On this page: You will recall that the First Law of
thermodynamics, expressed as ΔU = q + w, is essentially a
statement of the law of conservation of energy. The significance
of this law is that it tells us that any proposed process that
would violate this condition can be dismissed as impossible,
without even inquiring further into the details of the process.
For simple mechanical operations on macroscopic objects, the
First Law, conservation of energy, is all we usually need to
determine such things as how many joules of energy is required
to lift a weight or to boil some water, how many grams of
glucose you must metabolize in order to climb a hill, or how
much fuel your car needs to drive a given distance. But if you
think about it, there are a number of "simple mechanical
operations" that never occur, even though they would not violate
energy conservation. What do all these scenarios that conform to
the First Law but are neverthless never seen to occur have in
common? In every case, energy becomes less sp