
Ek = p²/2m
How can the uncertainty principle be applied to energy and time?
If the energy of a state is measured with an uncertainty ΔE, then the lifetime of the state is of order Δt such that:
ΔEΔt ≥ h/4π
Also applies to decaying particles where ΔE is the uncertainty in the measured value of energy released and Δt is the lifetime of the particle.
You can also replace the inequality with approximation.
What is some evidence for the wave nature of electrons?
When electrons are fired one at a time through a double slit, it creates an interference pattern, which suggests that the electron is interfering with itself. This can only be explained if the electron is also a wave.
Describe how the quantisation of angular momentum of the electron in a hydrogen atom led Neils Bohr to a successful understanding of the spectrum.
The Bohr model predicts that the electron in the hydrogen atom has discrete/quantised energy. This explains the absorption and emission spectra of hydrogen.
What must be conserved during pair production and annihilation?
- Momentum before and after must be the same. This is why two photons are produced in opposite directions.
- The total mass and energy of the system at the start must be equivalent to the total mass and energy at the end.
What is quantum tunneling?
Objects can ‘borrow’ energy, in order to pass barriers. This can be illustrated by the wave function, which may have a small (but non-zero) amplitude on the other side of the barrier, indicating a small chance of finding the object there.
How can you calculate the total energy of an electron for a particular orbital number?
E = -13.6/n² E = the total energy = Ek + Ep n = the orbital number (starting at 1)
