P2

How photocells work; (turn light energy into electrical energy)

-light energy absorbed by a photocell

-causes electrons to be knocked loose from silicon atoms in the crystal

-electrons flow freely in 1 direction producing a direct current

Output from a photocell depends on;

-light intensity   -surface area exposed     -distance from light source

Advantages and Disadvantages;

-robust (dont need much maintenance)    

-no fuel required/no cables             -no electricity produced when its dark/cloudy

-no pollution (no contribution to global warming)

-Sun is very hot and produces infrared radiation with a very short wavelength

-short wavelength passes through transparent glass 

-walls and floor absorb the radiation, warm up and re-emmit a longer wavelength

-Glass reflects the longer wavelength back inside the building 

Solar reflectors are moved by a computer to make sure they are always facing the sun.

Energy from the wind;

-Wind is a renewable form of energy but it does depend on the speed of the wind

-wind turbines dont work if theres no wind or if the speed isnt big enough

-wind farms dont contribute to global warming and they dont pollute the atmosphere but they can be noisy and take up a lot of space and people sometimes complain that they spoil the view

Current from a dynamo can be increased by;

-stronger magnet   -more turns on the coil     -rotating the magnet faster

Output from a dynamo can be displayed on an oscilloscope

Oscilloscope trace shows how the current produced by the dynamo varies with time

Time for one complete cycle is called the period of the alternating current

Power stations (in conventional power stations fuels are used to heat water)

-water boils to produce steam

-steam at high pressure turns a turbine

-turbine drives a generator

Simple generator consists of and coil of wire rotating between poles of a magnet

-coil cuts through magnetic field as it spins

-current is produced in the coil

Current can be produced if coild remains stationary and the magnet moves.

Generators at power stations work on the same principle.

Efficiency; measure of how well a device transfers energy

-Energy in a power statio is lost in the boilers, generator and cooling towers.

efficiency=useful energy output/total energy input

Infrared radiation is absorbed by the earths atmosphere

Carbon dioxide occurs naturally in the atmosphere as a result of;

-natural forest fires          -volcanic erupions        -respiration

-decay of dead plant and animal matter      -escaping from the oceans

Man made carbon dioxide is caused by buring fossil fuels, waste incineration, deforestation and cement manufacture. 

Water vapour=most significant greenhouse gas                                                                               -most water vapour occurs naturally                 -0.001% comes from human activity                     -half of the greenhouse effect is due to water vapour   -a quater is due to clouds

Methane is produced-organic matter decomposes in an environment lacking oxygen

natural sources=wetlands,termites and oceans

man-made sources= mining and burning fossil fuels, burying waste in landfills, rice paddies

-Electronmagnetic radiation from the Sun has a relatively short wavelength 

-Radiation absorbed by and warms Earth.

-Earth re-radiates energy as infrared radiation with a longer wavelength

-Longer wavelength radiation absorbed by greenhouse gases which warms the atmosphere

Dust in the atmosphere;

-smoke from factories reflects radiation from the town back to Earth=temperature rises

-ash cloud from a volcano reflects radiation from the Sun back into space=temperature falls

Most scientists agree that the evidence supports climate change. 

Average temperature has increased steadily during past 200 years.

Scientists dont agree on to which extent human activity has contributed to global warming

Series of transformers and power lines that transport electricity from the power station to the consumer

-transformers are used to step up the voltage to as high as 400 000 V.

