# Thermodynamics

Boyle's Law:

### PV/T = constant.

For a fixed quantity of gas, pressure is proportional to temperature and inversely proportional to volume.
Consider the molecular model as an explanation of the law.

• T - corresponds to an average speed of the molecules.
• P - the forces caused by the molecules in motion hitting the side of the container. It is comparable to the average kinetic energy of the molecules.

Ways to get (heat) energy out of a system:

1. Change in temperature - a steam engine works by heating and then cooling of steam.
2. Change in pressure - a windmill.
3. Change in volume - a gas rushing against a piston, gasoline engine (sudden creation of (hot) gases push the piston).

Phases of matter - heat energy disrupts the molecular configurations causing the molecules to move faster and thus changing the state of matter from solid to liquid to gas. Each "phase changes" requires heat energy to effect the phase change called latent heat.

Brownian Motion: The observed jiggling and random movement of microscopic particles caused by the impacts of moving molecules in the medium.

### Laws of Thermodynamics.

1. Conservation of Energy: The total energy in a system is constant; energy can neither be created nor destroyed. (The layperson might say, "You can't get something for nothing.")
2. Availability of "useful" energy: You cannot transfer heat from a lower temperature to a higher temperature without using energy; available useful energy dissipates; the randomness of a closed system increases. (The layperson might say, "You can't even break even.")
3. Reaching Absolute Zero: You cannot cool a substance to absolute zero in a finite number of steps. (So, as The layperson might say, "You can't hit rock bottom.") This is a consquence of another form of the law: all materials have zero entropy at absolute zero temperature.

Entropy: A measure of the randomness of a system. Also a measure of the useful energy in a system. The second law of thermodynamics implies that the entropy of a closed system is always increasing.

Maxwell's demon: An imagined way to get around the second law of thermodynamics and produce free work. See my detailed explanation of Maxwell's demon.

Heat of Combustion. The heat given off in oxidation reactions. It is usually measured in calories per mole.

Units of energy.

• calorie: Then energy needed to heat one cc of water one degree C.
• Kcal = 1000 calories.
• Joule = The amount of energy needed to move a mass 1 m against a force of 1 nt. One calorie is equal to 4.184 joules.

Potential Energy: energy stored in a system. For gravitational potential we have PE=mgh.

Kinetic Energy: The energy of an object due to its motion. KE=(1/2)mv2.