RMP Lecture Notes

Energy

One of our definitions of thermodynamics was "the basic science that deals with energy." So, what is energy?

the capability to produce an effect
the sum total of all capacity to do work

We're probably more familiar with the categories used when "counting" energy:

Potential Energy (E_P) -- depends on position within reference frame
Kinetic Energy (E_K) -- depends on movement within reference frame

Although both of these exist in both the macroscopic and microscopic reference frames, we'll generally use them only in the macro- sense.

Internal Energy (U) -- includes all chemical energy. It lumps together the microscopic energies, including E_P and E_K of molecular vibration, rotation, and translation. These can be expressed as functions of the temperature and density of the system and don't really vary much with the position or motion of the system. U thus depends on the thermodynamic state.

Note that energy may be stored (E_P, U) and/or transferred (heat, work). Heat and work are ways of transferring energy ("energy in transit") and so are measured in energy units.

It isn't really possible to identify a condition that has zero energy, because energy depends so much on our definitions and how we choose to make measurements. Consequently, all energy terms are relative to some reference state.

Physics has often defined work as force (needed to move an object) acting through a distance (moved). This is reflected in energy units:

These are the basic units of energy, heat, and work.

There is another set of units based on heat

calorie = heat needed to raise 1 gram of water by 1 degree Kelvin
BTU (British Thermal Unit) = heat needed to raise 1 pound of water by 1 degree Rankine

Strictly speaking, these units are not formally part of the unit systems. Practically, though, the BTU is probably used more often than the foot-pound.

Power is the rate of change/transport of energy.

Warning: kilowatt-hours measure energy not power

Review:

Force = mass * acceleration
Pressure = force / area
Work = force * distance
Power = energy / time

References:

Elliott, J.R. and C.T. Lira, Introductory Chemical Engineering Thermodynamics, Prentice Hall PTR, 1999, pp. 5-6.
Sonntag, R.E., C. Borgnakke, and G.J. Van Wylen, Fundamentals of Thermodynamics (6th ed.), John Wiley, 1998, p. 21-23.

R.M. Price
Original: 5/12/2003
Modified: 5/21/2004