Thermodynamic Laws

 








A thermodynamic system is one that
interacts and exchanges energy with
the area around it. The exchange and
transfer need to happen in at least two
ways. At least one way must be the
transfer of heat. If the thermodynamic
system is "in equilibrium," it can't
change its state or status without
interacting with its environment. Simply
put, if you're in equilibrium, you're a
"happy system," just minding your own
business. You can't really do anything. If
you do, you have to interact with the
world around you.

A Zeroth Law?

The zeroth law of thermodynamics will be
our starting point. We're not really sure
why this law is the zeroth. We think
scientists had "first" and "second" for a
long time, but this new one was so
important it should come before the
others. And voila! Law Number Zero!
Here's what it says: When two systems
are sitting in equilibrium with a third
system, they are also in thermal
equilibrium with each other.
In English: systems "One" and "Two" are
each in equilibrium with "Three." That
means they each have the same energy
content as "Three". But if THAT’S true,
then all the values found in "Three",
match those in both "One" and "Two".
It’s obvious, then, that the values of
"One" and "Two" must ALSO match. This
means that "One" and "Two" have to be
in equilibrium with each other.

A First Law

The first law of thermodynamics is a little
simpler. The first law states that when
heat is added to a system, some of that
energy stays in the system and some
leaves the system. The energy that
leaves does work on the area around it.
Energy that stays in the system creates
an increase in the internal energy of the
system.
In English: you have a pot of water at
room temperature. You add some heat to
the system. First, the temperature and
energy of the water increases. Second,
the system releases some energy and it
works on the environment (maybe
heating the air around the water, making
the air rise).

A Second Law

The big finish! The second law of
thermodynamics explains that it is
impossible to have a cyclic (repeating)
process that converts heat completely
into work. It is also impossible to have a
process that transfers heat from cool
objects to warm objects without using
work.
In English: that first part of the law says
no reaction is 100% efficient. Some
amount of energy in a reaction is always
lost to heat. Also, a system can not
convert all of its energy to working
energy.
The second part of the law is more
obvious. A cold body can't heat up a
warm body. Heat naturally wants to flow
from warmer to cooler areas. Heat wants
to flow and spread out to areas with less
heat. If heat is going to move from cooler
to warmer areas, it is going against what
is “natural”, so the system must put in
some work for it to happen.