Physics P2:

The advantages and disadvantages of photocells: 

• They're robust and don't need a lot of mainteance. 
• They don't need any fuel or long power cables. This links into the idea of sustainability and the environment. If they're able to take in energy from the sun, it is more environmentally friendly and therefore advantageous. Furthermore, they don't cause pollution or increase climate change. Finally, they're also a renewable energy source.
• The only disadvantage of photocells is that they won't produce electricity when its dark.

How photocells work:

• A photocell contains two pieces of silicion joined together to make a p-n junction. An electric field is created between the two pieces. One piece has an impurity added to produce an axcess of free electrons- n type silicon. The other piece has a different impurity added to produce an absence of free electrons- p type silicon. 
• Sunlight contains energy packets called photons. Photons cause free electrons to move producing an electric current.

• Alpha radiation causes most ionisation and gamma radiation the least. Ionisation produces charged particles.
• If Atoms contain the same number of protons and electrons- this means they're neutral.

Uses of radioactivity:

• Alpha radiation is used in smoke alarms.
• Beta or gamma sources are used in rolling mills to conrol thinkness.
• Gamma sources are injected into the body as tracers.

• Comets have elliptical orbits. They pass inside the orbit of Mercury and out beyond the orbit of Pluto.

Order of the planets:

• The Sun
• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune

• Asteroids are mini-planets or planetoids orbiting the Sun in a 'belt' between Mars and Jupiter. They're large rocks that were left over from the formation of the Solar System.
• All bodies in space, including planets were formed when clouds of gas and dust collapsed together due to gravitational forces of attraction.
• The mass of an object determines its gravitational force.. Asteroids have relatively low masses compared to the mass of Jupiter. Jupiter's graviational force prevents asteroids from joining together to form another planet, so they remain in a 'belt' as they orbit the sun.
• Sixty-five million years ago, an asteroid with a diameter of 10km struck the Earth. Dust from the collision passed into the atmosphere, blocking sunlight and lowering temperatures. Photosynthesis couldn't take place and herbivorous dinosaurs couldn't feed. This therefore led to the extinction of all dinosaurs.

• Compared to the near circular orbits of the planets, the orbit of a comet is nearly elliptical. Most 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. Solar winds blow the dust into the comet's tail so the tail always points away from the sun.
• The speed of a comet increases as it approaches the Sun and decreases s it gets further away. This is a result of changing gravitational forces.

• A medium sized star like the Sun, becomes a red giant: the core contracts, the outer part expands and cools and it changed colour from yellow to red. During this phase, gas shells called planetary nebula are thrown out.
• The core becomes a white dwarf shining very brightly but eventually cools to become a black dwarf.
• Large stars become red supergiants as the core contracts and the outer part expans. The core suddenly collapses to form a neutron star and there's an explosion called a supernova. Remnants from a supernova can merge to form a new star. The dense core of the neutron star continues to collapse until it becomes so dense it formas a black hole.
• The swriling cloud of gas and dust is a nebula. Nebula clours are pulled together by gravity into a spinning ball of gas, which starts to get hot and glow.
• The protostar is shining but can't be seen because of the dust cloud. Gravity causes the star to become smaller, hotter and brighter.
• After millions of years, the core temperature is hot enough for nuclear fusion to take place. As hydrogen nuclei join together to form helium nuclei, energy is released. The star continues to shine while there's enough hydrogen.
• Small stars shine for longer than large stars because although they have less hydrogen, they us it up at slower rates.

Generators: 

• The current from a dynamo can be increased by using a stronger magnet, increasing the number of turns on the coil or rotating the magnet faster.
• The voltage and frequency from a dynamo can be displayed on an oscilloscope.
• The formula to work out the frequency is: frequency Hz = 1 divided period.
• The generator at a power station works like a dynamo. 
• A simple generator sinsists of a coil of wire rotation between the poles of a magnet to produce a current in the coil.
• Ina power station, fuel is burned to heat water to product steam. Steam at high presusres turn a turbine which then drives a generator.

• A fuel burns in air to release enregy in the form of heat. 
• Biomass can be burned. It can be fermented to produce methane which is then burned. 
• Nuclear fuels don't burn. In a nuclear reactor, uranium atoms split and release lots of energy as heat. Nuclear reactions in power stations have to be controlled to avoid an exlosion.
• Fossil fuels are non-renewable and produce Carbon Dioxide, a greenhouse gas that contributest to climate change.
• Biomass energy sources are reneable but still product Carbon Dioxide when they're used as a fuel.

• Radiation from nuclear waste causes ionisation, which causes a change in the structure of any atom exposed to the radiation. 
• The cells of our bodies are made of many different atoms. DNA is an important chemical in a cell and can be changed when its exposed to radiation. The cell behaves differently to normal and this is called mutation. One effect of mutation os for a cell to divide in an uncontrolled way. This can lead to cancer.
• Waste from a nuclear reactor can remain radioactive for thousands of years.

Advantages of using nuclear power:

• Fossil fuel reserves aren't being used.
• There's no atmospheric pollution.
• No Carbon Dioxide is produced so no global warming effects occur.

Disadvantages of using nuclear power:

• uclear power stations have high maintenance costs, decommissioning costs and produce quanities of radioactive waste.
• There's a risk of a major accident similar to Chernobyl.