physics aqa unit 3 notes

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Stars and space
The history of the universe:
The universe was once a hot ball of glowing matter that has expanded to be
cold and dark apart from the stars, the light we see from distant galaxies has
taken billions of years to reach us!
The universe expanded and became transparent as radiation passed through
the empty space between its atoms, there is now background radiation, at this
point it was a dark expanding cloud of hydrogen and helium.
The force of gravity is the attraction of objects with a mass, the force increases
as the size of the object does and decreases with distance, Gravity pulled
matter into denser parts to form gigantic clumps like stars and galaxies:
however gravity between two helium nuclei was unable to pull the nuclei
The galaxies:
Our galaxy is the Milky Way and thousands of millions of stars are collected in
them, they are all moving away from each other and us, there are regions of
empty space between galaxies, which would take light many years to cross.
New stars from when gravity pulls debris together
The sun radiates energy because of hydrogen fusion and the energy released
keeps the star hot, the force of gravity that makes the star contract is balanced
by the outward pressure o radiation from its core. These focuses stay together in
balance until most hydrogen nuclei have been fused
The life of average stars:
A star will eventually cool, becoming reder and expand to form a `red giant'
the core will contract and helium will fuse to form carbon and oxygen, after all the
helium has fused the star will contract and its outer layers will be lost, they look
like a disk which we call planetary nebula. The remaining core is dense and
very hot because It's a white dwarf, this will eventually cool into a black dwarf
The life of a huge star:
The star will cool and expand to become a red supergiant and the core will
contract and fusion in the core forms elements with larger nuclei, when fusion
reactions are complete the star will rapidly cool an contract, the outer layers

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The most dense neutron star is a black whole, even light can't escape their
gravitational fields but we know they're there as they attract gas from near by
stars and matter accelerated towards them gives out visible Xrays.
*Explosions scatter stars.
*The force of the collapse fuses small nuclei into larger one.
239Pu a half life of only 24,000 years. This means that any 239Pu in the
ground would have decayed away in the 4.5 billion year history of Earth.…read more

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Put more turns in the coil or use more cells
* Put an iron rod through the coils (iron is a soft magnet)
No force will act on a conductor carrying an electric current in a magnetic field
if the conductor is not at right angles to the magnetic field or cutting through
field lines
Reversing the rotation direction:
*reverse magnetic field or current
*swap magnets
Why does a coil turn?
When a coil has a current passed through it the force acts on each…read more

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EASIER to do it that way. Easier meaning that the electrical
connections to the coil can be direct without the added complexity of making
them to a moving coil.
For a bigger current in electro magnetic induction:
move the wire faster
use a stronger magnet
use more turns of wire
When the magnet spins (the cycle dynamo): an alternating PD is induced in the
coil because the magnetic field lines cut across the wires of the coil, this p.d.…read more

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The transformer:
An alternating current across the primary coil produces a changing magnetic
field, the filed lines cut through the secondary coil and this induces an alternating
voltage in the secondary coil, the core is laminated for efficiency, it cuts out
induced currents in the iron layers, we don't use DC as it would give no change
in the magnetic field so secondary coil voltage would be zero.
The higher the p.d.…read more

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Light and sound
We see in mirror due to the reflection of light, with a plane mirror the law of
reflection is simple the incident ray angle to the mirror will be equal to the
reflected way angle. Angle of incidence = angle of reflaction ( the angle between
the reflected ray and the normal.…read more

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The rules for this are:
1) A ray parallel to the axes reflects up then goes behind mirror to meet the
principle focus point
2) A ray will reflect at the axes (mirror centre) to meet the law of relection but the
virtual ray will form the `ray' behind the mirror in a straight line.
When light passes from one transparent medium to another it changes speed,
which causes it to change direction.…read more

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