High voltage= reduced energy loss, reduced distribution costs, cheaper electricity for consumers

Transformers then used to step down the voltage to a more suitable level for the consumer

Current passes through a wire=wire gets hot    

greater current=hotter wire

transformer increases voltag-current is reduced so there is less heating and therefore less energy is lost to the environment

-we pay less for electricity during the night when not as much is needed but it still has to be produced

electrical appliances usually display a power rating in watts(W) or kilowatts(kW)

power=voltage x current              energy supplied=power x time

unit of electrical energy used in the home is the kilowatt-hour (kWh)

cost of electricity used=energy used x cost per kWh

Some energy sources are more appropriate than other  in particular situations;

-availability    

-ease of extraction

-associated risks

-effect on the environment

Atoms contain same number of protons and electrons (they are neutral)

Ionisation involves gaining/losing electrons

-when atom gains electrons it becomes negatively charged

-when atom loses electrons it becomes positively charged

Electromagnetic radiation comes in tiny ‘packets’ called photons.                          

If the photons have enough energy, they can break molecules into bits called ions. This is called ionisation. These types of radiation are called ionising radiation.

The ions produced when ionising radiation breaks up molecules can take part in other chemical reactions. If these chemical reactions are in cells of your body, the cells can die or become cancerous. Protein molecules may change shape and strands of DNA may break or change. This is the reason that ionising radiation can be damaging to health.

-Alpha, beta and gamma radiations come from the nucleus of an atom                                          -Alpha radiation causes most ionisation and gamma causes the least                                                -Alpha radiation is short ranged and easily absorbed by aluminium                                                      -Beta radiation has a range of about 1m and is absorbed by few mm of aluminium                            -Gamma radiation is much more penetrating=thick sheet of lead will stop gamma passing

Experiments can be done to identify different types of radiation from its penetrating power- background radiation must be taken into account.

Uses; Thickness of paper (beta) Amount of radiation passing through the sheet is monitored and pressure on the rollers adjusted accordingly                                                                                    -Gamma radiation kills microbes/bacteria so it can be used for sterilising medical instruments and it can be used to check for leaks in pipes/welds                                                                       - The alpha radiation ionises the air particles inside the smoke detector. This allows a small electric current to flow. If there is a fire, smoke particles going into the detector are hit by alpha radiation. This reduces the ionisation of the air particles causing the current to drop. The drop in current is detected by the smoke detector, setting off the alarm.

-Plutonium is a waste product from nuclear reactors which can be used to make nuclear bombs

-Low level radioactive waste can be buried in landfill sites=high level waste is encased in glass and buried deep underground or reprocessed

-Radioactive waste can remain radioactive for 1000's of years so it must be stord where it cant leak into natural underground water supplies (then lakes and rivers) 

-It isnt suitable for making nuclear bombs but it could be used by terrorists to contaminate water supplies or areas of land

Advantages of nuclear power;

-Fossil fuel reserves arent used and no greenhouse gases are discharged into the atmosphere

Disadvantages of nuclear power;

-Very high maintenance and decommissioning costs and the risk of accidents (e.g. Chernobyl)

Asteroids; mini-planets orbiting the Sun (most orbit around mars and jupiter)                               -They are large rocks left over from the formation of the Solar System

-All bodies in space (even planets) formed when clouds of gas and dust collapsed together due to gravitational forces of attraction

-Mass of an object determines it gravitational force and asteroids have a low mass compared to mass of jupiter so jupiters gravitational force prevents asteroids from joining together to form another planet

Evidence for Asteroids;  

Geologists examine evidence to support the theory that asteroids have collided with Earth;  

-near a crater thought to have been from an asteroid impact they found quantities of metal iridium not normally found in the Earths crust but common in meteorites                                       -many fossils are found below the layer of iridium but few are found above it                                 -tsunamis have disturbed the fossil layers,carrying some fossil fragments up to 300km inland

-Scientist believe origin of the moon was the collision between two planets in the same orbit

-Iron core of the other planet melted and joined with the Earth's core

-less dense rocks began to orbit and they joined together to form our moon

Scientific evidence  which supports this ides;

-there is no iron in the moon

-earth density= 5500kg/m3 moon desity 3300kg/m3

-moon has exactly the same oxygen composition as the Earth but rocks on Mars and meteorites from other parts of the Solar System have different oxygen compositions

