Physics OCR

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Using the suns energy

  • The sun is a big source of energy and we can use it for photocells.
  • Photocells generate electricity from sunlight.
  • They generate DC (direct current) which means the elctrcity flows the same way round. This is different from mains electricity.
  • Photcells are made from silicon - a semiconductor. When sunlight falls on the cell...
    • The silicon atoms absorb some of the energy, losing some electrons.
    • These electrons then flow round a circuit - which is electricity.
  • The current and power output of a photocell depends on its surface area, the intensity of light and the distance from the light source.
  • Advantages include - no moving parts, sturdy, long lasting and renewable.
  • Disadvantage - no sun no power.
  • Curved mirrors focus light and they can be used as a solar oven.
  • Large curved mirrors can be used to generate steam to produce electrcity
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Solar and wind power

  • Passive solar heating is when energy from the sun is used to heat something up directly.
  • E.g. heat can pass through glass which is absorbed by things in the room. The light from the sun has a short wavelengthso it can pass though glass in the room. However the heated objects emit infared radiation of a longer wavelength so it can't escape. This heats the room. 
  • Solar water heaters use passive solar heating.
  • Glass lets heat from the sun in which is then absorbed by the black pipes and hets up water. 
  • Sun is the reason we have wind - energy from the sun heats up heats the atmosphere which causes convection currents.
  • Wind turbines convert kinetic energy into electricity. The wind turns the blades which turns a generator.
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Producing and distributing electricity

  •  The National Grid is the network of pylons and cables that covers the country.
  • The aim of a power station is to convert one type of energy into electricity.
  • The first stage is to use the fuel to produce heta which generates steam - this is the job of the boiler.
  • The moving steam drives the blades of a turbine.
  • This rotating movement is converted into electricity by the generator using electromagnetic induction (see next card).
  • Most power stations are inefficent - energy is wasted as heat and noise.
  • Different power sources have advantages and disadvantages...
  • Fossil fuels - burnt to release heat energy. These fuels are available and are a concentrated source of energy. However burning them creates harful gases, also we buy most of it from other countries which is expensive.
  • Biomass - is stuff from plants (wood and straw) or animals (manure) that can be burnt or fermented to create methene. It's renewable but burning methene produces carbon dioxide but it is stuff that plants took out the atmosphere when they were growing so the process is carbon neutral.
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The dynamo effect.

  • Electromagnetic conduction is the creation of a voltage in a wire which is experiencing a change in magnetic field.
  • Using this to transform kinetic energy into electrical energy is called the dynamo effect.
  • There are two situation where you get this...
  • An electrical conductor moves through a magentic field or the magnetic field through an electrical conductor changes.
  • If the direction of movement is reversed the voltage/ current will be reversed.
  • To get a bigger voltage and current you can increase...
  • The strength of the magnet, the number of turns in the coil, the speed of the movement.
  • Generators rotate a coil in a magnetic field.
  • Every half turn, the current in the coil swaps direction.
  • This means the generators produce an AC current. Turning the coil faster produces morre peaks and a higher voltage an current.
  • The frequency of AC electrical supplies is the number of cycles per second and is measured in hertz. In the UK electricity is supplied at 50 Hz 
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Supplying electricity efficently.

  • To transmit a lot of electrical power, you need either a high voltage or high current, but a high current means the cables get to hot which is ineffecient.
  • It's cheaper to increase the voltage so it's transformed to 4000,000V. This keeps the current lowmaening less energy is wasted.
  • To increase the voltage you need a step-up transformer.
  • To bring the voltage down to a safe level you need step-down transformers.
  • This is the reason mains electricity is AC tranformers only work on AC.
  • Power stations aren't very effecient, to calculate their efficency you need these equations, there are two parts...
  • Total energy input = useful energy output + waste energy output.  
  • Efficiency = useful energy output / total energy input   x1000.
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Electrical power.

  • Running costs depend on an appliances power rating.
  • Power is measured in watts or kilowatts - where 1 watt means 1 joule of energy per second.
  • Power (w) = Voltage (v) x current (a).
  • Your electrcity meter records how much energy you use in units of kWh.
  • A kilowatt-hour is the amount of electrical energy converted by 1 kw appliance left on for 1 hour.
  • The higher the power rating and the longer you leave it on the more it costs.
  • Energy supplied (kWh) = power (kW) x time (hours).
  • Cost = nuber of units x price per unit.
  • Electricity supplied during the night (off-peak) is sometimes cheaper. Storage heaters take advantage of this - they heat up at night then release energy slowly throughout the day.
  • Advantages of using off peak electicity...
  • Cost effective for the electrical company - power stations can't be switched off. Cheaper for consumers.
  • Disadvantages...
  • Increased risk of fire - noboby watching appliances.
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The greenhouse effect.

