Quantum Mechanics Flashcards
To revise quantum mechanics
Define a photon
a packet (quanta) of energy
State the order of the electromagnetic spectrum, in terms of increasing frequency
Radiowaves
Microwaves
Infrared
Visible
Ultraviolet
X-rays
Gamma rays
What do all electromagnetic waves have in common?
- They all travel at the speed of light in a vacuum
- They are transverse waves.
How are electromagnetic waves emitted?
From a charged particle when it loses energy, this can happen when:
An electrom moves from a higher to a lower energy state
A fast moving electron is stopped e.g. x-rays
Which regions of the EM spectrum have photon energies greater than that of visible light?
UV, X-Ray, Gamma
Which regions of the EM spectrum have photon energies less than that of visible light?
IR, Microwave, Radiowave
Which EM wave as the shortest wavelength/highest frequency?
Gamma
Which EM wave as the longest wavelength/lowest frequency?
Radio
What are the wavelengths of the visible spectrum?
400-700nm
What is a typical order of magnitude wavelength of a gamma ray?
10⁻¹² m
What is a typical order of magnitude wavelength of a microwave?
10⁻² m
What is a typical order of magnitude wavelength of a radio wave?
10² m
What is the relationship between photon energy and frequency?
Directly proportional
What is the relationship between photon energy and wavelength?
inversely proportional
Define an eV
1 eV is the energy gained by an electron passing through a potential difference of 1 V
Describe the photoelectric effect experiment?
- An insulating rod is given a negative charge.
- The negative charge is passed onto a zinc plate.
- The charge moves down onto the gold leaf
- The gold leaf repels and deflects away from the center
- Various light source are shone onto the zinc plate
Describe the observations of the photoelectric experiment when using visible light
If a filament lamp (lower frequency) is used the electroscope doesn’t discharge (gold leaf doesn’t move)
When very intense visible light is used, no electrons are emitted (gold leaf doesn’t move)
Describe the observations of the photoelectric experiment when UV light is shone onto the metal plate
- When UV light (high frequency) light is shown on the zinc plate the leaf falls immediately so electrons are being lost from the plate
- If very low intensity UV light is used the discharge starts immediately
Describe the conclusions of the photoelectric experiment
- There is a threshold frequency (minimum frequency) below which no photoelectrons are released
- The light is not interacting as a wave as enough energy would be eventually be transferred and photoelectrons would be released
- light is interacting as single photons** with the **surface electrons in an one to one interaction
- If the photons have enough energy/frequency (E=hf) then a photoelectron is released.
- If the frequency of the wave is to small it won’t have enough energy to release a photoelectron with a one to one interaction
Describe the interaction in the photoelectric effect
A single photon is absorbed by a single surface electron in a one to one interaction
Which has a higher frequency visible light or UV
UV
Which has higher photon energy red of blue visible light?
Blue - higher frequency
Define threshold frequency
Threshold frequency – minimum frequency of radiation needed to release photoelectrons
The minimum frequency of radiation needed to release photoelectrons is called?
Threshold frequency
Define work function
Work function – energy an electron must gain to be released from the metal
The energy an electron must gain to be released from the metal is known as the?
Work function
Define intensity
The amount of energy arriving per second per m2
The amount of energy arriving per second per m2 is known as?
Intensity
What is the photoelectric effect?
When light of a high enough frequency falls on a metal surface and causes electrons to be emitted from the surface
In the photoelectric effect what is the affect of increasing the intensity of visible light?
No effect, the frequency will be below the threshold frequency so no electrons will be released
In the photoelectric effect , if UV radiation is used with a constant frequency but the intensity is increased, what is the effect?
More photoelectrons are released each second - If intensity is increasing but each photon has a constant energy (E = hf) then more photons must be arrving and interacting with electrons per second.
Each photoelectron will have a constant kinetic energy as the energy of the incoming photons is constant (E=hf)
In the photoelectric effect, if UV radiation of constant intensity was used but its frequency was increased what would be the effects?
Less electrons would be released each second - the same energy must be arriving per second but each photon has more energy - so less photons must be arriving per second
Each released photoelectron will have more kinetic energy as the incoming photons have more energy (E = hf)
Explain why the kinetic energy of the photoelectrons emitted has a range of values up to a certain maximum.
Electron close to the surace escape with the most Kinetic energy. Electrons deeper in the metal need to use energy to move to the surface before escaping
Explain why the kinetic energy of the photoelectrons emitted has a range of values up to a certain maximum.
Electron close to the surace escape with the most Kinetic energy. Electrons deeper in the metal need to use energy to move to the surface before escaping
What are the three main observations in the photoelectric effect that shows light can not be interactive as a wave?
A threshold frequency, photoelectrons having a maximum kinetic energy and photoelectrons emitted immediately
Why does a threshold frequency show light is not interacting as a wave?
If light was interacting as a wave electrons should be emmitted at all frequencies
Why does photoelectrons leaving immediately prove light cant be acting as a wave?
If light was acting like a wave energy could build up over a period of time and electrons could be released with a delay
What name is given to the voltage that can be applied to the terminals of a vacuumm photocell to just reduce the photocurrent to 0.
Stopping potential
Why does the photoelectrons having a maximum kinetic energy show that light is not interactive as a wave?
