Physics P2

Ocr Gateway Physics, P2

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Solar Cells

Sun gives out energy in the form of heat and light some energy stored in fossil fuels and some is used in a more direct way- solar cells and solar heating

Solar cells (photocells) generate electricity directly from sunlight

  • generate direct current (DC- flows the same way)
  • Solar cells are made of silicon- a semiconductor when sunlight falls on the cell
    • the silicon atoms absorb some of the energy, knocking loose some electrons
    • these electrons then flow round a circuit- electricity
  • power output depends on surface area and intensity on sunlight

Advantages- no moving parts (low maintenance) no power cables or fuel, its renewable, no greenhouse effects, long life

Disadvantage- no sunlight= no power, needs a large surface area, photocells are expensive, won't work at night, sunlight is weaker the furthur away from equator you are

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Passive solar heating

Heating buildings by passive solar heating

  • enters as infra red radiation
  • is absorbed by objects in the room, and it cannot escape back out
  • objects emit the infra red radiation and it can't escape so room gradually warms up
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Producing and distributing electricity

National grid connects power stations to consumers

Fossil fuelled power stations

  • fossil fuels heat water
  • water turns to steam
  • steam turns the turbine
  • turbine drives the generate
    • are inefficient as energy is wasted as heat and noise

energy: potential ---> heat ---> kinetic ---> electrical

Biomass= renewable- includes wood, straw, and manure and is burnt to produce methane gas

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Advantages and disadvantages

non renewable-

  • advantages- cheap, easy, reliable, short start up time
  • disadvantages- will run out, produce greenhouse gases, cost alot to decommission


  • advantages- zero fuel costs, don't produce CO2 or sulphur dioxide and saves fossil fuels
  • disadvantages- unreliable, expensive to build, ugly, don't produce much energy
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The Dynamo effect

Electromagnetic Induction= the creation of voltage in a wire which is experiencing a change in magnetic field

The dynamo effect-

  • use electromagnetic induction to transform kinetic energy into electrical energy
  • 2 different situations:
    • an electrical conductor (coil of wire) moves through the magnetic field
    • the magnetic field through an electrical conductor changes
    • If direction is reversed voltage is reversed too

to get a bigger voltage you can increase

  • number of turns in the coil
  • strength of the magnet
  • the speed of movement
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- rotate a coil in a magnetic field

  • every half turn the current in the coil swaps direction
  • this means that generators produce alternating current (AC)
  • Turning the coil faster produces higher peaks and a higher voltage
  • Frequency of AC electrical supplies is the number of 'cycles' per second and is measured in hertz (Hz)
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Suppling electricity efficiently

To transmit a lot of electrical power you either need a high voltage or a high current

  • higher current means cables get hotter
  • much cheaper to increase voltage
  • Need a step up transformer
  • Need big pylons with insulators to transmit electricity safely
  • to bring it down to safe usable levels in homes local step down transformers reduce the voltage
  • this is why mains electricity is AC- so that transformers work

efficiency equations=

fuel energy input= electrical energy output + waste energy output

Efficiency = Electrical Energy Output

Fuel Energy Input

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Electrical Power

Running costs depend on an appliance's power rating

Power= Voltage times Current

Kilowatt- hours (kWh) are Units of energy

A kilowatt hour is the amount of electrical energy converted by 1 kW appliance left on for 1 hour

Units of energy= power times time

cost= number of units times price per unit

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Off peak electricity

Storage heaters- take in heat during the night and release the heat slowly during the day


  • Cost effective for the electrical company- power stations can't be turned off at night
  • Cheaper for consumers if they buy electricity in off peak hours


  • increased risk of fire- more appliances going at night with no one watching
  • you start fitting your routine around cheap rate hours
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Wind farms

Wind turbines convert kinetic energy of moving air into electricity- wind turns blades which turn the generator


  • renewable
  • wind is free
  • preserves fossil fuels
  • doesn't pollute or increase greenhouse effect


  • takes up alot of space
  • wind strength varies
  • turbines spoil scenery- visual pollution
  • can be noisy
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Nuclear power

uses uranium as the fuel to produce heat


  • expensive to build and maintain
  • risks of leaks of radioactive materials
  • problem with radioactive waste
  • uranium is non renewable
  • target for terroists- plutonium can be used in bombs
  • expensive to decomission


