Quantum and Astrophysics

Question 1

Define electronvolt

Answer 1

One electronvolt is the energy gained by a single electron travelling through a potential difference of 1 volt

Question 2

Define a photon

Answer 2

A photon is a quantum of energy of electromagnetic radiation.

Question 3

What are features of a photon?

Answer 3

Zero rest mass
Zero charge
Travel at the speed of light in a vacuum
Not affected by electric or magnetic fields

Question 4

What is the word equation for the photoelectric effect?

Answer 4

Energy of a single photon (hf) = minimum energy to escape (φ) + maximum kinetic energy of emitted electron (KEmax)

Question 5

Define work function (φ)

Answer 5

The minimum energy required to remove an electron from the surface of a metal plate.

Question 6

Define threshold frequency

Answer 6

The minimum frequency of electromagnetic radiation that will remove electrons from the surface of a metal plate.

Question 7

How is energy carried by a ray of light?

Answer 7

By a stream of photons each with energy E=hf

Question 8

When is a star on the main sequence?

Answer 8

When it is fusing hydrogen nuclei into helium nuclei.

Question 9

Explain the relationship between time on the main sequence and mass of a star

Answer 9

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.

Question 10

What will the sun evolve into?

Answer 10

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.

Question 11

List the sun, a white dwarf and a red giant from most to least dense

Answer 11

White dwarf, sun, red giant

Question 12

List the sun, a white dwarf and a red giant from having the most to least luminosity (power)

Answer 12

Red giant, sun, white dwarf

Question 13

List the sun, a white dwarf and a red giant from the most to least intense surface (temperature at the surface)

Answer 13

White dwarf, sun, red giant

Question 14

What is a black-body spectrum?

Answer 14

An object with a surface temperature above absolute zero, emits electromagnetic radiation. Its spectrum contains a range of wavelengths and intensities.

Question 15

Define black-body for a star

Answer 15

A star absorbs all incident electromagnetic radiation and emits its own spectrum depending on its surface temperature.

Question 16

Define luminosity

Answer 16

The total energy emitted per unit time.

Question 17

What is one astronomical unit equal to?

Answer 17

1AU = 1.50x10^11m

Question 18

How do electrons move between energy levels?

Answer 18

By gaining or emitting a fixed amount of energy.

Question 19

Define ionisation energy

Answer 19

The amount of energy required to completely remove an electron from an atom.

Question 20

Define excitation

Answer 20

When an electron moves from a lower to higher energy level. It receives the exact right amount of energy by absorbing a photon.

Question 21

Define relaxation

Answer 21

An electron in an excited state will soon return to lower energy levels by emitting a photon so it loses energy.

Question 22

Define continuous spectra

Answer 22

A continuous spectra contains all colours, wavelengths and frequencies.

Question 23

How is a continuous spectra made?

Answer 23

By viewing a white light source through a diffraction grating.

Question 24

Define an emission line spectrum

Answer 24

A series of narrow, bright, coloured lines on a dark background.

Question 25

How is an emission line spectrum made?

Answer 25

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.

Question 26

Define an absorption line spectrum

Answer 26

A series of narrow dark lines on a continuous spectrum background.

Question 27

Why does each element have its own unique line spectrum?

Answer 27

Each element has its own unique line spectrum due to a unique set of energy levels.

Question 28

How is an absorption line spectrum made?

Answer 28

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.

Question 29

Define the Doppler Effect

Answer 29

The observed change in wavelength due to relative motion between a source and observer.

Question 30

State Hubble’s Law

Answer 30

The speed of recession of distant galaxies is directly proportional to their distance away.

Question 31

What is a feature of cosmic microwave background radiation?

Answer 31

It is virtually uniform in all directions.

Question 32

Define CMBR

Answer 32

Photons from the Big Bang, originally gamma photons, with their wavelengths stretched due to expansion and energy reduced due to cooling.

Question 33

Define the Cosmological Principle

Answer 33

On a large scale, the universe is homogenous and isotopic with the universal laws of physics.

Question 34

Define homogenous

Answer 34

The universe has uniform density on the large scale.

Question 35

Define isotopic

Answer 35

The universe looks the same in all directions on a large scale.

Question 36

What is the equation for current due to the photoelectric effect?

Answer 36

I = number of photons emitted per second x e

Question 37

Define stellar parallax

Answer 37

The apparent movement of a star due to movement of the observer’s position.

Question 38

Define a parsec

Answer 38

A star at a distance one parsec produces a parallax angle of 1 arc-second in a triangle of base one astronomical unit.

Question 39

Define red shift

Answer 39

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)

Question 40

Why does the KE of emitted electrons range from 0 to max?

Answer 40

Free electrons lose kinetic energy due to collisions as they escape from the metal.

Question 41

When does a main sequence star evolve?

Answer 41

When hydrogen in the core runs out.

Question 42

What is electron degeneracy pressure in a white dwarf?

Answer 42

A quantum effect preventing all electrons in the core from occupying the lowest energy levels at once.

Question 43

What is the Chandrasekhar limit?

Answer 43

A white dwarf has a maximum mass of 1.44 solar masses

Question 44

How is stability maintained by a white dwarf?

Answer 44

By electron degeneracy pressure.

Question 45

What is 1 arc-second equal to?

Answer 45

1/3600 of a degree

Question 46

What is the ground state energy level?

Answer 46

n=1

Question 47

If n=3.99 what is nmax?

Answer 47

n=3

Question 48

What do the letters in the equation dsinθ = nλ represent?

Answer 48

d - slit separation n - maximum order number λ - wavelength of monochromatic light

Question 49

What is the difference between red shift and blue shift?

Answer 49

Red shift - wavelength become stretched as the source is moving away from the observer. Blue shift - wavelengths become compressed as the source is moving towards the observer.

Question 50

Δ λ/λ = Δf/f = v/c what is λ?

Answer 50

λ - the actual wavelength of the spectral line in a laboratory

Question 51

What is the composition of the universe?

Answer 51

70% dark energy 25% dark matter 5% ordinary matter

Question 52

What is 1 solar mass equal to?

Answer 52

1.99x10^30 kg

Question 53

When can stars evolve into supergiants?

Answer 53

If their mass is greater than 8 solar masses.

Question 54

What is an equation for intensity of electromagnetic radiation of incident light on a metal plate?

Answer 54

I = (hf x N) / A N - number of photons per second