Quantum and Astrophysics Flashcards
Define electronvolt
One electronvolt is the energy gained by a single electron travelling through a potential difference of 1 volt
Define a photon
A photon is a quantum of energy if electromagnetic radiation.
What are features of a photon?
Zero rest mass
Zero charge
Travel at the speed of light in a vacuum
Not affected by electric or magnetic fields
What is the word equation for the photoelectric effect?
Energy of a single photon (hf) = minimum energy to escape (φ) + maximum kinetic energy of emitted electron (KEmax)
Define work function (φ)
The minimum energy required to remove an electron from the surface of a metal plate.
Define threshold frequency
The minimum frequency of electromagnetic radiation that will remove electrons from the surface of a metal plate.
How is energy carried by a ray of light?
By a stream of photons each with energy E=hf
When is a star on the main sequence?
When it is fusing hydrogen nuclei into helium nuclei.
Explain the relationship between time on the main sequence and mass of a star
A high mass star provides higher gravitational forces, the core is hotter and hydrogen is used up faster. A high mass star spends less time on the main sequence.
What will the sun evolve into?
From a main sequence star to a red giant to a white dwarf. It will spend a long time on the main sequence as it is a low mass star.
List the sun, a white dwarf and a red giant from most to least dense
White dwarf, sun, red giant
List the sun, a white dwarf and a red giant from having the most to least luminosity (power)
Red giant, sun, white dwarf
List the sun, a white dwarf and a red giant from the most to least intense surface (temperature at the surface)
White dwarf, sun, red giant
What is a black-body spectrum?
An object with a surface temperature above absolute zero, emits electromagnetic radiation. Its spectrum contains a range of wavelengths and intensities.
Define black-body for a star
A star absorbs all incident electromagnetic radiation and emits its own spectrum depending on its surface temperature.
Define luminosity
The total energy emitted per unit time.
What is one astronomical unit equal to?
1AU = 1.50x10^11m
How do electrons move between energy levels?
By gaining or emitting a fixed amount of energy.
Define ionisation energy
The amount of energy required to completely remove an electron from an atom.
Define excitation
When an electron moves from a lower to higher energy level. It receives the exact right amount of energy by absorbing a photon.
Define relaxation
An electron in an excited state will soon return to lower energy levels by emitting a photon so it loses energy.
Define continuous spectra
A continuous spectra contains all colours, wavelengths and frequencies.
How is a continuous spectra made?
By viewing a white light source through a diffraction grating.
Define an emission line spectrum
A series of narrow, bright, coloured lines in a dark background.
How is an emission line spectrum made?
Each line in the spectrum is due to photon emission as electrons move to lower energy levels. The gaps between the energy levels are fixed so the photons have fixed energy, frequency and wavelength.
Define an absorption line spectrum
A series of narrow dark lines on a continuous spectrum background.
Why does each element have its own unique line spectrum?
Each element has its own unique line spectrum due to a unique set of energy levels.
How is an absorption line spectrum made?
A continuous spectrum from a star passes through a cool gas. The gas absorbs certain photon energies producing excitation of electrons. Each photon energy absorbed corresponds to a gap between energy levels, so the element in the gas can be identified.
Define the Doppler Effect
The observed change in wavelength due to relative motion between a source and observer.
State Hubble’s Law
The speed of recession of distant galaxies is directly proportional to their distance away.
What is a feature of cosmic microwave background radiation?
It is virtually uniform in all directions.
Define CMBR
Photons from the Big Bang, originally gamma photons, with their wavelengths stretched due to expansion and energy reduced due to cooling.
Define the Cosmological Principle
On a large scale, the universe is homogenous and isotopic with the universal laws of physics.
Define homogenous
The universe has uniform density on the large scale.
Define isotopic
The universe looks the same in all directions on a large scale.
What is the equation for current due to the photoelectric effect?
I = number of photons emitted per second x e
Define stellar parallax
The apparent movement of a star due to movement of the observer’s position.
Define a parsec
A star at a distance one parsec produces a parallax angle of 1 arc-second in a triangle of base one astronomical unit.
Define red shift
Distant galaxies have the same spectrum but each line is shifted to longer wavelengths at the red end of the spectrum. (Distant galaxy is moving away from the observer)
