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A2 Physics > Topic 11 > Flashcards

Topic 11 Flashcards

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1
Q

Definition: Specific heat capacity

A

The energy needed to raise the temperature of 1kg of material by 1K.

(J kg⁻¹ K⁻¹)

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2
Q

Equation: Specific heat capacity *

A

ΔE=mcΔθ

ΔE = change in energy (J)

m = mass (kg)

c = specific heat capacity (J kg-1 K-1)

Δθ = change in temperature (K)

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3
Q

Definition: Latent heat

A

The energy supplied to change the state of a material with no change in temperature.

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4
Q

Definition: Specific latent heat

A

The energy required to change the state of one kilogram of a substance at a constant temperature.

(J kg⁻¹)

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5
Q

Equation: Specific latent heat *

A

ΔE=LΔm

ΔE = energy supplied (J)

L = specific latent heat (Jkg-1)

Δm = change in mass (kg)

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6
Q

What is vaporisation?

A

When a material boils/condenses.

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7
Q

What is fusion?

A

When a material melts/freezes.

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8
Q

What happens to the energy of atoms when a material melts/evaporates?

A

Kinetic energy increases but potential energy remains constant.

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9
Q

What happens to the energy of atoms when a material condenses/freezes?

A

Kinetic energy decreases but potential energy remains constant.

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10
Q

What happens to the energy of atoms in a material when its temperature increases without the material changing state?

A

Kinetic energy remains constant but potential energy increases.

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11
Q

What happens to the energy of atoms in a material when its temperature decreases without the material changing state?

A

Kinetic energy remains constant but potential energy decreases.

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12
Q

How do you calculate a rate of change of energy/temperature in the ΔE=mcΔθ equation?

A

Use ΔE=mcΔθ by considering the changes in one second (also knows as power).

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13
Q

What assumption do you make when using ΔE=mcΔθ?

A

That all of the energy is transferred into the material being heated and none is wasted.

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14
Q

What is meant by the internal energy of a liquid?

A

The sum of the kinetic energy and potential energy of its atoms.

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15
Q

What does Boyle’s Law state?

A

p ∝ 1/V

The pressure of a gas is inversely proportional to its volume (at constant temperatures).

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16
Q

What does Charles’ Law state?

A

V ∝ T

The volume of a gas is proportional to its temperature /K (at constant pressures).

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17
Q

What does Gas-Lussac’s Law state?

A

p ∝ T

The pressure of a gas is proportional to its temperature /K (at constant volumes)

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18
Q

How do you convert from degrees Celsius to Kelvin?

A

°c + 273 = K

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19
Q

Definition: Pressure

A

The force per unit area exerted on an object.

(Pa)

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20
Q

What is absolute zero?

A

When the temperature is 0K, causing the particles of an ideal gas to have no kinetic energy.

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21
Q

Equation: Ideal Gas Law

A

pV = NkT

p = pressure (Pa)

V = volume (m3)

N = number of molecules

k = boltzmann constant

T = temperature (K)

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22
Q

What assumptions do you make when using the ideal gas Law?

A

1) Gas behaves as an ideal gas.
2) Mass / number of molecules remains constant.
3) Temperature remains constant.

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23
Q

What is the relationship between degrees Celsius and Kelvin?

A

Δ°c = ΔK

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24
Q

How do you calculate a change in one of the variables of the equation pV=NkT?

A

Work out the initial and final values for that variable, and find the different between the two.

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25
Q

How do you ensure results for the temperature of water are accurate?

A

Allow time for thermal equilibrium to occur.

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26
Q

How do you prove a relationship of proportionality is correct from a graph?

A

Use the graph to find k at different values to show it doesn’t change.

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27
Q

Definition: Internal energy

A

The sum of the kinetic energy and potential energy of all of the molecules in a gas.

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28
Q

What is an ideal gas?

A

A gas in which the molecules only have kinetic energy, and no potential energy.

