Photocells are used to collect energy from the sun. They don't require much maintenance and need no fuel, so they don't contribute to global warming
However, they don't produce electricity when it's too dark
They contain two pieces of silicon joined to make a p-n junction
One piece has an impurity added to produce an absence of free electrons (p-type) and one has an impurity to produce many free electrons (n-type)
Photons from the sun cause electrons to move, creating a current
Another method of generating electricity from the sun is to use the energy to heat water and turn it into steam, driving a turbine. Mirrors are used to reflect sunlight onto a central point, computers are used to ensure these reflectors always face the sun
Using a curved mirror, light can be reflected to a focus
Passive solar heating:
The sun produces light of a short wavelength, which can pass through glass windows. The floor and walls absorb this and reradiate it at a longer wavelength (because they are cooler than the sun), which cannot pass through glass. This then stays inside the building, keeping it warm
Wind turbines:
Wind is produced by convection currents in the air, mainly caused by the sun
Wind energy is renewable, but wind turbines do not produce energy if there is not enough wind
Wind farms can be noisy, take up space and many people consider them ugly
Power stations:
Fuels are used to heat water, which boils into steam, which turns a turbine. This turbine drives a generator, which creates a current
Dynamos, currents and voltages:
Dynamos produce a DC current, which can be increased by using a stronger magnet, increasing the number of turns of the coil or rotating the magnet faster
An oscilloscope can be used to display the current output of a dynamo, and shows how it varies over time
Generators:
A generator consists of a coil of wire rotating between the poles of a magnet, or magnet rotating around a coil of wire
Current is produced in the coil
A generator produces alternating currenct (AC), whilst batteries produce direct current (DC)
Efficiency:
Efficiency of energy transfer, such as output of a power station is calculated with useful energy output ÷ total energy input
Greenhouse gases:
Some wavelengths of radiation are absorbed by the Earth, which re-radiates it as infrared (longer wavelength), which is absorbed by greenhouse gases, which warms the atmosphere
Carbon dioxide is created from natural causes, such as forest fires, volcanic eruptions and respiration, and man-made causes such as burning fossil fuels/waste and deforestation
However, the most significant greenhouse gas is water vapour and only 0.001% comes from human activity. Water vapour contributes to half of the greenhouse effect, another quarter is caused by clouds
Another greenhouse gas, methane, is produced when organic matter decomposes with a lack of oxygen, e.g. from wetlands, termites and oceans, and also from man-made sources such as fossil fuels and from landfills
Dust can reflect sunlight back into space but also trap heat in
Some scientists argue on the seriousness of global warming and how far humans are responsible
Most wavelengths of electromagnetic radiation can easily pass through the Earth's atmosphere, but some, such as infrared, are absorbed by some gases in the atmosphere
Power:
Power (W - watts) = voltage (V) x current (A)
In the home, the unit of electrical energy is kWh, the kilowatt-hour; cost of electricity used (p) = energy used (in kWh) x cost per kWh (p)
Energy used (kWh) = power (W) x time (hours)
Electricity is often cheaper at night when not as much is needed, but still has to be produced
The National Grid is a collection of transformers and power lines that transport electricity from power stations to consumers. It is often transmitted at high voltages (up to 400,000 V) and therefore a low current, to reduce energy loss and consequently reduce energy costs and increase efficiency
The greater the current, the hotter the wire. Step-up transformers like those used in the National Grid increase voltage and reduce current
Ionisation:
Atoms have the same number of electrons and protons, so they are neutral
When an atom is ionised, it gains or loses electrons. If electrons are lost, the charge becomes positive and vice-versa
This can cause strands of DNA to break/change in the body, and protein molecules may change shape
Radiations:
The nucleus of an atom forms alpha, beta and gamma radiations
Alpha causes most ionisation, is short ranged and is absorbed by a few sheets of paper
Beta has a range of around 1 m and is absorbed by a few mm of aluminium
Gamma is more penetrating and is mostly stopped with a few cm of lead or a few m of concrete. It causes the least ionisation, but still has dangerous effects
When measuring radiation, background radiation (such as from rocks and space) should be taken into account
Safely handling radioactive materials involves protective clothing, tongs or keeping distance, short exposure time and shielded and labelled storage
Uses of radiation:
Smoke alarms contain an alpha radiation source, which ionises oxygen and nitrogen atoms in air making them conduct. When smoke takes its place during a fire, the current is reduced and the alarm sounds
In a paper rolling mill, thickness can be controlled by passing beta radiation through and measuring the amount of radiation that passes through
Gamma radiation kills microbes so it can be used to sterilise medical instruments
Gamma can also be used as a tracer to track movement of substances in the body or detect leaks in pipes or welds
Nuclear power:
