Lecture 3 Energetics: Some basic thermodynamics

The First Law of Thermodynamics
The 1st law states that energy can neither be created nor destroyed, although it can be altered in form. In any system or process, the amount of energy going in must be the amount of energy going out (or otherwise accounted for). Note that heat is a form of energy.

The Second Law of Thermodynamics
The 2nd law is the law of entropy. Entropy is the degree of disorder in a system, the degree to which energy is useful. In any process, the net entropy must increase. If the entropy decreases as some part of a process (like plant growth), there must be some energy escaping somewhere else as a waste product that increases in entropy enough to make up the difference. Yes, because entropy is always increasing, this does means that the Universe is doomed to a very disordered future, assuming the Universe is a closed system.

Sensible heat:
Sensible heat is, I think, a fairly old-fashioned term that isn't really in common use anymore. I was a bit confused by some stuff I found on the internet and by what Prof. Currie said in class. Is the sensible heat the amount of kinetic energy the molecules have, or is it, in effect, the opposite of that? I though this was what Prof. Currie said, but his lecture slide on latent heat would indicate the opposite. It's possible that everything I've written below is backwards. Somebody please confirm this

All matter is made up of atoms and molecules which are in some kind of motion (even if the motion is too small to see). Kinetic energy is the energy of motion. Thus, every piece of matter has some kind of kinetic energy associated with it, and this is what the sensible heat refers to.

Recall that there are three basic states of matter: solid, liquid, and gas (Just ignore plasma and quark-gluon plasma and stuff like that. It's definitely not important for biological systems). The molecules in a solid are tightly bound together, liquid molecules are bound less tightly, and gas molecules are barely bound at all. This is important, as it relates to energy.

Think about water. When water is cold (below 0 Celsius), it takes the solid form: ice. At this low temperature, the water molecules have very little kinetic energy (they are moving very slowly), so it is possible for the molecules to form chemical bonds with one another. It becomes a rigid lattice of ice.

If you add energy to the system (recall that heat is energy), you can raise the temperature of the ice to above 0 Celsius. The kinetic energy of the molecules increases, and the melting point is the point at which the kinetic energy exceeds the strength of the bonds holding the molecules together. Thus, the rigid structure collapses, and the ice becomes water. It is important to note that this process requires an input of energy from outside the system. The reason ice melts spontaneously at room temperature is because, as required by the laws of thermodynamics, heat flows from the room into the colder ice. The water will end up at room temperature (and, technically, the room temperature will decrease slightly as the heat flows into the ice), and the water/room system will end up at equilibrium. If you conducted this experiment while sitting in a freezer, the only way to melt the ice would be to take a blowtorch to it or something (massive input of energy).

Finally, if you add even more energy to the system, you can reach the boiling point at 100 Celsius. The kinetic energy of the water molecules will increase enough to break down the remaining bonds, turning the liquid into a gas. Thus, gases have the highest sensible heat because the molecules have the most kinetic energy.

Note that at the temperature at which a phase change occurs (solid->liquid and liquid->gas), the substance actually consumes energy without the temperature increasing. This is because you need energy to break the bonds. After the phase change has finished, all subsequent energy added goes into just raising the temperature of the substance.

The latent heat of the substance is the amount of energy it takes to make the phase change. Hence, a gas has the highest latent heat because if it were to return to the liquid state, it would have to release some of that energy so it could reestablish the bonds. So, in a way, the gas state is sort of "storing" energy.

In case you wanted to know, if you pour in enough energy into something, you can break the bonds that hold the electrons to the nuclei of the atoms, and the substance becomes a plasma. So even though we know that "opposites attract" and electrons and protons like to stick to each other, the kinetic energy exceeds the strength of this attraction, and the electrons and nuclei swim around without sticking together. A quark-gluon plasma would be WAY beyond a regular plasma. This is the point after which not even protons can survive. The quarks that compose them would fly around unbonded. Nobody has ever observed a quark-gluon plasma to my knowledge, although they keep trying.

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