NEO; If the Earth and a NEO collided it could be the end of life on Earth 

-To avoid this one option would be to explode a rocket near to the NEO which could alter its course enough to miss Earth

-most comets pass inside the orbit of Mercury and well beyond the orbit of Pluto

-as the comet passes close to the Sun th ice melts and solar winds blow the dust into the comet's tail which always points away from the Sun

-scientists are constantly monitoring and plotting the paths of comets and other NEO's

-speed of a comet increases as it approaches the Sun and decreases as it gets further away; this is because of the changing gravitational attracting

-Comet=elliptical orbits (pass inside the orbit of Mercury and go out well beyond the orbit of Pluto

-Meteor made from grains of dust that burn up as they pass through the Earths atmosphere =they heat air around them which glows and the streak is know as a 'shooting star'

-Black holes are formed where large stars used to be-cant see a black hole as light cant escape from it-have a very large mass but a very small size (very dense)

-Moons orbit planets;planets orbit stars due to centripetal force acting on them                                                              centripetal force acts towards the centre of the circular orbit and gravitational attraction is the source of the centripetal force 

Distances in space=very large light travels 300000km/s light year=distance light travels in 1year

light from the sun take8mins to reach Earth                                                                                                 light from the nearest star takes 4.22 years

America plans a manned mission to Mars after 2020 costing £400 billion

Spacecraft carrying humans has to have large ammounts of food, water and oxygen

Astronauts can wear normal clothing in a pressurised spacecraft

Special spacesuits worn outside the aircraft;                                                                                   -dark visor stops astronaut being blinded       -suit is pressurised and has oxygen supply             -surface of suit facing towards Sun can reach 120C          -facing away as cold as -160C

Travelling in space astronouts are subjected to lower gravitational forces than on Earth

Unmanned probes can go where conditions are deadly for humans

Unmanned spacecraft costs less and doesnt put lives in danger

They must be very reliable because there is usually no way of repairing them when they break down

-almost all galaxies are moving away from each other with further galaxies moving faster    

-with newly invented telescope Galileo observed 4 moons orbiting Jupiter and this confirmed that not everything orbited the Earth and supported Copernicus' idea that planets orbit Sun

-Roman Catholic Church didnt support Galileo's model as they believed that the Earth was at the centre of the Universe=very long time before this was accepted    

-17th century Newton was working on his theory of universal gravitation which suggested that all bodies attract one another

-Today we believe that gravitational colapse is prevented because the Universe is constantly expanding as a result of the Big Bang

-Source of light is moving away from an observer its wavelength appears to increase which shifts light towards the red end of the spectrum-red shift

-When scientists look at light from the Sun there is a pattern of lines across the spectrum

-The same pattern is observed when they look at light from distant stars but it is close to the red end of the spectrum

-Scientists use this information from the red shift to work out the age of the Universe

-a nebula is a swirling cloud of gas and dust                         

-nebula clouds pulled together by gravity and as the spinning ball of gas starts to get hot it glows this protostar cant be seen because of the dust cloud

-gravity causes star to become smaller,hotter and brighter and after millions of years the core temp is hot enough for nuclear fusion to take place

-hydrogen nuclei joing together to form helium nuclei,energy is released and the star continues to shine while there is enough hydrogen

Small stars shine for longer than large stars because they have less hydrogen but use it up at a slower rate 

Medium-sized star; (Sun) becomes a red giant while the core contracts the outer past cools,changes colour from yellow to red and expands

-gas shells called planetary nebula are thrown out

-core becomes a white dward shining brightly but eventually cools to become a black dwarf

Large stars; become red supergiants as the core contracts the outer part expands and suddenly the core collapses to form a neutron star and there is an explosion called a supernova

-neutron stars are very dense        -remnants from a supernova can merge to form a new star

-core of the neutron star continues to collapse and becomes even more dense and could form a black hole

black holes- very large density and its large mass means it has a very strong gravitational pull