PHYS 213 :: Physics Illinois :: University of Illinois at Urbana-Champaign

James Scholar Assignment #2

This assignment has two parts:

  1. First, complete the following calculation. Consider a very simple model of a computer memory in which molecules are either found to reside in the left half of their memory cell (encoding a “0”), or in the right half (encoding a "1"). Imagine that we have a 10-bit register. Initially each cell is in the "0" state (i.e., all 10 particles are in the left side of their respective cells). After the computation, they are in either half of the cell (depending on the specific computation). This doesn't necessarily require any work. For example, if one simply pulls out the dividing wall between the "0" and "1" side, the particles can by free expansion move from the "0" state into the "1" state.

    Your task is to determine the energy cost to reset the 10-bit register to its initial state, where every particle is again in the "0" side of its cell. This can be done by using a piston to push the particles (~compressing the gas) so that they can only occupy the left side of the cell. Here are the two relevant questions for you to answer:
    1. What is the change in dimensionless entropy during this process?
    2. What energy is required to carry out the process (assuming the process takes place at a temperature T?
  2. Then, read the extremely interesting article SciAmDemons-Bennett, which links thermodynamics to information theory, and write a 1-2 page essay on it in which you:
    1. Summarize the problem of Maxwell's demon
    2. Summarize the several incorrect solutions to this paradox
    3. Explain the correct solution
    4. Finally, explain the connection between this topic, and the calculation above, to that of "reversible computing" (which you should research a little).

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