Thermodynamics Terms
Adiabatic Expansion (or Compression)
Expansion or compression of an ideal gas where there is no heat transfer between the gas and the outside world. Since there is no heat transfer, Q = 0 and W = ΔU. This can also be rewritten as
. Unfortunately, "adiabatic" is not aerobic, so this doesn't apply to workouts.Avogadro's Number, NA
The number of atoms in a mole. It is 6.022 × 1023 for any substance. A mole of carbon-12 contains 6.022 × 1023 atoms of carbon-12. A mole of pencils contains 6.022 × 1023 pencils. And so on, for literally anything you can imagine.Boiling Point
The temperature at which the liquid and gas phase exist in equilibrium. Each type of material has a unique temperature that is its boiling point. At temperatures above the boiling point, lots of gas is generated. Below the boiling point the material is predominantly liquid. Like English tea without the biscuits.Boltzmann's Constant, 
The equivalent of the ideal gas constant R on an individual particle level, calculated by 
.Boyle's Law
Boyle's law says that, for gases at constant temperature, PV = constant. In other words, if you increase the pressure on a gas, its volume will decrease (and vice versa). This isn't true for people, for better or worse.Celsius Scale
A temperature scale where 0˚ is the "ice point," or the temperature at which water freezes, and 100˚ is the temperature at which water boils.Closed System
A system in which no matter can leave (exit the system), but energy can. In physics, we often model processes as closed systems. If no energy can leave a closed system either, it's said to be an isolated system, akin to an isolation chamber.Conduction
The transmission of heat via molecular collisions. Picture molecules sharing energy as they bump into each other.Convection
The transmission of heat via currents. Think of wind, but it's also true on a molecular scale.Entropy
The degree of disorder in a system. It's the principle behind messy rooms not spontaneously cleaning themselves up: it's all downhill with entropy.Fahrenheit Scale
A temperature scale where 32˚ is the point at which water freezes, and 212˚ is the temperature at which water boils. Those numbers don't make a lot of sense, which is probably why nobody except the US uses this scale.First Law Of Thermodynamics
This law states that energy is conserved in a closed system. The change in the internal energy, ΔU, of a system relates to energy, Q, in the form of heat, added to the system, and in the form of mechanical work, W, that leaves, or is done by the system. Want an equation? Here's an equation: ΔU = Q − W. Here are some important sign conventions to remember:- Heat, Q, added to the system is always positive (+)
- Heat, Q, leaving the system is always negative (-)
- Work, W, done by the system is always positive (+)
- Work, W, done on the system is always negative (-)
Freezing Point
The temperature at which the solid and liquid phases of matter exist in equilibrium. This is a unique temperature for each material. Above its freezing point a material is mostly liquid, and below the freezing point it's mostly solid. Hence the story of Frosty the Snowman.Heat
Energy. More specifically, heat is energy that will always flow from a place of higher temperature to a place of lower temperature. It is measured in units of joules (J). It's named after…no, not this guy, it's this guy.Heat Transfer Rate, H
A measured amount of heat transferred (ΔQ) over a measured amount of time (Δt):
or 
, where: - ΔT: temperature difference between the ends of the object
- k: thermal conductivity of the material
- L: material length, and
- A: material cross-sectional area
Ideal Gas Law
The mathematical relationship between pressure, volume, the number of molecules, and temperature in an ideal gas, PV = nRT. What's this stuff stand for?- P is the absolute pressure of the gas in atmospheres (atm)
- V is the volume of the gas in liters (L)
- T is the temperature of the gas in Kelvin (K). Only Kelvin will do.
- n is the amount of gas in moles (mol)
- R is the universal gas constant,
Internal Energy
The internal energy of a system is the average energy of all of the molecules in that system. A warm object has high internal energy—its molecules zip about like there's no tomorrow. A cold object has slow-moving molecules, because of its low internal energy.Isobaric Expansion (or Compression)
Expansion or compression of an ideal gas under constant pressure, such that W = -PΔV.Isothermal Expansion (or Compression)
Compression or expansion of an ideal gas under constant temperature. For this process,
—and since the temperature stays constant, the internal energy (U) of the system does not change. Under these circumstances, the First Law of Thermodynamics changes from ΔU = Q + W to become W = -Q.Isovolumetric Expansion (or Compression)
Expansion or compression of an ideal gas where there is no change in the volume of the gas. There is no work done in this case, so W = 0, which gives us ΔU = Q.Kelvin Scale
A temperature scale ranging from 0 to infinity. Zero Kelvin is the lowest temperature possible, and represents a total absence of molecular motion. On the Kelvin scale, we don't talk about degrees Kelvin—it's just Kelvin. Just like it's just Beyonce or just Madonna.Kinetic Theory
This theory says that everything is made up of atoms that are continuously in motion (i.e., bouncing around). Heating an object gives these atoms more energy and therefore increased their speed. Cooling those atoms down makes them slower.Latent Heat (L)
The heat per kilogram required to change the phase of a material. The three types of phase changes each have different latent heats. The heat, Q, required to change the phase of an object with mass m is Q = mL.- Latent heat of fusion, Lf: the heat per kg required to change from solid to liquid or vice versa.
- Latent heat of vaporization, Lv: the heat per kg required to change from liquid to gas or vice versa.
- Latent heat of sublimation, Ls: the heat per kg required to change from solid to gas or vice versa.
Molar Mass
Cancer of the teeth. Just kidding. The molar mass of a substance is the weight of one mole of that substance. The molar mass of carbon-12 is
and contains 6.022 × 1023 atoms of carbon-12. The molecular mass of water is 
and contains 6.022 × 1023 molecules of water. And so on.Mole
A furry, blind animal that ruins everyone's lawn. Strike that. In chemistry and physics, a mole is a way to count atoms. The definition of a mole is based on the number of carbon-12 atoms in 12g of carbon-12. This just happens to be 6.022 × 1023 atoms. As painful as studying for a physics midterm can be, we count our lucky stars that we aren't the poor schmuck who had to count carbon atoms for a living.Open System
A system where both energy and mass can leave. As an open relationship, everything in the system just kind of does what it wants.Phase Changes
The six possible exchanges between solids, liquids, and gases. They are deposition, vaporization, condensation, sublimation, melting, and freezing.- A solid melts into a liquid or sublimes directly into a gas.
- A liquid can evaporate into a gas or freeze into a solid.
- A gas condenses into a liquid or deposits into a solid.
Second Law Of Thermodynamics
This law says that heat will always flow from warm to cold in a spontaneous process. Another way of putting it is that the entropy of a system can only increase or stay the same. Even though energy is conserved, we can never turn all of the available heat energy into mechanical work. Just like we can never turn back the clock. Well, as long as the clock is a metaphor for the passage of time, rather than an actual clock.Thermal Conductivity, k
A measure of how well a material conducts heat, measured in
. Each and every material has its own constant of thermal conductivity—it's like a fingerprint.