Study Guide

Thermodynamics Terms

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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 vapor pressure of a liquid is equal to the atmospheric pressure.

Boltzmann's Constant,

The equivalent of the ideal gas constant R on an individual particle level, calculated by .

Boyle's Law

The pressure of an ideal gas is inversely proportional to its volume if the temperature and amount of gas is held constant.

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.


Heat transfer between two objects that are touching


Heat transfer between objects and the liquid or air that surrounds them.


A measurable quantity, often noted as ΔS for kicks, that represents the disorder in a system or surroundings. More technically, knowing the entropy of a system can help us determine the energy available for work in a process. That's right, Nature makes all of her randomness available for good use.

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 = QW. 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 vapor pressure of a liquid is equal to the vapor pressure of the corresponding solid.


A means by which energy is transferred from a hot body to a colder body when the two are placed in thermal contact with one another.

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 product of pressure and the volume of an ideal gas is directly proportional to the number of moles of the gas and the temperature.

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

The mass in one mole of a substance.


A measurement of an amount of a substance, always equal to 6.023 × 1023 units.

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.

Thermal expansion

Heat causes materials to expand, because higher temperatures create greater molecular motion. Each material has a unique coefficient of expansion, any change to its length (or area, or volume) is proportional to the change in temperature.

Thermal equilibrium

The temperature all objects in a system reach after some time together. If two or more objects are placed next to each other, so that they are touching, after some time they'll both be the same temperature. For example, consider an ice cream cone outside on a hot summer day. The hot summer day wins hands down: there is so much air outside that the poor cone doesn't have a chance of cooling us down unless ingested into our own systems.

Zeroth Law of Thermodynamics

The law backing up thermal equilibrium. If Thing 1 and Thing 2 are in thermal equilibrium, and Thing 2 and Thing 3 are also in thermal equilibrium, then Thing 1 and Thing 3 are too. It's just a fancy way of saying that everything that has the same temperature has the same temperature.

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