Quantum Phenomena (topic 3) Flashcards
How do electrons inside a metal normally behave?
Move freely within the metal, colliding with each other and positive ions
Photoelectric effect (definition)
electron are emitted from the surface of a metal when electromagnetic radiation above a certain frequency is directed at the metal
What observations about the photoelectric effect suggest the particle nature of light?
- Only takes place above the threshold frequency - irrelevant of intensity
- Occurs without delay
Explanation of the photoelectric effect (Einstein)
Electron at the surface absorbs a single photon and gains energy equal to hf. If the energy received exceeds the work function then the electron can escape.
What happens to excess energy when an electron absorbs a photon (photoelectric effect)?
Becomes kinetic energy
Stopping potential (definition)
The mimimum electric potential needed to stop photoelectric emission by attracting electrons back to the surface (as plate becomes positively charged so potential difference between cathode and anode grows)
Maximum kinetic energy
= e x V(s)
which is charge of electron x stopping potential
What did max Planck suggest about atoms?
Their energy is quantised and only appears in multiples of h
What is the work function?
The minimum energy needed by a conduction electron to escape the metal’s surface when the metal is at zero potential (ground state)
What happens if an electron absorbs a photon with energy less that the work function?
It collides with other electrons and positive ions until the extra kinetic energy is lost
What is a vacuum photocell? How does it work?
Glass tube containing a metal plate (cathode) and smaller electrode (anode). EM radiation is emitted at the metal plate, which causes photoelectrons to be sent cathode > anode.
How is the number of electrons released measured in a vacuum photocell?
Microammeter - current is proportional to electrons per second transferred ( divide by charge of electron)
What is light intensity? How does it relate to current in a vacuum photocell?
Light intensity is a measure of energy per second carried by incident light, which is proportional to the photoelectric current and the number of photons per second emitted (as each electron absorbs one photon)
Why is intensity of light not relevant to maximum kinetic energy?
Each electron only absorbs one photon
On a graph of maximum kinetic energy against f, where is the threshold frequency?
X-intercept
Practical to measure ionisation energy for a gas atom
- electrons released from a heated filament to a positive metal plate, in a tube with gas (low pressure so electrons reach anode).
- p.d. is increased s that electrons get faster
- when electrons move fast enough to ionise gases, the current hugely increases as ionised electrons also reach anode
- work done (ionisation energy of gas atom) = charge x volts = e x tube potential difference
What is an electron volt?
The work done when an electron is moved through a p.d. of 1V (as E = QV and electrons have set charge)
How does excitation by collision work?
Using gas filled tubes with a metal grid between the filament and the anode, we can observe that gas atoms absorb energy from colliding electrons without being ionised at certain energies
Other than excitation, what happens to electrons in a gas filled cathode-anode tube?
- if a colliding electron does not have enough energy to cause excitation, it is deflected from the atom with no overall loss of E
- If the colliding electron loses all kinetic energy in the collision, the current due to flow of electrons is reduced
How can a gas filled tube be used to find excitation energies?
Finding the p.d where the current at the anode falls (as the colliding electrons dont have enough kinetic energy to reach anode). The p.d. is the same magnitude as the excitation energy in eV
Why is energy needed for excitation?
The electron moves further away from the positively charged nucleus
What is the ground state?
All electrons at the lowest possible energy levels (each shell only holds a certain number of electron shells, so ground state != all on lowest level)
Why does deexcitation happen?
an excited electron moves up a shell, which leaves a vacancy, which must be filled by an electron from an outer shell.
How does excitation by photon occur?
A photon of exactly the correct energy is absorbed by an electron, which can move up a shell if there is a vacancy.
What does it mean to fluoresce?
glow with visible light when absorbing UV radiation
How does fluorescence occur?
Atoms in the substance absorb UV photons and emit visible photons
How does a fluorescent tube light work?
- inner tube contains mercury at low pressure. Ionisation and excitation of the mercury atoms occur as they collide with each other and electrons from current in the tube
- Mercury atoms then emit UV photons (+ lower energy photons) when they deexcite
- UV photons are absorbed y atoms in the fluorescent coating, which emit visible photons when they deexcite
What are the two wavelengths either side of visible light?
- 400nm = violet
- 650nm = red
Why is a line spectra produced?
Photons of a specific energy are released, and energy is directly proportional to wavelength of a photon
What is the “dual nature” of light?
It can behave as a wave or as a particle, according to circumstance
What observations suggest light has a wave-like nature?
Diffraction, e.g. Young’s slits
(longer wavelengths diffract more)
What observations suggest light has a particle-like nature?
The photoelectric effect, which begins more or less immediately rather than building up energy from a wave
What observations suggest matter has a particle-like nature?
Electrons in a beam can be deflected by a magnetic field
What is the de Broglie wavelength?
The wavelength of matter which is equal to h/momentum
(hence wavelength can be changed by changing velocity)
What observations suggest matter has a wave-like nature?
When electrons in a vacuum tube are directed at a metal foil which acts as a diffraction grating due to its crystalline structure, they diffract and create a pattern of fluorescent rings
How is a beam of electrons created for the electron diffraction experiment? How can this be used to dictate de Broglie wavelength?
Electrons are attracted to a metal plate with a small hole in the centre. By increasing the p.d. between the emitting filament and the plate, the electrons get faster.
de Broglie wavelengthis proportional to momentum, which is dictated by the electron’s velocity.
