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Thermodynamics

Heat Engines

The working fluid (say, water for a steam engine) of the heat engine receives heat Qh from the burning fuel (diesel oil, for example) which converts it to steam. The steam expands, pushing on the piston so that it does work W; as it expands, it cools and the pressure decreases. It then traverses a condenser, where it loses an amount of heat Qc to the coolant (cooling water or the atmosphere, for example), which returns it to the liquid state. The second law says that, if the working fluid (in this case the water) is to be returned to its original state so that the heat-work process could begin all over again, then some heat must be rejected to the coolant. Since the working fluid is returned to its original state, there is no change in its internal energy, so that the first law demands that Qh - Qc=W. The efficiency of the process is the amount of work obtained for a given cost in heat input: E = W/Qh. Thus, combining the two laws, E = (Qh-Qc)/Qh. It can be seen therefore that a heat engine can never run at 100% efficiency.

It is important to note that the laws of thermodynamics are of very great generality, and are of importance in understanding such diverse subjects as chemical reactions, very low temperature phenomena, and the changes in the internal structure of solids with changes in temperature, as well as engines of various kinds.

See also Gases, properties of.

Resources

Books

DiLavore, Philip, Energy: Insights from Physics. New York: Wiley, 1984.)

Goldstein, Martin, and Inge F. Goldstein. The Refrigerator and the Universe. Cambridge: Harvard University Press, Cambridge, 1993.


David Mintzer

KEY TERMS

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Adiabatic process

—A process during which no heat is transferred between the system and surroundings is described as "adiabatic."

Avogadro's number

—The number of molecules present in one mole of whatever the compound is always equal to 6.0229 × 1023. It was named for the Italian physicist Amedeo Avogadro.

Boiling point

—The boiling point of a liquid is the temperature at which it boils, also the temperature at which its vapor condenses.

Calorie

—The amount of heat necessary to increase the temperature of water by one degree Celsius.

Celsius temperature (°C)

—The temperature scale on which the freezing point of water is 0° and the boiling point is 100°.

Change in phase

—Change in the form and characteristics of a substance, e.g., changes from gas to liquid, or liquid to solid.

Coefficient of linear expansion (β.α)

—The fractional rate of change in length of an object with a change in temperature.

Condensation temperature

—The temperature at which a gas changes into a liquid (equal to the boiling point).

Equation of state

—Relationship among the (experimentally determined) variables, which give complete information about the state of a system.

Fahrenheit temperature (°F)

—The temperature scale on which the freezing point of water is 32° and the boiling point is 212°.

First law of thermodynamics

—The internal energy of a system is increased by the amount of work done on the system and the heat flow to the system (conservation of energy).

Heat of condensation

—The amount of heat needed to be removed from a gas to change it to its liquid phase (equal to the heat of vaporization).

Heat of fusion

—The amount of heat needed to be added to a solid to change it to its liquid phase.

Heat of solidification

—The amount of heat needed to be removed from a liquid to change it to its solid phase (equal to the heat of fusion).

Heat of vaporization

—The amount of heat needed to be added to a liquid to change it to its gaseous phase.

Ideal gas

—A gas obeying the ideal gas equation of state, pV = nRT, where, e.g., p is in Newtons/meter2, V is in m3, n is the number of kilomoles of the gas, T is the temperature in K, and R = 8.31 kJ/kmolK.

Internal energy

—The change in the internal energy of a system is equal to the amount of adiabatic work done on the system.

Kelvin temperature (K)

—The Celsius temperature plus 273.15°C.

Kilomole (kmol)

—A quantity of matter equal to M kilograms, where M is the molecular weight of the substance, with carbon-12 being taken as M = 12 (one kilomole equals 1,000 moles).

Macroscopic theory

—A theory which ignores the molecular nature of matter.

Melting point

—The temperature at which a solid changes into a liquid.

Microscopic theory

—A theory which is based on the molecular nature of matter.

Second law of thermodynamics

—No process is possible whose only result is the transfer of heat from a cooler to a hotter object (Clausius statement). No process is possible whose only result is the conversion of heat into an equivalent amount of work (Kelvin Planck statement).

Solidification temperature

—The temperature at which a liquid changes into a solid (equal to the melting point).

Specific heat

—The amount of heat needed to increase the temperature of a mass of material by one degree.

Statistical mechanics

—The microscopic theory of matter for which the macroscopic theory is thermodynamics, or the molecular basis of thermodynamics.

Sublimation

—The change of a material from its solid phase directly to its gaseous phase.

Temperature (T)

—The (experimentally determined) variable which determines the direction of heat flow; the variable which is common to all equations of state.

Thermal equilibrium

—A condition between two or more objects in direct thermal contact in which no energy as heat flows from one to the other. The temperatures of such objects are identical.

Universal gas constant (R)

—The constant in the ideal gas equation of state (as well as elsewhere); equal to 8.31 kJ/kmolK.

Additional topics

Science EncyclopediaScience & Philosophy: Thallophyta to ToxicologyThermodynamics - Historical Background, Temperature, Expansion Coefficients, Thermostats, Water, Heat, The First Law Of Thermodynamics - Conservation of energy