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Physics > Reach for the Stars > Flashcards

Reach for the Stars Flashcards

1
Q

Light year

A

is the distance light travels in a vacuum in one year

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2
Q

1 astronomical unit (1AU)

A

the average distance between the Earth and the Sun which is 1.50x10^11m

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3
Q

Photosphere

A

a layer of extremely hot gases that emits most of the Sun’s electromagnetic radiation

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4
Q

1st method for measuring distances (only works for the Sun)

A

simple geometry can be used to measure distances to objects that are close enough to appear as a disc rather than just a point of light

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5
Q

What is the only star that appears as a disc to us?

A

the sun everything else is too far away

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6
Q

What is the second method for working out distances?

A

Using stellar parallax

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7
Q

What is parallax in relation to stars?

A

It’s when stars appear to move across the sky as the Earth rotates, the relative motion of the stars - one’s movement with respect to another seems to be zero, so we say constellations are fixed. You’d expect as the Earth moves around its orbit the nearer stars would move relative to the more distant background stars in much the same way nearby objects move more rapidly across the field of view compared to more distant objects

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8
Q

1 parsec (parallax second)

A

distance of an object that would have a parallax of one arcsecond when observed from Earth

1pc = 3.09x10^16m = 3.26 light years

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9
Q

Why does the Sun appear so much brighter than other stars when viewed from Earth?

A

Because it is much closer - as you move further away from the source of any radiation, the radiation becomes less intense, this is because the radiation is spreading out in all directions so the amount that will land on you will become less. Also how big a star appears depends on how far away it is

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10
Q

Intensity obeys an…

A

…inverse square law

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11
Q

How do we use parallax to measure the distance from the Earth to a star?

A

We know the distance between the Sun and Earth. Measure the angle 2x, the star appears to have moved 6 months apart to see how the star has moved relative to the ‘fixed’ background stars. The parallax angle is half this. Then using trigonometry we can work out the distance to the star. (Provided the angle is small and in radians otherwise the error is to big)

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12
Q

Intensity (energy flux)

A

the rate at which energy is transferred across a unit area perpendicular to the light beam

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13
Q

Luminosity

A

the rate at which the source radiates energy - the total energy it loses every second is its radiated power

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14
Q

Further away from the star…

A

the lower the intensity seen by the observer

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15
Q

Intensity =

A

total energy emitted from source per second / surface area of sphere

I = L / 4pid^2

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16
Q

Conditions for intensity equation

A

The radiation emitted from the star in one second will spread out so that it is shared evenly over the surface of the sphere. Travelling at a steady speed, if it is not absorbed or scattered en route the radiation will pass through the sphere in such a way that: ‘equation’

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17
Q

Black body radiation

A

radiation given off from a body because of its temperature (is determined by the thermal motion of its constituent atoms and molecules)

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18
Q

As objects get hotter they go from emitting infra-red (invisible) radiation to…

A

…red, orange, yellow and white light; the hotter the whiter. Also as the object gets hotter the intensity of radiation increases; the hotter the brighter

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19
Q

A better name for a black body is

A

ideal thermal radiator

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20
Q

What is an ideal thermal radiator?

A

an object whose radiation spectrum depends only on its temperature not its composition (could also be a perfect absorber - absorbs all the radiation that falls on it, none is reflected)

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21
Q

What does Wien’s law describe?

A

as the temperature of a black body is raised, the peak of its radiation spectrum shifts to shorter wavelengths

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22
Q

What is described by Stefan’s law?

A

raising the temperature also increases the amount of radiation emitted from the object’s surface, the total energy radiated each second from each square meter of surface is proportional to the fourth power of its absolute temperature

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23
Q

To find the luminosity of the object…

A

…multiply the emitted intensity by the object’s surface area

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24
Q

How do astronomers classify stars?

A

by their surface temperature

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25
Q

What do a Hertzsprung Russell diagrams compare?

A

luminosity against temperature

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26
Q

Where are main sequence stars found on a Hertzsprung Russell diagram?

A

diagonally from top left to bottom right - most amount of stars in a group

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27
Q

Where are red giant stars found on a Hertzsprung Russell diagram?

A

top right corner - they’re cool but very luminous, need to be large to emit this amount of radiation from their surface

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28
Q

Where are white dwarf stars found on a Hertzsprung Russell diagram?

