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OCR Physics B Unit 4 > 4.3 Our Place in the Universe > Flashcards

4.3 Our Place in the Universe Flashcards

1
Q

Name the planets of our solar system, in order.

A

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

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2
Q

What are comets made of? Describe their orbit.

A

Rock, ice and dust. Elliptical.

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3
Q

Explain how you would use radar to measure large distances from the Earth.

A

Send a short pulse of radio waves to be reflected back to Earth. Measure the time between emission and detection, t and use the equation 2d = c t to find d.

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4
Q

How can you measure the average speed of distant objects using radar?

A

Send two pulses repeated by a time interval. Find the difference in distance between the intervals and use it to calculate average speed.

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5
Q

State two assumptions made when using radar to measure large distances.

A

The radio waves are always travelling at c and the time taken to reach the object is equal to the time taken to return. (Also no relativistic effects)

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6
Q

Name two factors that affect the brightness of a star.

A

Its luminosity (energy given out in a given time) and its distance from us.

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7
Q

Give two factors that affect how bright a star appears to us.

A

Absolute magnitude and how far away it is.

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8
Q

What is apparent magnitude?

A

How bright a star appears from Earth.

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9
Q

Give the name and an example of objects that you can calculate the brightness of directly.

A

Standard candles, for example Cepheid variable stars.

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10
Q

Why are Cepheid variable stars standard candles?

A

Because their brightness changes in a predictable pattern.

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11
Q

What is the definition of an astronomical unit, AU?

A

The mean distance between the Earth and the Sun.

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12
Q

How many metres are there in an astronomical unit?

A

1.50 x 10^11

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13
Q

What is the definition of a light-year?

A

The distance that electromagnetic waves travel though a vacuum in one year.

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14
Q

How many AU in one light-year?

A

63000

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15
Q

How many metres are there in one light-year?

A

9.46 x 10^15

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16
Q

What is the distance between the Sun and the nearest star, Proxima Centauri in light-years?

A

4.2ly

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17
Q

What is the width of the Milky Way in light-years?

A

100,000ly

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18
Q

How can you calculate the size of the observable universe?

A

Multiply the age of the universe by the speed of light.

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19
Q

When a car is moving away from you, what happens to the sound waves and therefore the frequency of the sound heard from the car?

A

They are stretched out, giving a longer wavelength and there a lower frequency when they reach you.

20
Q

How does speed of the emitter change the sound wave being observed?

A

The greater the speed, the greater the change in wavelength.

21
Q

Explain the principle that allows us to use the Doppler effect to measure the speed of distant objects.

A

Atoms will emit and absorb radiation with same characteristic spectrum wherever they are. You can measure how much it has been shifted to find the object’s speed.

22
Q

When is an object’s emission spectrum redshifted?

A

When it is moving away from the observer.

23
Q

When is an object’s emission spectrum blueshifted?

A

When it is moving towards the observer.

24
Q

Give the equation used to find the velocity of an object from the radiation it emits.

A

v / c = ∆λ / λ

25
Q

Where is a continuous spectrum of radiation emitted from?

A

A very hot region of a star called the photosphere.

26
Q

Why are characteristic absorption spectra created by each atom?

A

Because every atom absorbs different parts of the spectrum.

27
Q

How can you use the pattern of absorption lines from a star to find its speed?

A

Record an absorption spectrum in a lab and compare the difference in wavelength.

28
Q

What is time dilation?

A

Time runs at different speeds for observers moving relative to each other. This is only noticeable at speeds close to c.

29
Q

What is the equation for “stopwatch time”, τ in terms of time measured by a moving observer, t?

A

t = 𝛾 τ

30
Q

Give the equation used to find the relativistic factor, 𝛾.

A

𝛾 = 1 / √(1 - (v^2 / c^2))

31
Q

What is recessional velocity proportional to?

A

Distance from Earth/

32
Q

The more redshifted light from a distance star is…

A

The faster the star is moving away from us and the further it is away from us.

33
Q

What is Hubble’s law?

A

v = H0 d (v is in km s^-1, d is in Mpc and H0 is in km s^-1 Mpc^-1)

34
Q

How many metres in one Mpc?

A

3.09 x 10^22

35
Q

What is the range of generally accepted values for Hubble’s constant, H0?

A

50 to 100 km s^-1 Mpc^-1

36
Q

What are the SI units for H0?

A

s^-1

37
Q

Why is Hubble’s law strong evidence for the Hot Big Bang Theory.

A

All objects are moving away from each other so they are likely to have originally come from one point.

38
Q

What is cosmological redshift caused by?

A

Space expanding and light waves being stretched along with it.

39
Q

What illusion is caused by the universe expanding uniformly?

A

The idea that the observer is at the centre of the universe.

40
Q

What is the age of the universe in terms of Hubble’s constant?

A

1 / H0

41
Q

What is the approximate age of the universe and the value of Hubble’s constant to go with it?

A

13.7 billion years. 75 km s^-1 Mpc^-1

42
Q

What is the Hot Big Gang Theory?

A

The theory that the universe started off very hot and dense and has since been expanding.

43
Q

How does the HBBT predict Cosmic Microwave Background Radiation?

A

It predicts that lots of EM radiation was produced in the very early universe. It should still be observed today, with it’s wavelengths redshifted to the microwave region.

44
Q

Give some properties of CMBR.

A

Continous spectrum corresponding to 2.73K. Isotropic and homogenous (same in every direction). Small fluctuations in temperature. Doppler shift caused by the Great Attractor.

45
Q

What are the tiny fluctuations in CMBR temperature caused by and what do they mean?

A

Tiny energy density variations which seeded galaxy formation.

46
Q

How does the HBB model account for the amount of helium in the universe?

A

The early universe was hot enough for hydrogen atoms to fuse.

OCR Physics B Unit 4 (5 decks)

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