Laws of Thermodynamics

Follow up of Heat

Prerequisite: chemistry, physics

Less on temperature, more on energy. Less on chemistry, more on physics~ Here is something we can all agree on:

The zeroeth law of dynamics was a bit of a "duh" statement that scientists overlooked. Thermal equilibrium can also be understood with concentration and electrostatic analogies. Molecules at molecular equilibrium are at uniform concentration. Charges at electrostatic equilibrium are at uniform voltage. Likewise, heat at thermal equilibrium are at uniform temperature.

There again, internal energy:

I forgot to include that heat = Q. And for internal energy, we will focus more on the format ∆U = (3/2)nR∆T.

Here is your old grandma in a brand new dress. The Law of Conservation of Energy combined with ∆U = (3/2)nR∆T, PV = nRT, and PV graphs can tell us many things~

An isothermal process is when the temperature is kept constant. Since ∆T = 0, it follows that ∆U = 0 because ∆U = (3/2)nR∆T. The graphs looks like 1/x.

An isobaric process is when the pressure is kept constant. Since ∆P = 0, W = P∆V. It is more apparent when written as "W = (F/A)(A*∆x)", bearing in mind that W = F∆x.

An isovolumetric (sometimes isochloric) process is when the volume is kept constant. Since there is no ∆x, W = 0.

The adiabatic process is when no heat flows in or out of the system. Recall that heat Q is "energy transfer due to ∆T" so "∆Q" is quite redundant. Since Q = 0, it follows that ∆U = -W because ∆U = Q - W. Here is a better adiabatic graph:

So why is it that there is a change in temperature if Q = 0? Recall that temperature is the average kinetic energy of molecules, and in the case of adiabatic processes the change in this kinetic energy is due to work.

Nothing mind boggling here. Molecules do not diffuse to a higher concentration. Charges do not move to a higher voltage. There is actually a section on Gibbs free energy in AP chemistry, which in a sense calculates the spontaneity of a reaction, but that is beyond the scope of AP physics.

Efficiencies are disappointing enough, but thermodynamics takes that disappointment one step further. Even efficiencies are not completely efficient, if you compare calculations with Q and with T.

The biggest annoyance with this law is the fact that 100% efficiency is impossible. When you convert heat to kinetic energy there is always a residue of heat at equilibrium, as seen in QH --> QL + W. Heat at equilibrium cannot do work! If you turn QL back into QH with yet inefficient work, you end up with less QH than you started with.

1) QH --> QL + W
2) QL + W --> QH' + QL'
in which QH' < QH
and QL' > QL

One day when all kinetic energy is spent and all QH falls to equilibrium, we will be in heat death. Dun, dun dun, there goes your dramatic finale! Moral of the story: kids, do not waste your kinetic energy~