Quantum Physics Flashcards
State the unit of work function
Joules
Explain why the kinetic energy of emitted electrons has a maximum value
Because the energy available depends on the frequency of the radiation and can be no more than the (hf)
Explain with reference to the work function why, if the frequency of the radiation is below a certain value ,electrons are no emitted
In order for electrons to be emitted from an electron well. A minimum amount of energy is needed , this is called the work function
State what is meant by work function
Minimum energy required to remove electrons from a metal
In fluorescent tune ,explain how the mercury vapour and the coating of its inner surface contribute to the production of visible light
mercury vapour at low pressure is conducting (1) atoms of mercury are excited by electron impact (1) producing (mainly) ultra violet radiation (1)
which is absorbed/ excites the coating (1)
which, upon relaxing, produces visible light
State what is meant by ground state
when electrons/atoms are in their lowest/minimum energy
An atom can also become exited by absorption of photons. Explain why only photons of certain frequencies cause excitation in a particular atom
electrons occupy discrete energy levels
and need to absorb an exact amount of/enough energy to move to a higher level photons need to have certain frequency to provide this energy or e = hf
energy required is the same for a particular atom or have different energy levels all energy of photon absorbed
What is meant by an exited atom
An electron has moved to a higher energy level
Describe the process by which mercury atoms become exited in a fluorescent tube
Current flows through the tube
The electrons collide with mercury atoms
Raising the electrons to a higher energy level in the mercury atoms
What is the purpose of the coating in a fluorescent tube
High energy photons are absorbed by the coating powder, this powder then emits photons in the visible spectrum
Incident photons have a variety of different wavelengths
Explain why fluorescent tube contains vapour at a low pressure
Mercury is a liquid at room temperature - in order for it to turn to a gaseous stable , the pressure must be kept low
6 marker - the photoelectric effect suggests that electromagnetic waves exhibit particle behaviour. Explain what the threshold frequency is and why it’s existence supports particle nature of electromagnetic waves
Threshold frequency - minimum frequency for emission of electrons
- if frequency below threshold frequency, no emission of electrons, even if intensity increases
- as energy of photon is less than work function
- wave theory cannot explain this as energy of wave increases with intensity
Ionisation may be caused by electron impact.
Explain how else might an atom be ionised ?
Impact by ionising radiation
What is ionisation energy
The minimum energy needed to move the electron from the atom completely from the ground state
Why do exited mercury atoms emit photons of characteristic frequencies
Mercury atoms have fixed energy levels
When electrons change levels they lose a specific amount of energy
Leading to photons of specific frequencies
What does the gradient of E=hd graph represent
Place constant
Explain the meaning of threshold frequency
Minimum photon frequency that will emit electrons from the metal surface
At a fixed frequency of incident radiation, the emitted electrons are collected and a current recorded - how can you increase this current ?
Increase brightness
Increases frequency of photons being emitted by light source
More photons being available to liberate electrons
Explain why no photoelectrons are emitted when a blue filter is replaced by a red filter
The photons energy for the red filter is lower than the work function of the metal so no electrons can be emitted
How does a fluorescent tube work
1) Contain mercury vapour, across which a high voltage is applied
2) this voltage accelerates fast moving free electrons that ionise some of the mercury atoms, producing more free electrons
3) when these collide with mercury atoms they are exited to higher energy levels
4) when these electrons return to their ground state they lose energy by emitting high-energy photons
5) phosphorus coating absorbs them and causes its electrons to emit visible light photons
Explain how the threshold frequency provides evidence against the wave theory of light
If light only acted as a wave , the electrons in the metal would slowly absorb incident light waves until they had enough energy to escape the metal - otherwise there would be no threshold frequency
The atom in ground state is given energy - state what happens to this energy
Electron in ground state will leave atom with the remaining energy
State what will happen to subsequent electrons when one leaves
Electrons in higher energy levels may fall down to replace vacancy left by emitted electrons
Give one example of particles acting as a wave
Diffraction - it can only be explained using waves. If light was acting as a particle , the light particles in the beam would either go through the gap
Explain why different frequencies are possible when electrons move back down to lower energy levels
Electrons may cascade down to lower levels through different pathways
Give an example of a use of a superconductor
bullet trains/powerful magnets
Why are energy levels always negative
It’s because the electrons are “bound” to the nucleus, and so you have to put energy in to move it up energy levels