A

bottom left - extremely hot but low luminosity

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29
Q

How can Hertzsprung Russell diagrams be used to find distance to stars?

A

by knowing temperature can find its luminosity and therefore use I = L / 4pid^2

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30
Q

Standard candles

A

objects of known luminosity (or can be reliably estimated)

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31
Q

Cepheid variables

A

variable stars that grow bright and dim in a regular cycle

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32
Q

Atomic number

A

number of protons and neutrons in the nucleus of an atom

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33
Q

Proton number

A

number of protons in an atom

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34
Q

Isotopes

A

atoms with the same number of protons but a different amount of neutrons - can be used to date rocks

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35
Q

Nucleons

A

protons and neutrons

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36
Q

Radioactive decay

A

general name for a processes in which nuclei rearrange themselves to become more stable by emitting radiation that has high enough energy to cause radiation (ionisation)

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37
Q

When will radioactive decay occur?

A

not all isotopes of an element are stable so they undergo decay to make them stable

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38
Q

What are the 3 types of radioactive emission?

A

alpha, beta, gamma

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39
Q

Transmutation

A

when the nucleus changes from one element to another

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40
Q

How must a decay reaction formula be balanced?

A
  • the sum of the lower numbers is unchanged, showing conservation of charge
  • the sum of the upper numbers is unchanged, showing conservation of nucleons
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41
Q

Beta minus decay

A

a neutron turns into a proton and an electron, the high speed electron is emitted from the nucleus - extremely high speed, so has enough energy to ionise atoms it encounters

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42
Q

Energy from radioactive decay

A

the release of energy that accompanies radioactive decay is vast compared with energies involved in chemical reactions

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43
Q

Beta plus decay

A

proton in the nucleus turns into a neutron and positron, the positron is emitted as well as a neutrino

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44
Q

Alpha decay

A

an alpha particle is emitted (2 protons and 2 neutrons)

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45
Q

Gamma decay

A

a nucleus becomes more stable by emitting a photon of gamma radiation –> very high energy EM radiation (no change in proton or neutron number just a loss of energy from the nucleus)

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46
Q

What do all alpha, beta and gamma emissions cause?

A

ionisation of air and other materials through which they pass

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47
Q

What must you remember to do when counting radiation from a source?

A

to remove the background radiation - subtract the background count rate

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48
Q

Background radiation

A

from naturally occurring rocks and cosmic rays, is very low level

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49
Q

How effective is an alpha particle at ionising?

A

very effective due to large mass and charge, they can readily interact with atoms and molecules causing ionisation - lose a lot of energy over a short distance so a short range in air

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50
Q

How effective is a beta particle at ionising?

A

Produce less ionisation than alpha over a given distance due to low mass and high speed, although are typically emitted with lower energies can travel much greater distances

51
Q

How effective is gamma radiation at ionising?

A

is uncharged and moves at the speed of light, relatively little ionisation so loses little energy when passing through matter and is very penetrating with a long range

52
Q

What are cloud chambers designed for?

A

to show the tracks of ionising radiation - radioactive emissions ionise air and this causes vapour to condense so vapour trails show particles tracks

53
Q

Features of radioactive decay

A
  • could happen at any moment, is spontaneous
  • it is independent of any external stimulus such as physical or chemical conditions
  • it is random, is impossible to predict when a particular nucleus will decay
54
Q

the rate of decay is also called

A

the activity

55
Q

What is the activity of a radioactive source measured in?

A

Becquerel, Bq - number of alphas, betas or gammas emitted per unit time

56
Q

Half life

A

time for the number of nuclei of that isotope/ radioactive isotopes/ counts to halve (some isotopes are very unstable and have short half-lives)

57
Q

What graphs does radioactive decay create?

A

an exponential - so can log it to get a straight line

58
Q

How is the Sun’s energy ‘made’?

A

by fusion of hydrogen –> helium –> bigger elements

59
Q

Fusion

A
  • 2 smaller nuclei come together to form a bigger nuclei releasing huge amounts of energy - pulling apart each reacting nucleus into its separate nucleons and reassembling them to make a single, larger nucleus - this involves a net release of energy
  • less energy needed to join daughter nuclei so energy released as heat
60
Q

Nucleon binding energy

A
  • the energy released when a nucleus is formed

- energy binding nucleons together, involves a release of energy

61
Q

What does binding nucleons together involve?

