OCR Physics 7 (All)

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Observing the Sky

Earth year: time taken to orbit the sun once

Earth month: time taken for the moon to obit once

Sidreal day: time taken for distant stars to return to the same position in the sky (23hrs 56mins), the Earth spins 360°

Solar day: time taken for the sun to return to the same position in the sky (24hrs), the Earth spins over 360°

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The Moon

Phases of the moon

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 Lunar eclipse: Moon moves into the Earth's shadow preventing light from reaching the moon so the moon appears to disappear

Solar eclipse: the moon is the same angular size as the sun and so it is able to block the sun.

This happens infrequently because the moon orbits at an angle

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Celestial Sphere

Used to determine where a star is (it doesn't rotate with the Earth)

Vernal equinox reference point Right ascention measures angle East from the vernal equinox Declination measures angle of the star above or below the celestial equator

Angles: (Degree = 3600 seconds) 60 seconds in a minute, 60 minutes in a degree, 360 degrees in a circle 


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Heliocentric Model

Heliocentric model Sun at the centre of the solar system

Geocentric model Earth is at the centre of the solar system

Evidence: Phases of Venus, retrograde motion of Mars (Earth surpasses Mars in it's orbit so Mars appears to move backwards in the sky)

(Angular size = conversion factor * width of image)

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Refraction Light changes direction when passing through a glass prism due to different speeds of wavelengths (light slows as it goes through dense materials)

Short wavelengths eg. violet change direction more because they slow down more


Wavespeed = frequency * wavelength

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Converging/Convex Lens

Lens with a slightly bent face which bends light to a focal point

Thick: short focal length, more powerful Thin: long focal length, less powerful

Focal length: distance from centre of the lens to focal point Focal point: where the parallel rays of light meet when passing through a lens 

Power (D) = 1 / Focal length


Rays of light that pass through the centre continue straight on. All rays converge at the focal point

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Mirror vs Lens

Lenses are heavy, which makes them difficult to move

It is also difficult to make lenses uniform

Lenses absorb some of the light radiation

Lenses produce chromatic aberration (different colours have different focal lengths)

Lenses only work well for visible light

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Eyepiece lens is more powerful (thicker) than the objective lens

Magnification = Fo / Fe  (See diagram above)

Most telescopes use a concave mirror


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Waves spread out (diffract) at the edges when they pass througha gap

Aperture: a hole in optics through which radiation travels (a gap)


Narrow gap/longer wavelength more deffractiojn

When: gap < wavelength, max. defraction ; gap = wavelength, little defraction ; gap > wavelength, no deffraction

High levels of deffraction produces blurry images and so telescopes should have as large an aperture as possible to produce sharp images

Diffraction grating: set of narrow evenly spaced parallel lines on a thin sheet of glass. Different colours emerge at different angles to produce several spectra

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Astronomical Distances

Parallax method The apparent movement of close stars against the background of stars

(http://www.schoolsobservatory.org.uk/sites/default/files/astro/paraerth.jpg)When d = 1 parsec ; p = one second

However: beyond 300 parsecs the angle is too small to measure and so can't measure the distance to most stars

Brightness method Further away stars appear dimmer but this assumes all stars have the same luminosity (temperature and size)

Cepheid variable stars are stars which vary in brightness those with higher luminosity have longer periods. They can  be used in the brightness method by comparing stars with the same apparent brightness to the luminosity.

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Scale of the Universe (Curtis vs Shapley)

Curtis Milky way is one of many galaxies (spiral nebulae = other galaxies)our galaxy is only 10 000 parsecs across with our sun close to the centre of the universe

Shapley Universe is one large galaxy (spiral nebulae = gas clouds) about 100 000 parsecs across with our sun and solar system far from the centre of the galaxy

Hubble Helped to solve Curtis/Shapley debate by proving objects were outside of our galaxy. Distant galaxies are moving away from us (further the galaxy the faster the movement) as shown by red shiftSuggests the universe began from one point which supports the Big Bang theory

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Scale of the Universe (Hubble's Law)

Hubble's law Recessional velocity = red shift Gradient = age of universe (estimate)

V = Hor

Velocity of galaxy Hubble's constant Distance to galaxy

1 / H= estimate for the age of the universe

Ho ≈ 2*10-18 


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Absolute zero = 0°K = -273°C

Kelvin to Celcius: +273

Celcius to Kelvin: -273

eg. 11°C = 284°K

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Gas Behaviour

P=Pressure  V=Volume  T=Temperature (in Kelvin)  k=constant

At kT : P * V = k

At kV : P / T = k

At kP : V / T = k

∝ 1/P



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Fusion: two nuclei join together to create a larger nucleus eg. in stars Hydrogen fuses to make Helium

Needs high temperatures and pressure to occur


Mass of the reactants > mass of the products

E = m c²  (Energy = mass * [speed of light]²)

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Star Spectra

Continuous Spectra

Hot objects emit radiation, they always emit more of some radiation than another this is called the peak frequency

If more energy is radiated the peak frequency is higher

Line Spectra

Electrons can move between energy levels when gaining/losing heat (lowest energy levels are close to the nucleus)

Absorption spectra: at high temperatures electrons move to higher energy levels by absorbing radiation of a particular frequency. This is shown as the gaps in the continuous spectrum

Emission spectra: in higher energy levels, electrons fall towards the nucleus (lower levels) emitting radiation of a particular frequency. This is shown as a series of bright lines. 

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Spectra can be used to work out the components of a star because energy levels in atoms are different for each element

Photosphere (surface of a star) emits a continuous spectrum, this passes through the star's gases which produes emission and absorption spectrum

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Stars begin as clouds of dust and gas, gravity causes denser regions to clump, the cloud then breaks up into protostars and they continue to collapse under gravity (smaller volume) which causes pressure and temperature to increase.

Temperature will eventually increase to be hot enough for fusion to occur releasing energy and preventing gravitational collapse. The star is now in the main sequence.

Fusion happens in the core of stars. The energy produced is transported to the photoshere (surface) by photons of radiation and convection currents.

Once all of the hydrogen at the centre of the star is used up, it is no longer in the main sequence. It grows to become a red giant or supergiant star. The surface cools.

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Life Cycle of Stars

1. Hydrogen runs out, sore shrinks, rest of the star expands, photosphere cools

2. Core shrinks again until pressure and temperature and pressure are so high helium fusion begins creating eg. carbon nitrogen and oxygen

3. Helium runs out so core is unstable and is compressed by the rest of the star

4. In a red giant no more fusion occurs  (not enough mass), core shrinks to become a white dwarf star these gradually cool and fade

5. Red supergiants continue to fuse heavier elements until the majority of the core is iron

6. The core then collapses and the star explodes as a supernova

7. The core collapses again to become a neuton star or a black hole

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Life Cycle of Stars


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Hertzsprung-Russell Diagram


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Space Telescopes

Our atmosphere only allows certain wavelengths of EM radiation through, light is refracted by water and absorbed by dust so telescopes are placed where these problams are minimised.

Telescopes in space are not effected by the atmosphere, so this is where a number are (except radio telescopes which are too big)

Problems: difficult to repair, expensive

Telescope locations: remote locations (low pollution), high elevation eg. mountain top (thin atmosphere), dry location (water in atmostphere refracts)

The locations also need to take into account: cost, access, environment and social factors.

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