  • no fossil fuels
  • plenty of uranium at the moment
  • doesn't contribute to greenhouse effect
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Nuclear radiation

Nuclear radiation causes ionisation

  • when an unstable nucleus decays it gives off nuclear radiation (alpha, beta or gamma)
  • all 3 kinds can cause ionistion which is when they knock electrons off atoms turning the atoms into positively charged ions and it can cause health problems

Alpha particles= 2 protons and 2 neutrons

  • can be stopped by paper or skin
  • travel a few cm's in air

beta particles= electrons

  • small and fast and travel a few m's in air
  • stopped by thin sheet of aluminium

Gamma= very high frequency electromagnetic waves- no mass, no charge

  • stopped by thick lead or concrete, huge distances in air
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Background radiation

=low level background radiation which is around us all the time

comes from:

  • substances on earth- air, food, building materials
  • radiation from space- cosmic rays from the sun
  • living things
  • radiation due to human activity
  • soil
  • rocks
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Alpha Radiation

- is used in smoke detectors

  • have a weak source of alpha radiation close to two electrodes
  • the radiation ionises the air and a current flows between the electrodes
  • if there is a fire the smoke absorbs the radiation stopping the current making the alarm sound
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Beta Radiation

- is used in tracers and Thickness gauges

medical use:

  • patient injected with radioactive source
  • its progress around the body can be followed by an external radiation detector
  • a computer converts the reading to a tv display showing where the strongest reading comes from
  • can show if the body is working properly
  • beta or gamma

thickness control (cardboard or paper) :

  • use a radioactive beta source because the paper would partly block the radiation
  • when the amount of detected radiation goes down- paper too thick
  • if the reading goes up- too thin- rollers adjust
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Gamma Radiation

has medical and industrial uses

  • used to treat cancer
    • directed carefully at the cancer at the right dosage so it doesn't damage the normal cells
  • used to sterilise medical instruments
    • kills the microbes
  • non destructive testing
    • used on planes, direct gamma at the jet engines and if too much gets through they know the blade is cracked
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Radioactive materials

protecting yourself: wear protective clothing, handle with tongs, keep in lead lined compartment, minimise exposure time, have hazard warning signs


  • high level- put in glass then steel containers
  • intermediate level- set in cement in steel drums, often stored underground
    • must be stored in a suitable place: must be geologically stable, must be no risk of leaking into groundwater, soil, rivers and plants (contamination)
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Earths magnetic field

It is like a big bar magnet- has north and south poles

the earth's south magnetic pole is actually the north pole

The earths molten core causes its magnetic field

  • core is made of molten iron which moves in convection currents
  • magnetic fields are generated whenever charged particles are moving around- get a magnetic field where ever electrical current flows
  • if take a coil of wire and let an electrical current flow through it, you'll get a magnetic field
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The Moon

  • scientists think the moon formed when another planet collided side on with earth
  • the theory: a smaller Mars- sized object crashed into earth and in the heat of the collision the dense iron cores of these two planet merged to form the earth's core
  • the less dense material was forced out as really hot dust and rocks which orbited around the earth until it came together and formed the moon
  • evidence:
    • moon has a lower density and no iron core
    • moon rocks contain few substance which evaporate at low temperatures suggesting it was formed of hot material
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Particles and Rays from the sun

Earth's Magnetic field shields us from charged particles by deflecting them away

cosmic rays= heavily ionising, consisting mostly of charged particles

solar flares= massive explosions that release vast amounts of energy and some of this energy is given off as gamma rays and X-rays, they also give off massive clouds of charged particles which are ejected at very high speeds which can cause disturbances

Problems caused by solar flares

  • damage artificial satellites- used for communication, weather forcasts, spying
  • the electrons in solar flares can cause surges of current in a satellite's electrical circuitry meaning it might need to be shut down
  • can disrupt electricity supplies
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Aurora Borealis

caused by cosmic rays

  • some charged particles in cosmic rays are deflected by Earth's magnetic field and spiral down near the magnetic poles
  • their energy is transferred to particles in the earth's atmosphere causing them to emit light- polar lights
  • the polar lights are shifting curtains of light that appear in the sky
  • called the northern lights (aurora Borealis)
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The solar system

