Complexity Explorer Santa Fe Institute

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Introduction to Renormalization

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6.2 The Thermal Physics of Plasma » Quiz Solution

How do we describe a plasma in thermal equilibrium?

A. the density of particles at any point depends upon (a) the local electric potential, (b) the number of particles of the opposite charge nearby.
B. the density of particular at any point depends upon (a) the number of particles of the same charge nearby, and (b) the system temperature.
C. the density of particular at any point depends upon (a) the number of particles of the same charge nearby, and (b) the number of particles of the opposite charge nearby.
D. the density of particles at any point depends upon (a) the local electric potential, (b) the system temperature.

Answer (D). It of course matters where the other particles are; so in one sense all four answers are correct if we're talking about correlations. But the question is about how we actually choose to describe all the correlations in the system: what quantities go into the function? It turns out that the trick to solving this problem is to summarize the information about the positions of all the other particles using the electric potential. This tells you the cumulative effect of where everything is on a test particle. The temperature then tells you how much the test particles care about being in a very high or very low energy state.