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CGP PHYSICS > Chapter 12 - Our place in the universe > Flashcards

Chapter 12 - Our place in the universe Flashcards

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1
Q

What does our solar system consist of?

A

The sun and all objects which orbit it

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2
Q

What shape is the orbit of a comet?

A

Elliptical

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3
Q

How can radar be used to work out the distance to a stationary object in the solar system?

A

-A short pulse of radio waves (travelling at the speed of light) is sent out and bounces off the object
-The pulse returns and is detected after an interval of time
-The distance is worked out using speed x time
d = 1/2 ct
Half the time interval is used so it is just the time there and not there and back

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4
Q

How can radar be used to work out the relative velocity between the Earth and an object in the solar system?

A
  • Two radar pulses are reflected off an object with a known period of time between pulses
  • The distance of the object at the time of each pulse is found using d = 1/2 ct
  • Speed = distance/time is used with change in distance and change in time to find the relative velocity of the object
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5
Q

What assumptions need to be made for radar ranging?

A
  • The speed of light is always constant

- The time taken to reach the object and the time taken to return are the same

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6
Q

For what objects can radar ranging be used?

A

Objects within the solar system

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7
Q

For what relative velocities can radar ranging be used?

A

Velocities much less than the speed of light

otherwise there will be relativistic effects

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8
Q

What two things does the brightness of a star depend on?

A

The luminosity and the distance from Earth

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9
Q

What is luminosity?

A

How much light energy is given out in a given time period

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10
Q

What is the term for how bright a star looks when seen from Earth?
What does this depend on?

A

Apparent magnitude

The absolute magnitude and the distance from Earth

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11
Q

For what kind of objects can you use brightness to work out distance?

A

Standard candles

Objects whose brightness can be calculated directly

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12
Q

What is a standard candle?

A

An object whose brightness can be calculated directly

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13
Q

Give an example of a standard candle?

A

Cepheid variable stars

their brightness changes with a known pattern

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14
Q

What units are often used to measure distances within the solar system?

A

Astronomical units

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15
Q

What is the symbol for astronomical units?

A

AU

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16
Q

What is an astronomical unit?

A

The mean distance from the Earth to the sun

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17
Q

What is the symbol for a light year?

A

ly

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18
Q

What is a light year?

A

The distance that electromagnetic waves would travel through a vacuum in one year

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19
Q

When doppler shift happen?

A

When there is a relative velocity between an observer and a source of waves

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20
Q

What happens to the frequency and wavelength when a source of radiation moves towards you?

A

Higher frequency, lower wavelength

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21
Q

What happens to the frequency and wavelength when a source of radiation moves away from you?

A

Lower frequency, higher wavelength

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22
Q

What is pitch?

A

Frequency

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23
Q

What is the formula which relates speed, wavelength and frequency?

A 
v = λ f
speed = wavelength x frequency
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24
Q

How can Doppler shift be used to measure the velocity of an object relative to Earth?

A

The spectrum of the radiation emitted by the object is compared to radiation emitted by the same atoms on Earth
The amount by which the spectrum is shifted gives the doppler shift and so the velocity can be calculated

25
Q

What is the equation for finding velocity from a doppler shift?
When can this equation be used?

A

v/c = Δλ/ λ

when v

26
Q

When does redshift happen?

A

When an object is moving away from the Earth

27
Q

When does blueshift happen?

A

When an object is moving towards the Earth

28
Q

What is a star’s absorption spectrum?

A

Radiation is emitted in a continuous spectrum, but atoms in the atmosphere of the star absorb specific wavelengths, depending on what kind of atoms they are.
This produces dark absorption lines in the spectrum.
There is a characteristic pattern for each atom and so the composition of the stellar atmosphere can be found

29
Q

At what speeds does time dilation happen?

A

Close to the speed of light

30
Q

What happens to time for two objects moving relative to each other?
What is this effect called?

A

Time will run at different speeds for the two different objects
(but this is only noticeable when the relative velocity is close to the speed of light)
RELATIVISTIC TIME DILATION

31
Q

What is the name for the time interval measured by a stationary observer?

A

Proper time/wristwatch time

t0 or Τ (tau)

32
Q

If there is a stationary observer and an observer moving at close to the speed of light, who will measure the longest time interval for the moving observer?

A

The person moving at close to the speed of light

33
Q

Why can the effects of relativistic time dilation be ignored when v

A

The effects aren’t noticeable because γ is very close to 1

34
Q

What is the equation for relativistic time dilation?

A

t = t0 x γ

35
Q

What is the symbol for the relativistic/gamma factor?

A

γ (gamma)

36
Q

What is the relativistic factor?

A

γ = 1 / √(1 - v^2/c^2)

37
Q

What is the recessional velocity?

A

How fast a galaxy is moving away from Earth

38
Q

What is recessional velocity proportional to?

A

Distance from Earth

39
Q

What is Hubble’s law?

A 
v = H0 x d
(H0 = hubble's constant)
40
Q

What is the symbol for Hubble’s constant?

A

H0

41
Q

What are the units of v, recessional velocity?

A

kms^-1

ms^-1 in SI units

42
Q

What are the units of d, distance from Earth?

A

Mpc (megaparsecs)

m in SI units

43
Q

What is the symbol for a megaparsec?

A

Mpc

44
Q

How many metres are there in one megaparsec?

A

3.09 x10^22

45
Q

What are the units of H0, the hubble constant?

A

kms^1Mpc^-1

46
Q

What are the SI units of the hubble constant?

A

s^-1

47
Q

What value does the rate of expansion of the universe depend on?

A

Hubble’s constant

48
Q

Is the universe expanding uniformly?

A

yes

49
Q

What is the difference between normal redshift and cosmological redshift?

A

Cosmological redshift is due to the expansion of space which radiation is moving through (as the universe expands), not due to a relative velocity

50
Q

If the universe has been expanding at the same rate for its whole life, how old is it?

A

t = 1 / H0

51
Q

Why can the exact age of the universe not be worked out?

A

The exact value of the Hubble constant is not known

52
Q

Approximately how old is the universe?

A

13 billion years old

53
Q

What is the limit on the size of the observable universe?

A

A sphere with radius (age of the universe) light years

54
Q

What is the hot big bang (HBB) theory?

A

The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) which has been expanding ever since

55
Q

What is cosmic background radiation?

A

Electromagnetic radiation produced in the very early universe which can still be detected
It has been redshifted to the microwave region and is uniform in every direction

56
Q

What are the properties of cosmic background radiation?

A
  • continuous spectrum corresponds to temperature 2.7K
  • isotropic and homogeneous (the same intensity in every direction)
  • Microwave
57
Q

What causes tiny fluctuations in the temperature of cosmic background radiation?
What are they needed for?

A

(very small) energy density variations in the early universe

Needed for initial ‘seeding’ of galaxy formation

58
Q

How does the hot big bang model explain the large abundance of helium in the universe?

A

The hot dense universe must have been hot enough for hydrogen fusion to produce helium

CGP PHYSICS (8 decks)

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