However, as briefly mentioned in class, additional variables can be added if needed to deal with an extra force not covered by variables S, V, and N. (For example, adding a VdQ term for charge particles). Let's say you wanted to model the change in energy for taking some constant volume of material and changing its shape (say, from a circle to a square). What "extra variable" do you need to make U a function of? Please also write the full differential for dU with this extra term included, and explain what that term means briefly