Physics - topic 5

Physics - topic 5

1. Radioactivity

   • Mass number: Total number of protons and neutrons. Atomic number: Number of protons.
     • Isotopes are atoms with the same number of protons but a different number of neutrons. The other isotopes tend to be radioactive which means they decay into other elements and give out radiation.
       • Radioactibity is a random process: Unstable nuclei will decay and in the process give out ionising radiation. When the nucleus does decay it will spit out one or more of 3 types of radiation: alpha, beta or gamma.
         • Atoms can gain or lose electrons. When an atom loses or gains an electron it's turned into an ion. This is known as ionisation. Alpha, beta and gamma are all type of ionising radiation.
           • Alpha particles are helium nuclei: they're made up of 2 protons and 2 neutrons. They're relatively big and heavy and slow moving. They have a strong positive charge. They're strongly ionising but dom't penetrate far into materials but are stopped quickly.
             • Beta particles are electrons emitted from a nucleus: They move quite fast and they're quite small. They have a negative charge. They're moderatley ionising and penetrate moderately far into materials before being stopped.
               • Gamma rays are a type of electromagnetic radiation: They're the opposite of alpha particles. They're weakly ionising and penetrate a long way into materials before eventually interacting with an atom.
                 • Alpha particles are blocked by paper and cardboard. Beta particles are blocked by thin aluminium. Gamma rays are blocked by thick lead.

2. Nuclear fission

   • The splitting up of uranium atoms
     • The chain reaction: A slow moving neutron is fired at an isotope of uranium - uranium-235. The neutron is absorbed by the nucleus - this makes the atom unstable and causes it to split. When the u -235 atom splits it forms 2 new lighter elements and thermal energy is released. Each time a uranium atoms splits up, it also spits out 2 or 3 neutrons, which can hit other uranium nuclei causing them to split also. This is a chain reaction.
       • Nuclear power stations are powered by nuclear reactors. In a nuclear reactor, a controlled chain reaction takes place in which uranium atoms split up. The heat energy released by nuclear fission is used to boil water to drive a steam turbine, which turns a generator to generate electrical energy.
   • The neutrons released by fission reactions in a nuclear reactor have a lot of energy. These neutrons will only cause other nuclear fissions if they're moving slow enough to be captured by the uranium nuclei in the fuel rods. The uranium fuel rods are placed in a moderator to slow down the fast moving neutrons. This creates a steady rate of nuclear fission, where one new neutron produces another fission. Control rods, often made of boron, limit the rate of fission by absrobing excess neutrons.

3. Nuclear fusion

   • 2 light nuclei collide at high speed and join to create a larger nucleus. Fusion releases a lot of energy.
     • Fusion only happens at really high pressures and temperatures (it doesn't happen at low pressures and temperatures due to the electrostatic repulsion of protons). So fusion reactors are really hard and expensive to try to build.
       • Cold fusion: nuclear fusion which occurs at around room temperature. Cold fusion would make it possible to generate lots of electricity, easily and cheaply.