On Aug 24, 3:07 pm, John Stafford <n...@droffats.net> wrote: > In article > <d6d4e6d1-7920-4163-b097-8e1445b02...@h19g2000yqb.googlegroups.com>, > > Darwin123 <drosen0...@yahoo.com> wrote: > > Most of thermodynamics can be visualized by imagining that entropy > > is an indestructible fluid. Temperature is a quantity proportional to > > the pressure this fluid can exert. One can think of entropy as > > analogous to electric charge, and temperature as analogous to voltage. > > Help me understand that. Given that the greater difference in > temperature between an object and the environment (think of a hot bar of > steel in an ambient atmosphere of 0c) then the faster the transfer of > energy from the bar to the atmosphere. > > How does that relate to voltage?
I am relating an analogy, not an equivalence. Similar equations are used in hydrodynamics, electrodynamics, and thermodynamics even though they relate to different things. Thus, similar pictures are used in hydrodynamics, electrodynamics, and thermodynamics. You have to keep in mind that the quantities are different, especially when they interact together. There are some differences, that have to be always kept in mind. However, if you know one field then you can use some of that knowledge in the other field. I take it you know a little about electrodynamics, since you are asking about voltage. So I will present a useful analogy between thermodynamics and electrodynamics. On an atomic level, electricity consists negatively charged particles called "conduction electrons" or positively charged bubbles called "holes". Of course, electricians don't use that knowledge. On a macroscopic level, electricity is a fluid as described by Benjamin Franklyn. The equation for power put out by a DC current crossing a circuit is: P=IV where P is power, I is current, and V is the drop in voltage across the circuit element. Of course, this assumes the current is constant. The energy used by the circuit element in a time, t, is E=Pt where Q is energy used up by the circuit element. The electric charge that passed through that device is q=It where I is electric current and t is the time. So, using these two equations E=qV Now, electric charge (q) is analogous to entropy (S). Used energy (E) is analogous to heat energy (Q), which by definition is the energy released from entropy. voltage (V) is analogous to temperature (T). So the result of substituting the electrodynamic quantities for thermodynamics is: Q=ST Often, this is presented as the "definition" of entropy which is: S=Q/T So entropy, S, has many properties similar to an electric charge. Here is a big difference. The total electric charge can not be destroyed and can not be created. This is called conservation of electric charge. The total entropy of a system can not be destroyed but can easily be created. This is called the Second Law of Thermodynamics. Calculating the work done by a heat engine turns out to be similar in many ways to calculating the work done by a circuit element.