P2 - Living for the future

Photocells:

• contains two pieces of silicon that form a p-n junction:
  • n-type: has an added impurity to produce an excess of free electrons
  • p-type: has an added impurity to produce an absence of free electrons
• photons (energy packets from the sun) cause the free electrons to move producing an electric current
• output of a photocell depends on:
  • the light intensity
  • the surface area exposed
  • the distance from the light source
• advantages of using photocells:
  • robust and don't need maintenance
  • they dont need any fuel or long power cables
  • cause no pollution and do not contribute to global warming
  • use a renewable energy source
• only disadvantage is that they do not produce electricity when it is too dark or cloudy

Passive solar heating:

• sun produces infrared radiation with a very short wavelength:
  • walls and floor inside buildings absorb radiation, warm up and re-radiate infrared radiation at a longer wavelength
  • glass is transparent to short wavelength radiation so it reflects this longer wavelength back inside the buildings

Heat from the Sun: Solar reflectors are moved to face towards the sun by computers

Wind energy:

• wind is renewable
• energy depends on speed of wind -> wind turbines do not work if the speed of wind is too great or if there is no wind
• wind farms do not contribute to global warming and do not pollute the atmosphere
• but they can be noisy, take up a lot of space and can spoil the view

Current from dynamo is increased by:

• using a stronger magnet
• increasing the number of turns on a coil
• rotating the magnet faster

Generator:

• consists of a coil of wire rotating between poles of a magnet
  • coil cuts through magnetic field as it spins which produces a current in the coil
• if coils remain stationary and magnet is moved, current is still produced

Power stations:

• water boils to produce steam at high pressure which turns a turbine that drives a generator

Efficiency = 

Natural sources of carbon dioxide:

• natural forest fires
• volcanic eruptions
• decay of dead plant and animal matter
• respiration

Man made sources of carbon dioxide:

• burning fossil fuels
• deforestation 
• cement manufacture

Methane is produced when organic matter decomposes in an environment lacking oxygen:

• Natural: wetlands, termites and oceans
• manmade: mining fossil fuels, digestive processes in animals, rice paddies and burying of waste in landfills

The greenhouse effect:

• half of the greenhouse effect  is due to water vapour and a quarter due to clouds
• electromagnetic radiation from the sun has a relatively short wavelength
• this radiation is absorbed by and warms the Earth which then re-radiates the energy as infrared radiation with a longer wavelength
• This radiation with a longer wavelength is absorbed by greenhouse gases which warms up the atmosphere

• Majority of scientists agree that evidence supports climate change : average temperature of the Earth has increased steadily during the last 200 years
• scientists do not agree on to which extent human activity is contributed to climate change and global warming

Measuring power:

• power = voltage x current
• energy supplied = power x time
• cost of electricity used = energy used x cost per kWh
  • we pay less for electricity in the night as it is not needed but it still has to be produced

National Grid - a series of transformers and power lines that transport electricity from power stations to consumers

Step-up transformers are used to step up the voltage to as high as 400000V to:

• reduce energy loss
• reduce distribution costs
• cheaper electricity for consumers
• transformers then step down the voltage at a safe level for consumers to use

A step up transformer increases the voltage so that the current is reduced which means there is less heating effect therefore less energy is lost to the environment

Ionisation : gaining or losing electrons

• gaining electrons = atom is negatively charged
• losing electrons = atom is positively charged
• formation of ions can disrupt behaviour of molecules inside the body

Advantages and disadvantages of nuclear power:

• Advantages: fossil fuels not used, no greenhouse gases discharged into atmosphere
• Disadvantages: very high maintenance costs, very high risk of accidents

Alpha radiation:

• short ranged, easily absorbed by a sheet of paper / card
• used in smoke alarms:
  • radiation ionises oxygen and nitrogen atoms in air, causes small current that is detected
  • when smoke fills detector in alarm, air is not so ionised, current is less and alarm sounds

Beta radiation:

• range of 1m, absorbed by a few mm of aluminium
• used in paper rolling mill:
  • amount of radiation passing through sheet is monitored and pressure of rollers are adjusted accordingly to contol the thickness of the paper

Gamma radiation:

• much more penetrating, few cm of lead stops most radiation 
• used for:
  • sterilising medical equipment as it kills microbes and bacteria and to check leaks in pipes

Nuclear waste:

• plutonium is a waste product from nuclear reactors, used to make nuclear bombs
• low level waste - buried in landfill sites, high level - encased in glass, buried deep underground

The solar system:

• meteors are made up of grains of dust that burn up as they pass through the Earth's atmosphere
  • they heat the air around them which glows and forms a streak (shooting star)
• Black holes are formed where large stars used to be:
  • cannot see a black hole because light cannot escape from it
  • large mass but small size

Centripetal forces:

• act towards the centre of the circular orbit
• gravitational attraction is the source of the centripetal force

Unmanned probes:

• can go where conditions are deadly for humans, cost less and do not put lives at risk
• have to be very reliable because there is no way of repairing them when they break down

Asteroids = mini planets / planetoids orbiting the Sun:

• most orbit between Mars and Jupiter
• large rocks left over from the formation of the solar system
• the mass of an object determines its gravitational force and Jupiter's large gravitational force prevents asteroids from joining together to form a planet

The origin of the moon:

• the Moon was a result of the collision between two planets
• the iron core melted and joined with the Earth's core and less dense rocks formed the Moon
• scientific evidence:
  • density of Earth = 5500 kg/m3 while density of Moon = 3300 kg/m3
  • no iron in Moon
  • Moon has exactly the same oxygen composition as the Earth

Asteroids - found quantities of iridium near a crater which isn't found in Earth's crust, but common in meteorites

Comets:

• pass inside the orbit of Mercury and well beyond the orbit of Pluto
• as the comet passes close to the Sun, the ice melts and solar winds blow dust into comet's tail which always points away from the Sun
• the Sun has a larger gravitational force and so as the comet approaches the Sun, the speed of the comet increases and as it moves further away from the Sun, its speed decreases

Near Earth Objects (NEOs):

• if an NEO was on a collision course with the Earth, life would end on Earth
• to avoid this, one option is to explode a rocket near the NEO to alter its course away from the Earth
• you would have to do this at a safe distance from Earth to ensure the impact doesn't affect Earth

Models of the universe:

• Galileo's model: not everything orbited the Earth, he discovered 4 moons orbiting Jupiter which supports Copernicus' model that planets orbit the Sun
• the Roman Catholic Church did not support Galileo's model as they believed that the Earth was at the centre of the universe
• in the 17th centruy Newton was working on his theory of universal gravitation which suggested that all bodies attract one another
• today we believe that gravitational collapse is prevented because the universe is constantly expanding 

Red Shift:

• when observing distant stars, the pattern of lines across the spectrum are closer to the red end of the spectrum as they have a longer wavelength, causing red shift
• this proves that the universe is expanding

Life history of a Star:

• nebula (swirling cloud of gas and dust) clouds are pulled together by gravity 
• gravity causes the star to become smaller, hotter and brighter and after millions of years, nuclear fusion takes place which join together hydrogen nuclei to form helim nuclei
• small stars shine longer than large stars as they have less hydrogen but use it up slower

The end of a star's life:

• medium sized stars become a red giant, the outer part cools, changes from yellow to red and expands
• planetary nebula are thrown out, core becomes a white dwarf, cools to become black dwarf
• large stars become red supergiants, core contracts, outer part cools, core collapses to form a neutron star and there is an explosion (supernova) and the core continues to collapse to become even more dense and eventually into a black hole
• balck holes have a very large mass concentrated into a small volume which means that it has a very strong graviattional pull