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~

Heat

Follow up of Temperature and Kinetic Theory

Prerequisite: chemistry

Scored me a four in AP chemistry. The first thing that tripped me up was titration (but those were good days, joking about titrating ก๋วยเตี๋ยว). Tied at second place with solubility, was calorimetry. Now I wish someone had told me this:

Sure I knew what temperature was, otherwise I would not have even gotten a three. What I did not understand was heat. It is not a difficult concept but the teacher (and the textbook) probably took it for granted and only explained temperature.

I like to think that the relation of heat and temperature is similar to that of moles and concentration, or even charge and voltage. Molecules go from high to low concentration. Charges go from high to low voltage. Likewise, heat goes from high to low temperature.

Not sure why internal energy is introduced in this chapter. Will bring this up in the next~

Latent heat is just a fancy name for heat of phase change, whether it be freezing, melting, condensing, or evapourating. It is very important because it is actually rather easy to overlook in a calculation. At least for me. And we have our old friend specific heat down there, familiar and just as important.

Physics is obsessed with rates, so there:

AP is only likely to contain conduction. The rest, again, are nice to know.

Head on to the finale of the thermal physics trilogy.. Laws of Thermodynamics~

Temperature and Kinetic Theory

Prerequisite: chemistry

My regular physics class actually did not cover any thermal physics because most were covered in chemistry. Even so, my regular chemistry teacher was *cough cough* so I learned most stuff from AP chemistry. Here are the stuff I have left to gleam, which are mostly just equations. They are not particularly important in AP, but they are good to know~

When change in temperature causes change in length or volume. Can be written in the format on the right.

Thermal stress is a form of pressure. This is not likely to appear in AP as Young's modulus is not covered. Whatever that is. It has something to do with elasticity.

Here is another form of PV = nRT. Despite this, the classic nR form is still more common in AP than this Nk form.

I came across this vaguely in AP chemistry. Now it is a lot more clear. Geez, why did the textbook not put it this way:

where k is Boltzmann's constant.

More stuff. Nice to know, but not vital.

Also nice to know. The final answer can be in any (mass / time) unit depending on what you use in your substitution.

That symbol J.. physicists are really running out of alphabets to use! They finished the Greek letters, might as well use Chinese characters next. Sure, we have plenty of Chinese characters~

Next in the thermal physics trilogy is Heat