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Homework Help: Homework Help! Essay Questions!

  1. Jun 14, 2006 #1
    Hello, slight problem. I have a sheet full of essay questions to do and I need help to see that I'm on the right lines to answering them. I'm pretty clueless and I'm only doing core physics as an extra class at uni since I enjoyed it at school. Anyway if any one can give me some help with the following questions I'd be very grateful!


    1. Electrons in metallic objects are virtually free to move. When a metallic object is placed inside an electric field, constant in time, electrons redistribute so that the local excess of negative or positive charge on the surface of the box compensates for the external field. As a result, the total electric field inside the box is null and objects inside the box do not feel external electric fields.
    Gravity is a central force very much similar to the electric force between static charges.
    Do you think it will be possible to create a "box" that could screen external gravitational fields? Explain your answer.

    2. It is known that there are small asteroids with very irregular shapes. What are the forces that give these asteroids their shapes? What shape do you think a neutron star that does not rotate around its axis can have?

    3. It is known that living cells can, to some extent, repair their DNA. Do you think this process can be done without using some form of energy?

    4. Quantum effects are normally important only on atomic scales or smaller. Gravity is the dominant force for very massive bodies. Under what conditions will quantum effects become important for gravity?

    5. Using thermodynamic arguments give reasons for the existence or otherwise of perpetual motion machines. Illustrate your answer by proposing a perpetual motion machine and showing how it either works or fails. (A perpetual motion machine is a machine which, once started, will continue working forever unless interfered with by an external body.)

    6.Consider if it is possible to store information in a linear system.

    7. In order to measure the distances of very distant Galaxies we need to find inside those Galaxies stars, such as the Cepheid variables or some supernovae, whose behaviour in time allows us to estimate the amount of energy that they radiate. Suppose that one day we will know the physics of other Astronomical objects well enough to use their detection as a new way to measure distances beyond the limits of the present techniques. Suggest a procedure to test these new techniques.

    So far I have for No.5 "The Law of Conservation of Energy (First law of Thermodynamics) says that energy can be neither created nor destroyed. It can only be changed from one form to another. This is further supported by Einstein, who showed us that E=MC squared. In other words, Energy and mass are interchangeable, and the flow of energy into a system must match the flow of energy out of a system." It is impossible to create a perpectual motion machine. Imagine that we have two steel balls of equal mass suspended from a beam on equal lengths of string. When hanging in a relaxed position, the two steel balls just touch. If we lift the first steel ball and pull it out to the extent it's string allows, we have loaded that ball with potential energy. If we then release the ball, it will drop and strike the other ball, passing it's energy onto the second ball and sending upwards in an arc.
    The reason these two balls don't continue clicking and clacking for eternity is because they "leak" energy in a number of ways. Through heat generated by friction when the balls pass through the air. Through the generation of sound when they click"
    Last edited: Jun 14, 2006
  2. jcsd
  3. Jun 14, 2006 #2


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    Staff: Mentor

    I'd suggest that you jot down a couple sentences in answer to each question, and we can offer suggestions or corrections to help you expand out from the 2-sentence answers to longer ones. Fun questions, BTW.

    Your answer for #5 is not bad, but I'm not sure the conservation of energy angle helps. And what kinds of thermodynamics have you covered so far in class?
  4. Jun 15, 2006 #3
    Last edited by a moderator: May 2, 2017
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