Quantum physics Flashcards
What is the photoelectric effect?
The emission of electrons from a metal surface when electromagnetic radiation shone on the metal surface is above a certain frequency
What are the electrons emitted called?
Photoelectrons
What are the three main conclusions drawn from experiments?
- No photoelectrons are emitted when the radiation is below the threshold frequency
- The number of photoelectrons emitted per second is proportional to the intensity of the radiation
- As the frequency increases so does the maximum kinetic energy and it is unaffected by the intensity
What is the threshold frequency?
The minimum frequency required for the photoelectric effect to occur.
What is intensity?
It is the energy transferred per second hitting a given area of the metal
Why couldn’t the photoelectric effect be explained by the wave theory?
According to the wave theory
- The energy carried is proportional to the intensity of the beam
- The energy carried would be evenly spread over the wavefront
- Each free electron would gain a bit of energy from each incoming wave
- Each free electron would gradually gain enough energy to leave the metal surface
What did Einstein suggest about electromagnetic waves?
They exist in discrete packets called photons
How can the photon energy be calculated?
E=hf
What did Einstein suggest about the interaction between the photons and the electrons in a metal surface?
- There was a one to one particle-like interaction
2. A photon would transfer all its energy to one, specific electron
How was the photoelectric effect explained by the photon model?
According to the photon model
- When light hits the metal surface, the metal is bombarded by photons
- If one of the photons collides with one free electron, the electron will gain energy equal to hf.
What do the electrons need to overcome before they can escape the metal surface?
the work function
What is the work function?
The minimum energy needed for an electron to escape from the metal surface and this depends on the metal
What would happen if the photon energy is below the work function?
No electrons emitted
What would happen if the photon energy is equal to the work function?
The electrons would just be emitted
What would happen if the photon energy is greater than the work function?
The electrons would be emitted with kinetic energy
How would you calculate the threshold frequency?
threshold frequency = work function/Planck’s constant
How would you calculate the maximum kinetic energy?
Maximum kinetic energy = (Planck’s constant * frequency) - work function
What is the stopping potential energy?
The minimum energy needed to stop electrons from being emitted
What can be calculated using the stopping potential energy?
The maximum kinetic energy
Why can the maximum kinetic energy be calculated using the stopping potential energy?
- The emitted electrons have to do work against the applied P.D
- The stopping potential is the P.D needed to stop the fast-moving electrons with max kinetic energy
- The work done by the P.D in stopping the fastest electrons in equal to the energy the electrons are carrying
How would you calculate the maximum kinetic energy using the stopping potential energy?
KEmax = eV
max kinetic energy = charge of an electron * stopping potential energy
How is work done calculated?
work done = P.D * charge
What do electrons exist in, in atoms?
Discrete energy levels
What is the ground state?
The lowest energy state of the atom
What is excitation?
The movement of an electron to a higher energy level
What are the two types of excitation?
Electron collision
Photon absorption
What is de-excitation?
The movement of an electron to a lower energy level
Describe the process of excitation by electron collision
- A delocalised electron has a certain amount of kinetic energy
- The delocalised electron collides with an atomic electron
- The atomic electron gains the energy of the delocalised electron
- If the energy gained is enough, the atomic electron will move to a higher energy level
- The delocalised electron continues with less kinetic energy
Describe the process of excitation by photon absorption
- An electron can jump to a higher energy level by absorbing a photon
- The photon energy must be exact
- If the energy is greater or less than the energy required, the photon isn’t absorbed
Describe the process of de-excitation
- Electrons fall to a lower energy level, if there is a vacancy
- Energy is emitted as a photon of a certain frequency
- Different de-excitation levels emit different wavelengths
How can the energy emitted during de-excitation be calculated?
The difference between two energy levels = the photon energy
change in E = E2- E1 = hf
What is ionisation?
The removal of an electron from an atom
What is the ionisation energy?
The amount of energy needed to completely remove an electron from the atom from the ground state
What is the electron volt?
The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt.
Explain how the fluorescent tube works
- Electrons collide with mercury vapour atoms when a current is passed through a fluorescent lamp
- Excitation only happens when the electrons have sufficient energy
- UV photons are emitted when the atomic electrons de-excite
- The tube is coated with phosphor which absorbs the UV photons
- The UV photons excite the phosphor atoms
- When the atoms de-excite, they de-excite at different de-excitation levels emitting visible photons
Why is mercury used?
Mercury is a liquid at room temperature and therefore easier to vapourise
Why is the gas kept at a low pressure?
At low pressure, the gas is conductive
What is a line spectrum?
A series of discrete bright lines against a dark background which are of certain wavelengths
How is a line spectrum formed?
When electrons in excited gas de-excite to lower energy levels, the photons emitted appear as discrete lines of certain wavelengths
What is an absorption spectrum?
A series of dark lines on a continuous spectrum
How is an absorption spectrum formed?
- When white light is shone through a cool gas, most of the electrons will be in their ground state.
- The electrons absorb photons of exact energy equal to the difference between two energy levels.
- Photons of exact energy are absorbed and the electrons excite to higher energy levels
- The absorbed photons appear as dark lines on a continuous spectrum
What can the line spectrum be used for?
To identify elements as the energy levels are unique to each atom
What can the absorption spectrum be used for?
To identify which elements are present in stars
How does light behave as a wave?
Light produces interference and diffraction patterns
These are both wave properties
How does light behave a particle?
The photoelectric effect
Einstein said that EM waves existed as photons
Due to the one-to-one interaction, one photon would give all its energy to one electron
What was De Broglie’s theory?
Wave-particle duality
If wave-like light showed particle properties, particles like electrons should be expected to show wave-like properties
How do you calculate De Broglie’s wavelength?
De Broglie’s wavelength = h/mv
What shows the wave nature of electrons?
Electron diffraction
What was observed to indicate that electrons have wave-like properties?
When accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal,diffraction patterns are observed.
In electron diffraction experiments, what happens when a smaller accelerating voltage is used?
The electrons are slower
Therefore, the rings are more widely spaced
What happens when the momentum of an electron is increased?
It’s De Broglie wavelength is shorter
The rings are less widely spaced
What happens when particles with a greater mass are accelerated to the same speed as the electrons?
They show a tightly packed pattern
Due to their mass, they have a greater momentum and a shorter De Broglie wavelength
What happened before the wave-particle duality theory was accepted?
Other scientists had to evaluate the theory through peer review
Then, it was tested with experiments
Once there was enough evidence, the theory was validated by the scientific community
How has scientists’ understanding of the nature of matter changed over time?
it has changed through the process of hypothesis and validation
In general, what is the wavelength of electrons accelerated in a vacuum tube?
The wavelength is about the same size as electromagnetic waves in the x-ray part of the spectrum
What happens when the intensity of the radiation shone on the metal surface is increased?
The number of photoelectrons emitted per second increases for a given area
