Available Energy
Energy is conserved — the total never changes. But not all energy can do useful work. The fraction of energy that can actually drive a process, run a machine, or power a living cell is called the available energy (also called free energy, or exergy in engineering). The rest is unavailable — locked up in disordered thermal motion at a temperature too uniform to flow anywhere.
The governing concept is entropy. The second law of thermodynamics says: in any spontaneous process, total entropy (disorder) increases or stays the same. This places a ceiling on efficiency. Heat engines (the steam engine, the internal combustion engine, power plants) cannot convert heat to work at 100% efficiency; some heat must be dumped to a colder reservoir. The maximum possible efficiency is set by the Carnot limit: η = 1 − T_cold/T_hot.
Why This Matters Beyond Engines
The available energy concept is not just about heat engines. It governs why food must be continually eaten: metabolism is a chemical heat engine, and the free energy in food molecules is progressively degraded toward equilibrium. When the chemical gradients flatten out — when all the ATP is spent, all the sugars are oxidized — there is no available energy left even though the total energy is unchanged. The atoms are still there; the energy is still there; but it is now uniformly distributed as heat, and heat at uniform temperature can do no work.
The same principle explains the arrow of time. Ice in a glass melts but doesn't spontaneously re-form. Coffee mixes but doesn't unmix. These are one-way processes not because energy prohibits the reverse, but because the reverse would require a spontaneous decrease in entropy — an astronomically improbable state. The laws of physics are time-symmetric at the microscopic level; the one-way character of ordinary experience is statistical.
Total vs Available
Feynman notes the distinction explicitly: we have a total conservation law for energy, but the useful fraction keeps diminishing. This is why the energy crisis is not about running out of energy in the conservation-law sense — the energy in coal or oil does not disappear when you burn it. It becomes heat at low temperature. The problem is that low-grade thermal energy dispersed into the atmosphere is practically unavailable for anything useful. You haven't lost energy; you've lost organization.
Connections
- conservation-of-energy — total energy is conserved even as available energy degrades
- heat-as-atomic-motion — unavailable energy is thermal energy in disordered form
- e-equals-mc-squared — nuclear energy offers very high energy density; the available fraction from fission/fusion is relatively high