  • The Earth is surrounded by an atmospher made up of various gases.
  • The gases filter out certain types of radiation from the sun.
  • The Eartth abosrbs EM radiation from the sun which warms the Earths surface. The Earth then emits some of this back out to space.
  • Most of the radiation emited from the Earth is infared. A lot of this infared is absorbed by atmospheric gases like carbon dioxide, methane and water vapour.
  • These gases the radiate heat back to the Earth.
  • This is called the greenhouse effect.
  • Humans are causing an increase in greenhouse gases...
  • Carbon dioxide - People use more enrgy which we get from burning fossil fuels. More land is needed so trees are chopped down.
  • Mathane - Cattle farming has increased to feed the growing population. Decaying waste in landfill is increasing.
  • Water vapour - mostly comes from natural resources e.g. oceans seas rivers and lakes. Power station produce water vapour.
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Global warming and climate change.

  • Since we started burning fossile fuels the level of carbon dioxide in the atmosphere has increased.
  • The global temperature has risen due to this.
  • Changes to the weather can have human and natural causes.
  • Also changing temperature can have large effects on the weather...
  • Hot regions suffer more extreme weather e.g. longer hotter droughts.
  • Hurricanes form over warm water so warmer water means more hurricanes.
  • Human causes of temperature change...
  • The rising carbon dioxide levels caused by humans affect the greenhouse effect.
  • Soot and gases produced by factories can reflect heat back down to earth.
  • Natural causes...
  • Ash and gases thrown into the atmosphere by volcanos can refelct radiation from the sun back into spacecausing the Earth to cool down.
  • Changes in our orbit round the sun can cause ice ages.
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Nuclear radiation.

  • When an unstable nucleus decays it gives of nuclear radiation.
  • The three types of radiation are alpha beta and gamma. They all cause ionisation - when atoms lose or gain electrons turning atoms into ions.
  • Positive ions are formed when atoms lose electrons and Negetive when they gain electrons.Ionisation can can also start chemical reaction between differentatoms.
  • Alpha particles are big, heavy and slow moving...
  • They're stopped quickly - they don't penetrate far into materials.They can be stopped by paper or skin. This means they're strongly ionising.
  • Beta particlesare electrons so they are small and fast....
  • They can penetrate moderately before colliding so they are moderately ionising. They can be stopped by a thin sheet of metal.
  • Gamma rays...
  • After creating a and b particles the nucleus might need to get rid of some extra energy it does this by emmiting a gamma ray.
  • They have no mass or charge and can penetrate a long way into materials meaning they are weakly inonising
  • They can be stopped by thick concrete or lead. 
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Uses of nucleur radiation.

  • Alpha radiation is used in smoke detectors...
  • They have a weak source of a radiation close to two electrodes. The radiaiton ionises the air and a current flows between the electrodes but in a fire, the smoke absorbs the radiation - the current stops and the alarm sounds.
  • Beta radiation...
  • If a radioactive substance is injected into a patient its progress can be followed using an external radiation dtector. A computer converts the reading to a TV dispaly showing where the strongest reading is.
  • Doctors use these tracers to see if the body is working properly. They use Beta because it can pass out the body.
  • Beta is also used in thickness control. You direct radiation through paper being made. If the detected radiation changes it means the product isn't the right thickness so the rollers adjust. The substance can't decay.
  • Gamma rays kill living cells so can be used to cure cancer by directing the radiation at cancer cells.
  • They can also be used to sterilise equipment by killing microbes.
  • Check turbine blades - if too much radiation gets through there's a crack.  
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Nuclear power.

  • A nuclear power station works in the same way as other power sattions but with nuclear fission to create steam.
  • In nuclear fission, atoms in the nuclear fuel are split releasing lots of heat energy.
  • Advantages...
  • It makes lots of energy without producing carbon dioxide.
  • The reactions release more energy than chemical reactions.
  • Nuclear fuel is cheap.
  • There is still lots of uranium available.
  • Disadvantages...
  • Power staions are expensive to build and take a long time to start up.
  • Processing uranium before you use it causes pollution.
  • There is a risk of leaks which can be dangerous.
  • The radioactive waste produced is dangerous and hard to dispose of.
  • Reprocessed uranium creates plutonium which can make nuclear bombs.
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Danger from radioactive material

  • Radioactive sources need to be stored safely and when using them humans should cover up theri skin and keep at arms length.
  • Industrial workers wear full protective suits to prevent radioactive particles being inhaled. Lead lined suits, lead/concrete barriers and lead screens keep workers safe.
  • Radioactive waste is difficult to dispose of...
  • Most waste is low level and be disposed of by burying it in secure landfill.
  • High level waste is dangerous because it stays radioactive for a long time. It is often sealed into glass blocks which are then sealed in metal canisters.
  • However its hard to find suitable places to bury,the site has to be geologically stable.
  • Also people living nearby can objectso nuclear waste is kept on site.
  • Some radioactive waste can be reprocessed - reused.
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The solar system

  • Our solar sytem consists of the sun and the planets, Mercury, Venus, Earth, Mars, (Asteroid belt), Jupiter, Saturn, Uranus, and Neptune orbiting it.
  • It's held together by gravitational attraction...
  • Things only change direction when a force acts on them.
  • Gravity pulls everything in the universe towards everything else.
  • The pull of gravity on stars and planents is huge and it makes the planets orbit the stars.
  • The pul of an object gets smaller the further away you go. This is why the Earth orbits the sun rather than a bigger star further away.
  • Circular motion us always caused by a froce towards the centre of the circel. For plantes, moons etc in an orbit this force is provided by gravity.
  • A force that causes a circular motion is called a centripetal force.
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Asteroids and comets.