A

it involves a release of energy

62
Q

What is the binding energy per nucleon a measure of?

A

the stability of a nucleus - the greater the binding energy, the more difficult it is to break the nucleus apart

63
Q

Fission

A
  • the splitting of a very massive nucleus (usually unstable) into lighter fragments (2 less massive nuclei) and some neutrons - releases energy
  • daughter have less nuclei than parent so energy released
64
Q

In a fusion reactor what is used to keep the plasma within the reactor?

A

a magnetic field

65
Q

For a fusion reactor on Earth what is needed to get a hydrogen nuclei to fuse?

A

requires them to be moving close together at high speeds - this is because of the positive charge, unless they’re moving very fast the electrostatic repulsion ensures they never come close enough to react and the strong force to attract them

66
Q

What is required for fusion on Earth?

A

high temperatures and densities

67
Q

Einsteins’s equation

A

E = m x c^2

68
Q

Mass deficit or mass defect

A
  • loss of mass when associated with nuclear reactions

- the actual mass of an atom is different to the measured mass, the difference is the mass deficit

69
Q

When a nucleus is formed from nucleons what always arises?

A

a mass deficit

70
Q

For a given nucleus: mass deficit =

A

sum of masses of individual nucleons - mass of nucleus

binding energy = mass deficit x c^2

71
Q

Kepler’s laws are empirical

A

they describe how the planets move but they give no explanation of why they move like this - for that we need Newton’s work

72
Q

Kepler’s laws

A

1st - planets move in elliptical orbits
2nd - line between a planet and the Sun sweeps out equal areas in equal times
3rd - relates orbital radius to orbital period

73
Q

Newton’s great insight:

A

there is an attractive force of gravity between all masses no matter where they are in the Universe

74
Q

Force of gravity is proportional to

A

mass of each object and 1 over distance between them

75
Q

Gravitational force =

A

G x m1 x m2 / r^2

G - gravitational constant
m1 - mass of the first object
m2 - mass of the second objects
r^2 - distance between objects squared

76
Q

Gravitational field strength

A

gravitational force per unit mass

g = Fgrav / m –> close to Earth Fgrav is its weight

77
Q

Gravitational field

A

region in which a mass experiences a gravitational force

78
Q

On a spherical objects the gravitational field…

A

…in towards the object

79
Q

Gravitational potential

A

at a point is defined as the work done per unit mass against the gravitational field in bringing the mass from infinity to that point
Vgrav = -Gm/r

80
Q

Binary star

A

2 objects (stars) of comparable mass moving relative to a fixed point where they’re always on opposite sides

81
Q

Absolute zero

A

0Kelvin or -273 degrees celsius

82
Q

Pressure law

A

pressure directly proportional to temperature (constant volume)

83
Q

Charle’s law

A

volume directly proportional to temperature (constant pressure)

84
Q

Boyle’s law

A

pressure is inversely proportional to volume (constant temperature)

85
Q

An ideal gas is one…

A

…that exactly obeys the gas laws

86
Q

Ideal gas equation

A

pV = NkT

p - pressure
V - volume
N - number of particles
k - Boltzmann constant
T - temperature
87
Q

To find the (root) mean square speed

A

-square all the individual speeds
-sum all the squares
-divide by the total number of particles/ speeds
(-take the square root of the result)

88
Q

Internal energy

A

total of the kinetic and potential energy

89
Q

Specific heat capacity

A

amount of energy required to raise 1kg by 1 degrees celsius (or Kelvin)

90
Q

Redshift

A

The ratio of the change in wavelength as a fraction of the wavelength measured in the lab. Provided the wavelength change is small this is very nearly the same as the ratio of change in frequency

91
Q

Hubble’s law

A

v = H0 x d

z = H0 x d / c

92
Q

Open Universe

A

the Universe will continue expanding forever

93
Q

Closed Universe

A

the Universe will eventually stop expanding, contract and end up in a Big crunch or ‘Bounce’ and start expanding again

94
Q

Parallax

A

change in angular position of two stationary points seen by an observer due to the motion of the observer (shift of object with respect to background due to us moving)

95
Q

Where are supergiant stars found on a Hertzsprung Russell diagram?

