Physics 2.6

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  • Created by: Laura
  • Created on: 11-01-13 18:20

Which two fissionable substances are used?

In nuclear reactors the two fissionable substances which are used are Uranium- 235 and Plutonium- 239.

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What is nuclear fission?

Nuclear fission is the splitting of an atomic nucleus, commonly the atomic nucleus is either Uranium- 235 and Plutonium- 239.

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What is needed to start nuclear fission?

Nuclear fission is started by a high energy neutron being fired to an atomic nucleus and then being absorbed. This makes the atomic nucleus unstable and causes it to split into two smaller nuclei, which is fission.

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What is produced in fission?

Fission produces two smaller nuclei (for example when fission occurs with uranium- 235 you produce a Barium nucleus and a Krypton nucleus), more high energy electrons and a whole heap of energy.

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Diagram of a fisson chain reaction look like?

(http://www.bcscience.com/bc10/images/0_quiz-7.3-03.gif)

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What happens in nuclear fission?

(http://i.livescience.com/images/i/000/031/224/i02/shutterstock_14083729.jpg?1348088322)A high energy electron is fired at an atomic nucleus and is absorbed. This makes the nucleus unstable causing it to begin to split into two smaller nuclei. This process causes huge amounts of energy to be released (as heat energy) and more high energy electrons to be produces (as well as the new nuclei). The new high energy electrons can go on to hit and be absorbed by more original nuclei causing them to split and produce two new nuclei, energy and more high energy electrons. This is how the chain reaction of fission is formed and continues untill all of the original nuclei have split.

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Where does fission occur in nature?

Fission occurs naturally when any unstable isotope or atom decays, they split because they have too high a mass to be stable so may be more stable as two lighter isotopes.

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How are stars and planets formed?

Planets and stars are formed from a nebula, which is a massive cloud of dust. Due to the gravitational forces between the pieces of dust they begin to group together and as the group becomes larger their gravitational force increases so that they attract more and more pieces of dust, which are slowly compressed, due to gravity, to form rocks. The rocks continue to attract more dust because of their high gravitational force until they grow to become the size of a planet or protostar.

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How do protostars become main sequence stars?

By creating a protostar you are forcing (from gravity) huge amounts of material together and this increases the temperature and denisity of the protostar allowing nuclear fusion to begin.

Nuclear fusion is the joining together of two or more atomic nuclei (in stars the atomic nuclei is hydrogen) and this releases huge amounts of energy. Eventually this will cause the attractive gravitational forces to balance with the outward pressure from the expanding hot gases and the star will become stable.

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What is the life cycle of a star like our sun?

1.All stars star out as nebulas. The gravitational forces between the material in the nebulas causes the material to join together.
2. As this process continues the temperature and density of the joined material increases which causes the fusion of protons to hydrogen atoms to begin, this creates a protostar.
3. As the fusion continues the outward pressure from it balances out the gravitational forces holding the protostar together. This means that the star is stable and is in the main-sequence of its life.
4. During the main-sequence the fusion of protons to create hydrogen, then hydrogen to create helium and so on to create all the elements in the periodic table up to iron causes the outward pressure to become greater than the gravitational forces and the star rapidly grows into a red giant.
5. Whilst the star is a red giant the amount of nuclei which are being fused decreases (because they have already been fused into an element) and this means that the gravitational forces become greater than the outward pressure, so the star collapses under the pressure of its own gravity into a white dwarf.
6. The density of the star now is so great that the temperature of it causes the star to be white hot, however as no energy is being produced from fusion the star eventually cools and loses its brightness, turning it into a black dwarf.

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Life cycle of a star up to 5x the size of the sun?

All stars start out as nebulas, and for larger stars the same process occurs as stars a similar size to our sun up to the main-sequence of the star. 
During the main-sequence the fusion of protons to create hydrogen, helium and all the elements in the periodic table up to iron causes the outward pressure to become greater than the gravitational forces and the star rapidly grows into a super red giant, as there is a larger volume of fusion occuring as the star is bigger, and this producesmore outward pressure.
5. Whilst the star is a super red giant it loses energy because most of the nuclei have already been fused into new elements, so the rate of fusion decreases, meaning the energy produced during fusion decreases also. This loss in outward pressure causes the gravitational force to become greater but as the star is so large the rapid cooling and shrinking from this causes the star to explode into a supernova.
6. The explosion of the supernova causes massive amounts of energy, dust and gas to be released into space, and forms elements which are above iron in the periodic table.
7. If the original star was up to 3x the size of the sun the remnants of the supernova will form a neutron star, which is a dense clump of neutrons.
7. If the original star wasup to 5x the size of the sun the remnants of the supernova will form a black hole, which has such a great gravitational field that even light is absorbed.

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How are the elements formed?

Elements up to iron on the periodic table are formed through fusion in protostars, main-sequence stars, red dwarfs and super red dwarfs. The fusion stars off as two protons (two hydrogen) to become a helium isotope, and then on to fuse different combinations of isotopes and elements to create every element in the periodic table up to iron.

Elements after iron on the periodic table are formed during the explosion of a super red dwarf, so in a supernova. This is because the energy released in a supernova is great enough to fuse the heavier atoms together to create all the elements after iron in the periodic table.

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What is nuclear fusion?

Nuclear fusion is the joining together of two or more atomic nuclei to form a larger nucleus.

For example two protons (two hydrogen) with a lot of energy can be fused into a hydrogen atom and electron, as one proton is turned into a neutron and electron (the electron is given off) whilst the other stays as a proton. Then another proton can be added to create an isotope of helium (two protons and one neutron), which produces a lot of energy and gamma radiation as the proton stays as a proton. Another proton can also be added through fusion to create a helium nuclei (two protons and two neutrons), which will produce a lot of energy again and a helium atom, as the proton will be turned into a neutron.

During nuclear fusion more energy is prduced than is needed to create the fusion, as well as either an electron (if the proton added is turned into a neutron and electron) or gamma radiation (if the proton added stays as a proton).

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