# HSC Physics Revision 2

Here are some more points to help students maximise their HSC Physics mark.

1. BCS Theory of Superconductivity This theory was developed by Bardeen, Cooper and Schrieffer in 1957. Below the critical temperature two electrons interact forming a bound pair known as a Cooper pair that does not collide with the atoms in the lattice and so there is no electrical resistance. As the first electron approaches the lattice the positive ions are distorted creating a concentration of positive charge that attracts the second electron. BCS theory states that the total momentum of a Cooper pair must be zero so the second electron must be moving in the opposite direction to the first. A common misconception is that the members of the pair must be close together. This is incorrect as at close distances the members of the pair would be repelled by their electrostatic repulsion. Cooper pairs do not lock together permanently. They change partners wihin the group of electrons and the net result is a movement through the material without collision. Any collision with the lattice or increase in temperature above the critical temperature destroys the superconducting state. Always draw a diagram showing the Cooper pair and the distorted lattice.
2. Quark Question What is the change in quark composition when a neutron decays?
3. Gravitational Potential Energy An apple is at rest on the surface of the Earth. What is the gravitational potential energy of the Earth-apple system? Is it zero?
4. Turning the Handle of a Generator When a the handle of a generator is turned with no resistor in the circuit it is "easy" to turn the handle. When a resitor is placed in the circuit it is "hard" to turn the handle. Why?
5. Proton and an Electron A proton and an electron are both released from rest in a uniform electric field. Compare the size of the momentum of each particle after each moves the same distance.
6. Fermi's Initial Experimental Observation of Nuclear Fission Do not confuse this dot point with Fermi's later work in 1942. Fermi was awarded the Nobel Prize in Physics for 1939 "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reaction brought about by slow neutrons". Fermi did not discovery nuclear fission. He discovered the role of slow moving neutrons in triggering increased activity in uranium salts but did not connect this with the splitting of the uranium, which he later regretted.

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