the way that it does. The Laws of Thermodynamics give us the whats and whys of heat flow.
The laws of thermodynamics are a bit strange. There are four of them, but they are ordered zero to
three, and not one to four. They weren’t discovered in the order in which they’re numbered, and
some—particularly the Second Law—have many different formulations, which seem to have
nothing to do with one another.
There will almost certainly be a question on the Second Law on SAT II Physics, and quite
possibly something on the First Law. The Zeroth Law and Third Law are unlikely to come up,
but we include them here for the sake of completion. Questions on the Laws of Thermodynamics
will probably be qualitative: as long as you understand what these laws mean, you probably won’t
have to do any calculating.
Zeroth Law
If system A is at thermal equilibrium with system B, and B is at thermal equilibrium with system
C, then A is at thermal equilibrium with C. This is more a matter of logic than of physics. Two
systems are at thermal equilibrium if they have the same temperature. If A and B have the same
temperature, and B and C have the same temperature, then A and C have the same temperature.
The significant consequence of the Zeroth Law is that, when a hotter object and a colder object are
placed in contact with one another, heat will flow from the hotter object to the colder object until
they are in thermal equilibrium.
First Law
Consider an isolated system—that is, one where heat and energy neither enter nor leave the
system. Such a system is doing no work, but we associate with it a certain internal energy, U,
which is related to the kinetic energy of the molecules in the system, and therefore to the system’s
temperature. Internal energy is similar to potential energy in that it is a property of a system that is
doing no work, but has the potential to do work.
The First Law tells us that the internal energy of a system increases if heat is added to the system
or if work is done on the system and decreases if the system gives off heat or does work. We can
express this law as an equation:
where U signifies internal energy, Q signifies heat, and W signifies work.
The First Law is just another way of stating the law of conservation of energy. Both heat and work
are forms of energy, so any heat or work that goes into or out of a system must affect the internal
energy of that system.
EXAMPLE