There are a number of equivalent forms of the Second Law, each of which sounds quite different
from the others. Questions about the Second Law on SAT II Physics will invariably be qualitative.
They will usually ask that you identify a certain formulation of the Second Law as an expression
of the Second Law.
The Second Law in Terms of Heat Flow
Perhaps the most intuitive formulation of the Second Law is that heat flows spontaneously from a
hotter object to a colder one, but not in the opposite direction. If you leave a hot dinner on a table
at room temperature, it will slowly cool down, and if you leave a bowl of ice cream on a table at
room temperature, it will warm up and melt. You may have noticed that hot dinners do not
spontaneously get hotter and ice cream does not spontaneously get colder when we leave them
out.
The Second Law in Terms of Heat Engines
One consequence of this law, which we will explore a bit more in the section on heat engines, is
that no machine can work at 100% efficiency: all machines generate some heat, and some of that
heat is always lost to the machine’s surroundings.
The Second Law in Terms of Entropy
The Second Law is most famous for its formulation in terms of entropy. The word entropy was
coined in the 19th century as a technical term for talking about disorder. The same principle that
tells us that heat spontaneously flows from hot to cold but not in the opposite direction also tells us
that, in general, ordered systems are liable to fall into disorder, but disordered systems are not
liable to order themselves spontaneously.
Imagine pouring a tablespoon of salt and then a tablespoon of pepper into a jar. At first, there will
be two separate heaps: one of salt and one of pepper. But if you shake up the mixture, the grains of
salt and pepper will mix together. No amount of shaking will then help you separate the mixture of
grains back into two distinct heaps. The two separate heaps of salt and pepper constitute a more
ordered system than the mixture of the two.
Next, suppose you drop the jar on the floor. The glass will break and the grains of salt and pepper
will scatter across the floor. You can wait patiently, but you’ll find that, while the glass could
shatter and the grains could scatter, no action as simple as dropping a jar will get the glass to fuse
back together again or the salt and pepper to gather themselves up. Your system of salt and pepper
in the jar is more ordered than the system of shattered glass and scattered condiments.
Entropy and Time
You may have noticed that Newton’s Laws and the laws of kinematics are time-invariant. That is,
if you were to play a videotape of kinematic motion in reverse, it would still obey the laws of
kinematics. Videotape a ball flying up in the air and watch it drop. Then play the tape backward: it
goes up in the air and drops in just the same way.
By contrast, you’ll notice that the Second Law is not time-invariant: it tells us that, over time, the
universe tends toward greater disorder. Physicists suggest that the Second Law is what gives time
a direction. If all we had were Newton’s Laws, then there would be no difference between time
going forward and time going backward. So we were a bit inaccurate when we said that entropy
increases over time. We would be more accurate to say that time moves in the direction of entropy
increase.
Third Law
It is impossible to cool a substance to absolute zero. This law is irrelevant as far as SAT II Physics