Quantum Flashcards
What is the photoelectric effect?
Light incident on a metal surface causes electrons to be emitted from the surface
Why are electrons emitted from this surface by shining green and blue light on it? (not red)
Blue and green light are above the threshold frequency of this metal
So the photons of light have an energy > work function (φ)
Why are no electrons emitted when red light shines on this metal?
The red light photons are below the threshold frequency
So the energy of each photon < work function (φ)
Why does making the red light brighter not cause electrons to be emitted? (Photoelectric effect)
Electrons in the metal interact with photons in a 1-1 interaction
They only absorb photons which have an energy > work function (φ)
- Why do both light source cause electrons to be emitted? (from the surface)
- What is different about the electrons emitted due to the blue light?
- Both light sources have frequency above the threshold frequency (f0) of the metal
- The electrons emitted due to the blue light have a greater maximum kinetic energy (because blue photons have a greater energy from E=hf)
What does threshold frequency (f0) of a metal mean?
The minimum frequency of the incident light needed to cause electrons to be emitted from the surface
- What can you say about the green light incident on this metal?
- What difference does the brighter lamp make?
- The green light is above the threshold frequency so the photelectric effect happens
- The brighter lamp causes more photons of light to collide with electrons so more photons are emitted per second (But the electrons have the same maximum kinetic energy)
You are shining a light (above f0) on a metal. How do you:
- Increase the maximum kinetic energy of the emitted electrons?
- Increase the number of emitted electrons per second?
- Increase the frequency of the light source
- Increase the brightness of the light source
This is a graph for the photelectric effect. What information do the 3 features of the graph provide?
- Y-intercept
- X-intercept
- Gradient
- Y-intercept = - work function
- X-intercept = threshold frequency
- Plancks’ Constant
This is the photoelectric effect graph for a metal
Plot a line on this graph for a metal with a higher threshold frequency
- Y-intercept (φ) decreases
- X-intercept (f0) increases
- But the gradient (h) is constant
If you shine a really bright light on a metal but the light is below the threshold frequency why will electrons never be emitted?
Electrons interact with the photons in a 1-1 interaction
But only if the photon has an energy > work function
No red light photons have an energy > work function
So electron emission will never occur
What is the definition of the work function (φ) of a metal?
The minimum energy required to liberate an electron from the surface of a metal
How is the work function (φ) related to the threshold frequency (f0) of a metal?
When light (above f0) is incident on a metal surface how is the maximum kinetic energy of emitted electrons calculated?
Difference between the energy of each photon and the work function (φ)
For the gold leaf experiment (to show the photoelectric effect):
- How do you make the gold leaf rise?
- Why does the gold leaf fall?
- A charged rod transfers additional electrons to the plate causing repulsion between the stem and gold leaf
- Electrons are liberated from the metal surface (by light above f0) so the stem and leaf become neutrally charged again
Define the electron volt
The kinetic energy gained by 1 electron passing through a potential difference of 1 volt
How do you convert between electron volts and Joules?
eV → J : multiply by 1.6x10-19
J → eV : divide by 1.6x10-19
How is the maximum kinetic energy of photoelectrons (emitted during the photoelectric effect) measured?
- Connect the system to a circuit
- Place a battery opposing the current produced by the emitted electrons
- Measure the stopping potential when the total current = 0
- Ekmax = eVs
During the Photoelectric effect why are electrons with a range of kinetic energies emitted?
Electrons deeper down require more energy to rise to the surface before being liberated
(Electrons at the very top of the surface are emitted with maximum kinetic energy)
What are the 3 types of line spectra and how are they produced?
- Continuous - Produced by blackbody (sort of a star)
- Emission - Produced by an excited gas (electrons de-excite)
- Absorption - Produced by a continuous spectrum passing through a cold gas (electrons are exited to higher energy levels and absorb photons)
What are the key ideas of the Bohr model of the atom?
- Electrons can only travel in allowed orbitals (energy levels)
- Electrons can emit or absorb energies to instantaneously transition between orbitals
- Electrons cannot exist between orbitals
How could an electron excite from the n=2 → n=4 energy level?
It must absorb an energy = the difference between levels (By photon or electron collision)
How could an electron de-excite from n=3 → n=1 energy level?
It must emit an photon of energy = the difference between levels
How is the energy of a photon calculated?
Why do different gases (made of different elements) have different emission spectra?
- Each element has a different set of orbitals (with different energy levels)
- So each element has a different set of electron de-excitation energies
- The different de-excitation energies produce photons with different frequencies (E=hf)
How would you show the 488nm hydrogen emission line corresponds to a de-excitation from n=4 → n=2?
- Calculate then energy difference between the energy levels
- Convert energy difference to Joules
- Convert to f or λ (E=hf or E=hc/λ)
What is the ionisation energy of an atom?
The energy required for an electron to to become liberated from an atom
Equal to the energy of the ground state
What is wrong with this?
Never use work function when talking about energy levels. Ionisation and work function are different.
How is excitation by photon different from excitation by an electron?
- Photon energy = Difference between energy levels
- Electron energy ≥ Difference between energy levels
How many photons (of different wavelengths) can be emitted from this hydrogen atom?
6 possible transition so 6 different photons
Why is this mercury vapor in the fluorescent tubes kept at low pressure?
So a large enough current (of incident electrons) can be sustained
How does fluorescence work in a tube light?
- Mercury atoms excite by absorbing electrons from the current
- When the Mercury atoms de-excite they emit UV photons
- UV photons are absorbed by and excite the phosphor coating
- When the phosphor coating de-excites it emits visible light
When do particles exhibit properties of waves? (refraction, diffraction and polarisation)
When their Debroglie Wavelength is similar to the size of the gap they are passing through
What does this experiment show?
Wave-Particle duality
Electron diffraction through graphite to form maximas (bright rings) and minimas (dark rings)
How is the Debroglie wavelength λdb of a particle calculated?