  • When the solar system was forming the rocks between Mars and Jupiter didn't orm a planet because of the graviational attraction of Jupiter.
  • This left asteroids - they usually stay in their orbit but can sometimes enter different ones.
  • Meteors are rocks or dust that enter the Earths atmosphere which usually burn up.
  • Sometimes though some of the meteor crashes into the Earths surface as a meteorite.
  • The dust and smoke can block out the sunlight causing species to become extinct.
  • We can tell that asteroids have collided with the Earth because of craters but also...
  • Layers of unusual elements in rocks
  • Sudden change in fossil numbers as species suffer extinction.
  • Comets are balls of rock dust and ice which orbit the sun in very elongated ellipses. They come from objects orbiting the sun beyond the planets.
  • As a commets approaches the sun its ice melts leavinga tail of gas.
  • Comets speed up as they approach the sun because of the gravitaional pull.
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Near earth objects (NEOs) and the moon.

  • NEOs are asteroids or comets which might be on collision course with Earth.
  • Astronomers monitor them so when they find one they can monitor its trajectory (path) and see if its going to hit us.
  • Scientists think that our moon was formed when another planet collided side on with th Earth.
  • The theory is that some time after the Earth was formed a smaller object crashed into it.
  • In the heat of the collision, the dense iron cores of these two planets merged to form the Earths core.
  • The less dense material was thrown as really hot dust and rocks which then orbited the Earth and eventually came together to form the moon.
  • The evidence for this is...
  •  The moon has a lower density than the Earth and doesn't have a big iron core whereas the Earth does.
  • Moon rocks contain few substances which eveporate at low temperature - suggesting the noon was formed from hot material.
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Beyond the solar system.

  • We are in the milky way galaxy and like most things in the universe galaxies rotate.
  • The whole universe has more than a billion galaxies that really far apart.
  • Once you get outside our solar system, the distances between stars and galxies are so big that kilometers aren't enough.
  • So scientist use light years instead.
  • A light year is the distance that light travels through a vacum in one year.
  • When a really big star uses up all its fuel it explodes leaving a black hole.
  • Astronomers detect black holes by observing x-rays emitted by hot gases from other stars as they spiral into the black hole.
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Exploring the solar system.

  • We can explore space using manned spacecraft..
  • The solar system is big so sending manned space craft can take years.
  • The spacecraft would need to carry lots of fuel making it heavy It would be difficult keeping the astronauts alive and healthy for that time...
  • The space craft would have to carry food, water and oxyge.
  • You'd need to regulate the temperature and remove toxic gases.
  • the spacecraft would have to sheild the astronauts from radiation
  • Long periods in low gravity causes muscle wastage and loss of bone tissue.
  • It can be psychologiacally stressful.
  • Sending unmanned probes is easier.
  • Advantages...
  • They don't have to carry food, water and oxygen.
  • They can withstand conditions that would be lethal to humans.
  • With no people taking up room more instruments can be fitted.
  • They're cheaper - they carry less and don't have to come back to Earth.
  • If a probe crashes or burns no one gets hurt
  • Disadvantages...
  • They can't think for themselves and do maintenance. 
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The origin of the universe.

  • The universe seems to be expanding...
  • Looking at light from distant galaxies we see that the frequencies are lower than they should be - they're shifted towards the red end of the spectrum.
  • Measurement of red-shift suggest that all the distant galaxies are moving away from us whichever way you look.
  • More distant galaxies have greater red-shifts meaning they are moving away faster.
  • Another observation is that there is microwave radiation from all directions.
  • Scientists can detect low frequency microwave radiation coming from all parts of the universe.
  • It's known as cosmic background radiation.
  • As the universe expands and cools the radiation cools and drops in frequency.
  • All this evidence shows that the galaxies are moving away from each other suggesting that something must have started them going e.g. an explosion.
  • The Big Bang - initially all the matter in the universe started at one point then it exploded and started expanding.   
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Galileo and Copernicus

  • Most ancient greek astronomers believed that the sun, moon planets and stars all orbited the sun in perfect circles - geocentric or ptolemaic model.
  • The Copernican model replaced this model in the 1600s it states that the Earth and planets all orbit the sun which is at the centre of the universe. It said that they orbited in perfect circles.
  • Galileo found one of the best peices of evidence..
  • In 1610 he was observing Jupiter using a telescope when he saw some stars in a line near the the planet. He noticed they never moved away from Jupiter which suggested they weren't stars but moons. This proved the Ptolemaic model wrong.
  • Galileo also saw that Venus has phases - where the amount of the planet that's lit by the sun seems to change over time. If the Ptolemaic model was right these changes would be smallbecause Venus would always be in front of the sun. However if the Copernican model was right Venus could move in front or behind the sun so the changes in how much it lit up would be big.
  • The current model is the same as copernicus but we now know the sun isn't the centre of the universe and that plantes orbit the sun in more elliptical orbits.
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