T2. Heat Vs. Internal Energy

Heat is too often viewed as a form of energy (analogous to kinetic energy, potential energy, mass energy, etc.). It is measured, to be sure, in energy units; and when it is added to or subtracted from a system, that system gains or loses energy. But heat is best understood as a mode of energy transfer. Specifically, it is energy transfer by molecular collisions. (Work, in contrast, is energy transfer by a force acting over a distance, often at the macroscopic level.) If warm alcohol is added to cold water, the alcohol molecules lose energy and the water molecules gain energy. Energy has been transferred from alcohol to water, and the transfer, heat, has been effected by a multitude of molecular collisions. Heat, like work, is more a name for an energy currency than for a “real” energy. It is “energy in transit.”

Scientists call the energy content of a substance its internal energy, reserving the word heat for the gain or loss of internal energy brought about by molecular collisions. In the earliest days of thermodynamics, heat was used to designate both energy content and energy exchange. This was quite a natural point of view when heat (once called caloric) was itself regarded as a substance, stored in objects and capable of flowing from one object to another. Now, however, it is preferable to separate the two ideas. The main reason for this is that there are a variety of ways in which the internal energy of a system can be caused to change, heat being only one among the ways. Consider a certain quantity of air contained in a metal cylinder with a piston at one end. How can we increase its internal energy? One way is to heat the cylinder. The warm inner side of the cylinder transfers some energy to the trapped air as molecules in the air collide repeatedly with molecules in the metal surface. This is heat transfer. Another way is to move the piston inward. The force required to move it multiplied by the distance it moves is work, and this work is transferred to the internal energy of the air. The temperature of the air rises, just as if it had been heated. Colloquially we do say that it has been heated, and that it is hotter. Technically, we should say that its internal energy and its temperature have increased. When work is done on the air, it gains energy from the macroscopic bulk motion of the piston. When heat is transferred to the air, it gains energy from the microscopic motion of the molecules in the container.

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