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29
Q

How is pressure increased inside a container?

A

1) Volume decreases (or) molecules gain more kinetic energy.
2) This causes more collisions between the molecules and the wall of the container.
3) Therefore there is a greater rate of change of momentum.
4) This causes a larger force to be exerted in the walls of the container.
5) Pressure increases.

30
Q

How is internal energy increased?

A

Heat the body.

Do work on the body.

31
Q

How do you work out the internal energy of a gas?

A

Find the sum of kinetic energy of all of the molecules in a gas.

32
Q

Equation: Mean kinetic energy {Ek} *

A

{Ek} = ½m{c²} = ³/₂kT

{Ek} = mean kinetic energy (J)

m = mass (kg)

{c2} = mean velocity2

k = Boltzmann constant

T = Temperature (K)

33
Q

What is the pressure of an ideal gas at 0K?

A

Zero because the mean kinetic energy of a molecule of an ideal gas is directly proportional temperature, so at 0K the molecules have no kinetic energy.

34
Q

Which temperature scale do you have to use when using specific heat apacity equations?

A

Kelvin

35
Q

What happens to black body radiation as you heat an object?

A

The intensity increases and the peak wavelength λmax decreases.

36
Q

What is an ideal black body radiator?

A

A black body which absorbs and emits all wavelengths.

37
Q

Equation: Wien’s Law *

A

λmax T = 2.898 x 10-3

λmax = peak wavelength (m)

T = temperature (K)

38
Q

Equation: Stefan-Boltzmann Law *

A

L = σAT4

L = luminosity (W)

σ = stefan-boltzmann constant (5.67 x 10-8 Wm-2K-4)

A = surface area (m2)

T = temperature (K)

39
Q

What does the graph of the black body spectrum look like?

A
40
Q

Equation: Inverse square law of intensity *

A

I = L /4πd2

I = Intensity (Wm-2)

L = luminosity (W)

d = distance (m)

41
Q

Definition: Standard candle

A

An object of known luminosity.

42
Q

What does the Hertzsprung-Russell diagram look like?

A

The higher up the main sequence a star is, the larger its mass.

43
Q

How can astronomers determine the luminosity of a star?

A
  1. Measure the intensity and distance of the star.
  2. Use L = 4πd2I
44
Q

How are standard candles used?

A
  1. Measure the brightness of the standard candle from Earth.
  2. Use the inverse square law to calculate the distance to the standard candle.
45
Q

Why does trigonometric parallax become unreliable if the star is far away?

A

The parallax angle becomes too small to measure.

46
Q

What is trigonometric parallax?

A

The change in position of a star against the background of more distance stars.

47
Q

How can you increase the precision of trigonometric parallax calculations?

A

Use a space telescope as this willl mean the light does not have to pass through the Earth’s atmosphere.

48
Q

How can the light emitted from a star be used to determine the temperature of a star?

A
  1. Determine the λmax from the light.
  2. Use Wein’s Law to determine the temperature.
49
Q

What does the λmax tell us about a star?

A

The colour that it will appear in the sky.

50
Q

What is the difference between luminosity and intensity?

A

Intensity is the brightness of light as it appears on Earth, whereas luminosity is measyred sat the surface of the star.

51
Q

Equation: Doppler shift law *

A

z = Δλ/λ = Δf/f = v/c

z = red shift

Δλ = change in wavelength (m)

λ = original wavelength (m)

Δf = change in frequency (m)

f = original frequency (m)

v = velocity (ms-1)

c = speed of light (3.00 x 108 ms-1)

52
Q

Equation: Hubble’s law *

A

v = Hod

v = recession velocity (ms-1)

Ho = hubble’s constant (s-1)

d = distance (m)

53
Q

How do you work out the age of the universe using the Hubble Law?

A

t = 1 / Ho

t = age of universe (s)

Ho = hubble constant (s-1)

54
Q

What are the two extreme conditions necessary for fusion in the core of a star?