Waves could transfer any amount of energy so there would be no limit on kinetic energy.
Describe the stopping potential experiment
- Photoelectrons are emitted from a metal surface.
- They then have to travel towards the negative terminal of a battery.
- This means they have to do work against the potential difference.
- The stopping potential is the p.d. needed to stop the fastest electrons with the maximum kinetic energy.
- The work done by the p.d. is equal to the initial energy of the electrons.
- W = VQ
In the photoelectic effect experiment, from a graph of kinetic energy against frequency what can be found out?
Gradient = h
x intercept = threshold frequency
y intercept = - work function
Describe excitation
An electron can move up an energy level by gaining energy (e.g. via heating or collisions with other particles or absorbing photons). This is called excitation.
Describe how excitation happens via collision with electrons
Electrons collide with atoms transfering some, none or all of its energy.
Descibe excitation via electron collision when the electron has the same energy as the difference between energy levels?
If the electron has the same energy as the difference between energy levels then it will transfer all of its energy and the electron will stop.
Describe excitation via electron collision when the electron has more energy than the difference between energy levels?
If the electron has more energy than the difference between the energy levels, then excitation will occur and the eletron will take the excess energy away as kinetic energy.
Describe excitation via electron collision when he electron has less energy then the difference between energy levels
If the electron has less energy than the difference between the energy levels then it will be deflected by the atom with no loss in kinetic energy or excitation.
Describe excitation by using photons
Electrons in an atom absorb a photon and all of its energy. This can only happen if the energy of the photon is exactly equal to the energy the electron would need to gain to move to a higher energy level.
If the photons energy is smaller of larger than the difference between the two energy levels then it wont be absorbed.
Describe relaxation
An electron can move down to a lower energy level by losing energy. This is done by releasing a photon. This is called relaxation.
Define ionisation
If an electron gains enough energy it can escape the atom. This is called ionisation.
How much energy is assigned to an electron that has just been ionised?
O J
Why do energy levels have negative values?
- For an electron that is bound to the atom, energy has to be added to release it.
- Work has to be done to move an electron to a higher energy level
- But we define an electron as just being ionized as having zero energy.
- So we say that the electron in an energy level has negative energy.
Explain how fluorescent tubes work
- Fluorescent tubes contain mercury vapour across which a high p.d. is applied.
- The high voltage accelerates free electrons and ionise some mercury atoms
- The free electrons collide with electrons in the mercury atoms the electrons are excited to a higher energy level.
- When these electrons relax they lose high energy photons in the UV range.
- A phosphorous coating on the inside of the tube absorbs these photons, which excites the electrons to a higher energy level.
- These electrons then relax and release lower energy photons of visible light
What substance fills a flurescent tube and what type of photons does it emit?
Mercury - it emits UV photons
What substance coats a flurescent tube and what type of photons does it emit?
Phosphorous and it emits visible light photons
How is a line emission spectrum produced?
Given out by gas atoms which have been excited through discharge of electricity or by heating to high temps.
Explain why line spectra are seen as discrete lines
Line spectra are seen as discrete lines as they contains only specific energies, frequencies and wavelengths.
The electrons in the atom can only orbit at certain distances (shells/levels) from the nucleus, i.e. the radius of the orbit is “quantized”
An excited electrons will jump down (relax) to a lower energy level.
In jumping down the electrons emit a photon of energy equal to the difference in the energy between the two levels. (E= hf)
What is a continuous spectrum?
A continuous spectrum is a spectrum in which all wavelengths/frequencies are present between certain limits.
How is a continuos spectrum produced?
It is given out by hot glowing objects. All wavelengths are present because the electrons are not bound to atoms and are free so they are not confined to specific energy levels.
What is an absorption spectrum?
A continuous spectra with dark bands
How is an absorption spectrum created?
Atoms of hot gas absorbing particular frequencies or wavelength as white light passes through.
State some wave properties
- Refraction
- Diffraction
- Interference
- Polarisation
State some particle properties
- Travels in straight lines
- Photoelectric effect
- Carrying a charge/mass
- Deflection by an electric or magnetic field
Describe an experiment that showed electrons to behave like a wave
Electron diffraction through graphite - Electrons fired pass through a graphite target. A series of bright rings can be seen on a phosphorescent screen.
State the evidence that electrons can act like waves
Electrons are being diffracted** as they pass through the graphite. Electrons are **constructively interfering** to produce bright rings. Electrons are **destructively interfering to produce dark rings. The same pattern build up with just one electron at a time.
What is the condition required for noticeable diffraction?
The wavelength must be similar to the size of the gap it is passing through
Describe the two main methods that cause excitation
- Collision with electrons
- Absorbing a photon
How does excitation by collision differ from excitation by absorption?
In collision the incoming electron can transfer a proportion of its kinetic energy, in absorption the incoming photon has to give up all of its energy.
Which theory of light cannot be used to explain the photoelectric effect?
Wave theory
What name is given to the ability of something to exhibit wave behaviour and particle behaviour?
Wave-particle duality
Give one piece of evidence for electrons exhibiting wave behaviour
Electron diffraction
Give one piece of evidence for light exhibiting particle behaviour
Photoelectric effect
