[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe what's meant by nuclear fishion and a chain reaction you should then be able to describe how nuclear fish may be controlled or uncontrolled and finally you should be able to describe what happens in nuclear fusion and all of this is for triple physics students only so we're going to start by looking at nuclear efficient the first key idea you need to understand is that in some elements the nucleus is large and unstable and good examples of these are uranium and plutonium now I should point out that this only applies to certain isotopes of these elements but for the exam you're not required to name those Isotopes in nuclear fish the nucleus of these elements splits now this can happen spontaneously in other words by itself but that's quite rare normally for fishing take place the nucleus first has to absorb a neutron so here I'm showing you a number of uranium nuclei when a uranium nucleus absorbs a neutron this triggers the nucleus to undergo fishion in other words split like this when the nucleus splits it forms two smaller nuclei roughly equal in size scientists call these the daughter nuclei it also emits two or three neutrons plus gamma radiation energ is also released during the fishing reaction for example all of the fishing products have got kinetic energy now the really important part about this process is what happens to the neutrons emitted during fishing the neutrons can now be absorbed by more uranium nuclei and Trigger fishing again like this and again when these nuclear undergo fishing they emit two or three more neutrons each and these can then go on to trigger fishing themselves so in a very short time a huge number of uranium nuclei have undergone fishing and an enormous amount of energy has been released scientists call this a chain reaction and it's really important that you learn that now scientists can control the fishing chain reaction and use the energy released to generate electricity we find this in nuclear reactors however the explosion in a nuclear weapon is caused by an uncontrolled fision Chain Reaction Okay so we've seen what happened a nuclear fishion but what about nuclear fusion this is a totally different process in nuclear fusion two light nuclei for example hydrogen are joined to form a heavier nucleus some of the mass of the nuclei can be converted into energy and that's released as radiation now nuclear fusion is the process that releases energy in Stars such as the sun it's important to understand that nuclear fusion is not a chain reaction we only use that expression when talking about nuclear fish remember that you'll find plenty of questions on nuclear fish and on nuclear fusion in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by nuclear fish and a chain reaction you should then be able to describe how nuclear fish may be controlled or uncontrolled and finally you should be able to describe what happens in nuclear fusion [Music]