When describing a thermodynamic system, which of the following is a good description of "internal energy"?
Internal energy is the same as thermal energy, meaning the energy due to randomly directed microscopic motion.
Internal energy is the energy the system has when it is one part of a larger system.
Internal energy is the energy the system would have even if it were stationary and alone in the universe.
Internal energy is the energy inside each atom of the system, as represented by $E = mc^2$.
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