A
  • Very high temperature
  • Very high density
55
Q

How is the main sequence lifetime of a star affected by its mass?

A
  1. Large mass stars have a higher gravitational force and hence a greater density in their core.
  2. This means the rate of fusion is higher and hydrogen is used up faster.
  3. Therefore the main sequence lifetime of a star is shorter.
56
Q

Why do stars eventually move off the main sequence?

A
  1. Eventually, it runs out of hydrogen in its core.
  2. The temperature of the core then falls and the gravitational force causes the core to contract.
  3. This releases energy, causing the star to expand, and its temperature to fall.
  4. Therefore, a red giant is formed.
57
Q

Why can some standard candles only be used to determine distances to nearby galaxies?

A

Because their brightness is too small to measure.

58
Q

How do you use trigonometric parallax to calculate distances to nearby stars?

A
  1. Use trigonometry to calculate the distance to the star using the diagram below.
  2. You need to know the radius of Earth’s orbit around the sun.
59
Q

How can you calculate the distance to more distant stars?

A
  • Standard candles
  • Cepheid variables
60
Q

What do we know about a galaxy if its light has been red-shifted?

A

It is moving away from us.

61
Q

What is the lifecycle of the sun?

A
  • Main sequence stage:

The Sun fuses hydrogen into helium. Then hydrogen fusion ceases, causing the core of the sun to cool and collapse.

  • Red giant stage:

The sun then expands and becomes a red giant and the core becomes hot enough to fuse helium. Helium fusion then ceases and the core collapses again.

  • White dwarf stage:

The temperature doesn’t rise enough for further fusion to begin and the star becomes a white dwarf.

62
Q

How do old star clusters differ from young star clusters?

A

Young star clusters:

  • Consist mainly of main sequence stars.

Old star clusters:

  • Less heavy main sequence stars.
  • Many red giants.
  • Some red giants have evolved into white dwarfs.
63
Q

How did hubble deduce that distant galaxies are receding?

A

The wavelength change is greater for further away galaxies. Using Doppler’s equation, we can prove that these galaxies therefore have a larger velocity and are moving away from us.

64
Q

What is a main sequence star?

A

One which fuses hydrogen in its core.

65
Q

How does microwave noise provide evidence for an expanding universe?

A
  1. This noise originates from the big bang and its wavelength is linked to the temperature of the universe.
  2. As the universe expands, its temperature decreases.
  3. Therefore, the wavelength of this noise decreases and therefore the frequency increases.
  4. This shows a red shift has occurred.
66
Q

Why are scientists uncertain about the ultimate fate of the universe?

A
  1. It is difficult to make accurate measurements of distances to galaxies, meaning the value for Hubble’s constant is uncertain.
  2. Existence of dark matter produces uncertainty of the value of the average density of the universe.
67
Q

What is dark matter?

A

Matter which has mass but doesn’t emit electro-magnetic radiation.

68
Q

What effect does dark matter have on the universe?

A

It increases the average density of the universe meaning it is less likely to be a closed universe.

69
Q

Why did astronomers propose the existence of dark matter?

A

Observations of galaxies indicated that they must contain more matter than could be seen.

70
Q

What is red-shift?

A

The increase in wavelength recieved from a source due to the fact that it is receding from the observor.

71
Q

How is red-shift used to determine the velocity of a galaxy relative to Earth?

A
  1. Measure the frequency/wavelength of the light from the galaxy.
  2. Compare this to the frequency/wavelength for a source on Earth.
  3. Use the Doppler formula.
72
Q

How does density affect the fate of the universe?

A
  • average density > critical density

Closed: the expansion would stop and the universe wuld contract again.

  • average density = critical density

Open: expansion rate would eventually decrease to zero, but not contract again.

  • average density < critical density

Flat: expansion continues forever.

A2 Physics (14 decks)

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