A

top mid to right

96
Q

When do we use standard candles instead of parallax?

A

when they’re too far away, the parallax angle is too small to measure

97
Q

How can we determine the age of objects?

A

comparing the isotope composition

98
Q

What’s the difference between beta positive and beta negative decay?

A

for both the mass number stays the same
for negative - proton number increases by one
for positive - proton number decreases by one

99
Q

Alpha radiation

A
  • 2 protons and 2 neutrons
  • +2 charge
  • mass of 4
  • high ionising ability
  • low penetration, stopped by paper
  • is affected by electric and magnetic fields but not as much as beta
100
Q

Beta radiation

A
  • high speed electron (or positron)
  • -1 charge
  • can ionise but not that strong
  • stopped by foil, few metres in air
  • bent strongly by magnetic and electric fields
101
Q

Gamma radiation

A
  • electromagnetic wave
  • zero charge
  • zero mass
  • very weak ionising ability
  • reduced by lead or concrete
  • unaffected by magnetic or electric fields
102
Q

total binding energy =

A

nucleon binding energy x number of nucleons in nucleus

103
Q

How does fission work

A
  • normally plutonium/ uranium, neutron fired at unstable nucleus, this creates two new nuclei and more neutrons
  • its a chain reaction
  • gives off energy and neutrons
  • to control it boron rods are put in
  • creates nuclear waste which is hard to get rid of
104
Q

gravity goes both ways

A

-we’re attracted to it and its attracted to us

105
Q

eclipsing binary

A

as they orbit they will eclipse each other at some point so the brightness will vary

106
Q

2nd way binary stars show themselves is to observe spectrum of the star

A
  • as it moves towards/ away from us, the lines in its observed spectrum become doppler shifted
  • star recedes, lines shifted to longer wavelengths
  • star approaches shifted to shorter wavelengths
107
Q

Ideal gas assumptions

A
  • identical molecules in constant random motion
  • molecules never stop so collisions between molecules and sides, on average, are elastic (no KE lost)
  • because gases can be compressed easily the volume of the individual molecules is negligible compared to the volume they occupy when moving
  • therefore molecules are further apart, so we assume that there are no forces on the molecules except when they collide
  • the molecules have zero size and are identical
108
Q

Outer galaxies are moving…

A

…faster than inner ones

109
Q

Why is the Universe expanding?

A

because the space is expanding not the things in it moving

110
Q

What causes the shift in wavelength in galaxies?

A

the expansion of space itself

111
Q

How is Hubble’s law used to estimate distances?

A

by measuring redshift from galaxies that are too far away to have their distances measured by other methods

112
Q

Key facts about the Big Band

A
  • from a single point
  • expanded (explosion)
  • still expanding
113
Q

What is the steady state theory?

A

that the Universe always existed but is expanding

114
Q

CMBR - Cosmic Microwave Background Radiation

A
  • radiation left over from the Big Bang, is shifting to red end of EM spectrum, longer wavelengths
  • helps us age the Universe
115
Q

Why did the Big Bang theory win out over steady state?

A

Steady state provided competition for the origins of the Universe but the Big Bang theory won out due to more evidence supporting it

116
Q

What does the Big Bang tell us?

A
  • helps us age the Universe

- know how fast the Universe it expanding

117
Q

How old is the Universe?

A

14 billion years old

118
Q

What elements are normally used in fusion reactors?

A

deuterium and tritium

119
Q

What errors are there in measuring distances?

A
  • once over 30 million light years away, flux becomes too low and can’t be seen so Cepheid’s variables can’t be used
  • the greater the distance, the more difficult it is to find standard candles –> uncertainty in Hubble’s constant
120
Q

Electrostatic is a lot…

A

bigger than gravity

121
Q

Which force is more noticeable?

A

gravitational because it holds everything together - most matter tends to be made of oppositely charged particles which don’t need electrostatic forces

122
Q

Critical density

A

the Universe will keep expanding until a certain point then will stay that size

123
Q

Dark matter

A

is what’s holding matter in place but we can’t see it - is only a theory

Physics (12 decks)

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