Physics P7 OCR 21st century

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P7 ­ Further Physics
P7.1 Naked eye astronomy
The Sun rises in the east and sets in the west, moving across the sky during the day. It takes 24 hours
for the Sun to move from its highest point in the sky (at noon) back to its highest point the following
A sidereal day is the time it takes for the Earth to rotate 360° on its axis ­ it is the time taken for a
star to return to the same position in the sky ­ it's about 23 hours and 56 minutes
A solar day is the time taken for the Sun to appear at the same position in the sky ­ it's 24 hours
Solar and sidereal days are different because the Earth orbits the sun as well as spinning on its axis
The Earth orbits the Sun in the same
direction as it spins ­ so the Earth needs to
spin slightly more than 360° before the
sun appears at the same position in the sky
Distant starts and the Sun are directly
over an observers head
After 360° distant stars are over the
observers head again
But it takes till this position for the Sun to
be overhead again
The moon seems to go more slowly than
the Sun, taking about 25 hours to appear
at the same position in the sky
This is because the Moon orbits the Earth in the Sun in the same direction as the Earth is rotating
As the Earth moves around the Sun, the direction we face changes slightly each day - this means we
can see a slightly different patch of the sky each night - i.e. different stars
An Earth year is the time takes it takes the Earth to orbit the Sun once, so on the same day each year
you should be able to see the same stars in the night sky.
The Moon's appearance changes during its 28 say orbit of the Earth ­ this is called the lunar cycle
The moon doesn't glow itself ­ it only reflects light from the Sun ­ only half facing the sun is lit up
leaving the other half in shadow
As the Moon orbits the Earth we see different amounts of the Moon's dark and lit up surfaces
When the whole of the lit-up surface is facing the Earth we see a full moon and a new moon when
the dark half faces up

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Solar eclipse ­ A sloar eclipse happens when the
moon is (purely by chance) just the right size and
distance away that when it passes between the Sun
and the Earth an can block out the Sun.…read more

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Eclipses don't happen very often ­ the moon orbits the Earth at an angle to Earth's orbit around the
Sun so most of the time the Sun, Moon and Earth don't line up to cause eclipses
Partial eclipses happen a bit more often as they don't have to line up perfectly for this.
Mercury, Venus, Mars, Saturn and Jupiter are all planets that can be seen with the naked-eye ­ to the
naked eye, the planets look a lot like stars.…read more

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A converging lens uses refraction to bend the light rays coming out of it inwards ­ this has the effect
of focusing the light to a point called the focal point
As a light ray hits the surface of the lens and passes from
air to glass it slows down ­ this causes the light ray to
bend towards the `normal'
When it hits the glass to air boundary on the other side it
speeds up and bends away from the normal
The curvature of…read more

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Some objects in the sky are so distant and faint, only a tiny amount of radiation from them reaches us
­ to collect enough of the radiation from these objects to see them you need a telescope with a
huge objective lens (or mirror) ­ the diameter of the objective lens is called the aperture
The bigger the aperture the more radiation can get into the telescope and the better the image
When waves move through a narrow gap or past an obstacle, they…read more

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P7.3 Mapping the Universe
Parallax is an apparent change in position of an object against a distant background - it makes closer
stars appear to move relative to distant ones over the course of a year.…read more

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In the 1920s there was a debate about the size and structure of the Universe, led by two famous
American astronomers ­ Shapley and Curtis
Through telescopes, people had seen some faint, fuzzy objects they had called nebulae ­ some of
these objects looked spiral-shaped but some were just blobs ­ Shapley and Curtis argued about
what these nebulae were and where they were
Harlow Shapley's Argument Heber Curtis' Argument
Shapely believed the Universe was just one Curtis thought the universe was made up of…read more

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This suggests that the whole universe is expanding from a single point which supports the big bang
theory ( all the matter and energy in the Universe was compressed into a very small space)
Scientists believe the Universe began with a `big bang 14 thousand million years ago
P7.…read more

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If you increase the temperature of something, you give its particles more kinetic energy ­ they
move more quickly and vibrate more
The coldest that anything can ever get is -273°C ­ this temperature is known as absolute zero ­ at
absolute zero atoms have as little kinetic energy as it's possible to get
Absolute zero is the start of the Kelvin scale of temperature ­ To convert form °C to K add 273
P ressure ×V olume = C onstant
Pressure = C onstant…read more

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When a star starts nuclear fusion reactions it is called a main sequence star and it is stable ­
hydrogen is being converted via fusion into helium in the core.…read more


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