Quantum Phenomena Flashcards
What is the photelectric effect?
Photoelectrons are emitted from a metal after light above the threshold frequency is shone on it.
What factor affects the threshold frequency?
Different metals have different threshold frequencies.
What is the photon model of light?
EM waves travel in discrete packets called a photon, which have energy directly proportional to the energy of the wave.
How do the electrons absorb photons?
- If the wave is above the threshold frequency
- Each electron can absorb a single electron
- The number of photelectrons emitted per second is proportional to the intensity of the light but the kinetic energy of the emitted electrons stay the same.
What is the work function of a metal?
The minimum energy required for electrons to be emitted from the surface of a metal. (φ)
How do you calculate the energy of a photon?
E = hf
or
E = hc/λ
h = Planck’s constant (6.63 x 10-34 JHz-1)
f = frequency (Hz)
c = speed of light (3 x 108 ms-1)
λ = wavelength (m)
How do we determine the kinetic energy of a photoelectron emitted from the surface of a metal?
KE = hf - φ
When does photoemission occur?
As soon as light with a frequency greater than the threshold frequency is shone onto the metal surface, regardless of intensity.
What is the stopping potential?
The potential difference required to apply across the metal to stop the photoelectrons with the highest kinetic energy (at the surface) from escaping.
What do we use the stopping potential for?
E = eV
where
E = kinetic energy of the electron (J)
e = charge of the electron (1.6 x 10-19 C)
V = stopping potential (V)
What is eV a measure of? How can you convert it into a simpler unit?
- Unit of energy
- Convert to Joules by multiplying by charge of an electron
Where do electrons exist in the atom?
Discrete energy levels
What is excitation?
Excitation occurs when an electron absorb a photon with enough energy in order to move it up to a higher discrete energy level (known as an excited state).
What occurs in de-excitation?
A photon is emitted with the same amount of energy as the energy difference between the energy levels that the electron moves between.
What is ionisation?
Ionisation occurs when an electron gains energy by absorbing a photon with enough energy to allow to escape from the outer shell. This causes the atom to have a net positive charge.
What experiment proves wave properties of electrons?
Diffraction grating experiment
Why is it easier to demonstrate wave properties of electrons than protons?
It is easier to obtain a stream of electrons.
Why do some electrons that collide without atoms maintain their kinetic energy whilst others lose energy?
- One electron does not have enough kinetic energy to excite an electron.
- The other electron provides enough energy to allow excitation of the electron to occur.
What is meant by an excited atom?
An atom with an electron in a higher energy level than their ground state.
What is the process of fluorescence in a fluorescent tube?
1) A partially evacuated tube with low density mercury gas and is coated with a phosphor powder coating on the inside of the tube has a high voltage applied across the tube.
2) This causes electrons to flow from the cathode to the anode - producing an electron beam.
3) These electrons collide with mercury atoms causing the electrons in them to be excited and move to a higher energy level.
4) These electrons then de-excite to their ground state due to the instability of their excited state and emit a photon in the UV wavelengths.
5) These photons collide with the phosphor powder coating atoms on the surface of the glass causing them to get excited.
6) When these electrons de-excite, they emit photons in the visible light wavelengths.
What is the ionisation energy of an atom?
The minimum energy required to remove an electron from an atom from its ground state. It is usually measured in eV.
How doe the energy levels change as we move away from the ground state?
Decreasingly negative energies until the ionisation level where the energy value is 0 V.
Why do electrons de-excite after they are excited?
They are most stable in their ground states so de-excitation makes them more stable.
Explain how light emission spectra work. (6 marks)
1) Electrons can only exist in discrete energy levels.
2) If an electron gains energy, it can move to a higher energy level, as it is excited.
3) The electron then de-excites back to its ground state. It emits a single photon of EM radiation.
4) The photon energy is equal to the difference of energy in the excited and ground states.
5) Photon energy = hf, so the frequency depends on the energy level differences.
6) Each element has a specific set of energy level differences, hence causing them to emit a unique set of frequencies.
What are line absorption spectra and how do they differ from line emission spectra?
- They test which frequencies of light are absorbed by a element.
- A single electron can gain energy from a single photon, meaning it only absorbs certain frequencies of light.
- The photons released from de-excitation are emitted in all directions, causing the intensity in the original direction to reduce.
Why do we use line spectra?
No two elements produce the same set of spectral lines, so we can uniquely identify elements by their line spectrum.
How do we know that light behaves as both a particle and a wave?
Particle - photoelectric effect
Wave - diffraction or interference in a Young’s Slits Experiment
How does the photoelectric effect prove that light behaves as a particle (photons)?
- EM waves carry energy in discrete packets called photons. Photoelectrons are only emitted when photons are above the threshold frequency.
- Waves do not explain this as wave theory suggest the energy absorbed by each electron would allow photoelectrons to be emitted if the exposure time is long enough.
- It also expects that kinetic energy of emitted electrons increase with intensity but this is not true.
How did de Broglie prove that electrons can be have as a wave?
Diffraction pattern produced when a beam of electrons is directed at a thin, graphite film. If it were acting as a particle, we would see even distribution uniformly across the screen.
What do we observe when we diffract a beam of electrons?
We see concentric circles form. A larger accelerating voltage reduces diameter of a ring.
Why do we use a graphite screen for electron diffraction? Why do we see a diffraction pattern form on the screen?
- Crystalline structure allows many slits between atoms allowing a diffraction pattern to form
- Fluorescent screen with phosphor coating emits photons to see the circular pattern.
What is the equation that relates wavelength and momentum?
p = h/λ
p = momentum
h = Planck’s constant
λ = wavelength
Why does de Broglie’s wavelength prove the diffraction pattern?
- As momentum increases, the wavelength decreases
- This causes the concentric rings to get closer together
- This aligns with the electron diffraction with smaller radius with increasing voltage
How do find kinetic energy from de Broglie’s equation?
λ = h/√2mE
λ = wavelength
h = Planck’s constant
m = mass
E = kinetic energy
How does peer review validate a theory?
Other scientists check if the experiment is repeatable and the results are reproducible. This allows us to check validity, originality and significance.
What would we be expecting if the electrons were acting only as particles and are passed through a diffraction grating?
They would scatter uniformly.