Our solar system orbits around the sun which is a star

Sun, Mercury, Venus, Earth, Mars, (asteroid belt), Jupiter, Saturn, Uranus, Neptune

The solar system is held together by gravity

  • pulls everything in the universe towards everything else
  • pull of an object gets smaller the further away you go
  • if the earth wasn't already moving it would have been pulled by gravity directly towards the sun
  • circular motion  is cause by a force (pull) towards the centre of the circle
  • A force that causes circular motion= centripetal force

the suns gravity, the speed of the planets own motion and movement keep planets in orbit

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there is a belt of Asteroids orbiting between Mars and Jupiter

  • The rocks between Jupiter and Mars didn't form a planet
  • leaving millions of asteroids- piles of rubble and rock, in orbit of the sun
  • asteroids usually stay in their orbits but sometimes they're pulled or crash into different ones
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-are rocks or dust that enter the earth's atmosphere

as they pass through the earth's atmosphere they burn up

when meteorites collide with earth they cause:

  • hot rocks to fly upwards and outwards
  • wildfires spread
  • dust and smoke fill sky blocking sunlight
  • this leads to the cooling of the climate and extinction of animals

We an tell if an asteroid has collided with earth as there are big craters, unusual elements in the rock, sudden changes in fossil numbers

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  • Have elliptical orbits
  • are a ball of ice and dust
  • when they are going towards the sun they warm up and the frozen material starts to evaporate and electrically charged particles steaming outwards from the sun cause the tail to glow
  • they speed up when going towards the sun as there is greater pull from the sun's gravity
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Near Earth Objects

= NEOs

Near earth objects are asteroids or comets which might be on a collision course with earth

use powerful satellies and telescopes to find them

they can calculate an objects trajectory and find out if its heading for us

to get rid of them:

  • explode it with a bomb
  • fire missiles at it
  • if its a comet- use large mirrors to melt the ice so that its mass decreases and the path changes
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Beyond the Solar System

we are in the milky way galaxy

the whole universe has more than a billion galaxies

distances in space are measured using light years (the distance that light travels through a vacuum)

Stars can Explode leaving black holes:

  • very large mass
  • not visible
  • not even light can escape
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Exploring the solar system

manned space travel

disadvantages: need lots of food, water and oxygen, need to regulate temperature, causes muscle wastage, psychologically stressful, need lots of fuel, expensive

Unmanned probes: can gather information on temperatures, magnetic and gravitational fields and radiation levels

Advantages: don't have to carry food, water or oxygen, can withstand conditions which would be lethal to humans, more instruments can be fitted in, cheaper

Disadvantages: can't think for themselves and spontaneous decisions can't be made, can't repair themselves

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Origin of the Universe

Universe is expanding:

  • light from distance galaxies is shifted to the red end of the spectrum
  • the measurements of red shift tell us that distant galaxies are moving away
  • lines are shifted to red end and light has a longer wavelength
  • more distant galaxies have greater red shift than nearer ones meaning they are moving away faster- universe in expanding

Theres microwave radiation from all directions:

  • scientists can detect low frequency radiation coming from all directions and all parts of the universe
  • it's known as the cosmic background radiation
  • as the universe expands and cools the background radiation 'cools' and drops in frequency
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The Big Bang

  • universe occupied a very small space then it exploded and space started expanding and the expansion is still going on
  • the big bang theory lets us guess the age of the universe- 14 billion years old
  • estimates are difficult because its hard to tell how much expansion has slowed down
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Life of a Star

Clouds of dust and Gas --> Protostar ---> main sequence star ---> Red giant

Small stars ---> planetary nebula and a white dwarf

Big stars ---> supernova --> neutron star or a black hole

1. stars form of dust and gas

2. force of gravity causes to spiral together, gravitational energy ---> heat energy

3. temperature gets high enough, hydrogen nuclei undergo thermonuclear fusion

4. hydrogen runs out and star swells (red giant)

small stars: become unstabel and ejects outer layer of dust and gas- planetary nebula

leaves behind a hot dense core- white dwarf which cools down and disappears

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Big stars

Big stars glow brightly again and undergo more fusion and expand and contract several times forming heavier elements in various nuclear reactions- explode= supernova

exploding supernova throws out outer layers of dust and gas leaving a very dense core- neutron star

if star is big enough= black hole

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