Nuclear power generation does not directly create greenhouse gases and fossil fuel reserves are not used
However, it has high maintenance costs, waste must be disposed of, and there is a risk of accidents
Nuclear waste:
A waste product from nuclear reactors is plutonium, which can be used to create nuclear bombs
High level radioactive waste is buried underground, enclosed in glass. It could alternatively be reprocessed or buried in landfill sites if it's only low-level waste
This waste remains radioactive for thousands of years, it can contaminate underground water sources and could therefore be used by terrorists in this way
The acceptable radioactivity level may change over time, so this must be accounted for when disposing of nuclear waste
Comets:
Comets are mostly made of ice and have elliptical (oval shaped) orbits, the speed is greatest when it is closest to the sun (pericentre) and slowest at the farthest point (apocenter)
When close to the sun, ice melts, which is blown by solar wind to create the comet's tail, which points away from the sun
Meteors:
Meteors are made of dust that burn up as they pass through Earth's atmosphere, heating the air around them which glows, creating a 'shooting star' appearance
Black holes:
Black holes are formed from some large, old stars. Light cannot escape from them, so they are not visible
Black holes have a high density; they have a small size but large mass
Centripetal force:
Satellites (bodies orbiting other bodies, such as the Earth or Moon) orbit because of centripetal force towards the centre of the orbit, caused by gravitational attraction
Distances:
Light travels at 300,000 km/s (671 million miles per hour), light from the sun takes 8 minutes to reach the Earth, and light from the nearest star takes 4.2 years
Space exploration:
Manned missions are very expensive, unmanned spacecraft cost less and do not risk astronaut's lives. However, there is no way of repairing them if they break down so they must be reliable and contain backups for essential equipment such as batteries and computers
Manned spacecraft must carry or produce food, water and oxygen and be pressurised, which enables astronauts to wear normal clothing inside
Outside the spacecraft, spacesuits must be worn: - Visors stop them from being blinded from bright objects (e.g. the sun, as there is no atmosphere to reduce its effect) - They must be pressurised and contain an oxygen supply - The surface facing the sun can reach 120 °C and surface facing away from the sun can reach -160 °C
Asteroids:
Asteroids are planetoids (mini-planets) orbiting the sun, most are found in the asteroid belt between Mars' and Jupiter's orbits
They are rocks left over from the formation of the solar system
Jupiter's gravitational force prevents asteroids from combining to create another planet. All bodies in space were formed when dust and gas collapsed together due to gravity
The origin of the moon:
Most scientists believe that two planets in the same orbit collided, the iron core of one merged with Earth's core, and the less dense material joined to form the moon
There is no iron in the moon and the average density of the moon is lower, and the moon has the same oxygen composition as Earth's, supporting this idea
Evidence for asteroids:
Near to asteroid craters on Earth, iridium, a metal not found in the Earth's crust, has been found
Fossils are often mostly found below the layer of iridium but not above it
Tsunamis caused by the impact have carried fossil fragments inland
NEOs:
Near Earth objects (NEOs) which could damage the Earth are being monitored by scientists
If one is on a collision course with Earth, an explosion could be used by humans to alter its course and protect life on Earth
If one is seen as dangerous, it should by surveyed by telescope and monitored by satellites
The expanding universe:
Most galaxies are moving away from each other with galaxies far away moving fastest, suggesting a finite, expanding universe
When a source of light is moving away, its wavelength appears to increase, which shifts light to the red end of the spectrum, red-shift
Models of the universe:
Originally it was beleived that planets orbited the Earth, the Ptolemaic model (Claudius Ptolemy, around 100 AD)
Galileo observed four moons orbiting Jupiter with a telescope in 1610, confirming that not everything orbits Earth, supporting Copernicus' 1540s idea that the planets instead orbit the sun
The Church did not support this model as they believed that the Earth was at the centre of the universe
Newton's theory of universal gravitation suggested that all bodies attract one another
We believe that the expansion started by the Big Bang prevents a gravitational collapse
Stars:
Small stars shine for longer because they use hydrogen up at a slower rate
A nebula is a swirling cloud of gas and dust, which glows as it is pulled together by gravity
As it is compacted by gravity and it becomes hotter and brighter, eventually, the core temperature is hot enough for thermonuclear fusion, where hydrogen nuclei join to form helium nuclei
A medium sized star (e.g. the Sun) becomes a red giant, as the outer part cools and expands, whilst gas shells, planetary nebula, are thrown out. The core becomes a white dwarf (very bright) but eventually cools to become a black dwarf
Large stars become red supergiants, the core collapses to become a neutron star and there is a supernova (explosion). Neutron stars are dense
Remains from supernovae can merge to form a new star
The core of a neutron star can become even more dense and form a black hole