THE LAWS OF THERMODYNAMICS

Charles Percy Snow once summed up the laws of thermodynamics in a book called “The Two Cultures and the Scientific Revolution”. In his words the laws are as follows:

1^st Law

You can't win!!!

2^nd law

You can't break even

3^rd Law

You can't get out of the game.

Although this appears a little misleading once you understand the laws of thermodynamics these words take on new meanings and provide a easy way to remember the laws themselves.

The First Law of Thermodynamics

The first law is really just a statement about conservation of energy and it is as follows:

Energy can neither be created nor destroyed!!

OR

ΔU = Q + W

(Recall the U is internal energy, Q is heat and W is work)

This means that you can never get more out of a system then what you put into the system ,“You can't win”. A careful thing to note about this law is the sign on W. Chemists tend to use a negative sign rather than a positive sign, this is merely a convention and is determined by whether W is work done on the system or by the system

The Second Law of Thermodynamics

This law can be defined in a couple of ways:

• Heat flows from a hot object to a colder object.

• A given amount of heat can not be changed completely into energy to do work.

The first definition may seem kind of obvious to you but I will give you a simple example of why it is required.

When you hold a rock in the air it has gravitational potential energy. If you release that rock that potential energy turns into kinetic energy as the rock falls, finally when the rock strikes the ground that energy is converted into heat and remains in the rock as internal energy. Now try to think of the reverse from happening, why does a rock on the ground jump into the air and stay at a certain height when it is heated. According to the first law of thermodynamics this would be a perfectly fine situation, the rock could fly into the air and still conserve energy. The second law of thermodynamics explains why this situation does occur.

The second definition tells us that we can never get what we put into a system out as usable work this is where CP Snow applied his definition “You can't break even!!!”.

The Third Law of Thermodynamics

The energy of a system tends towards a constant as absolute zero is approached.

OR

You can't reach absolute zero and you can't remove all the energy from a system.

Absolute zero is -273 degrees Celsius. This is the point in which all motion of molecules stops. The reason that absolute zero can not be reached is not simple but I will try here to give a brief explanation.

Recall that heat flows from a hot object to a cold object. So in order to remove heat from something you must have a heat sink at a lower temperature. So to get to absolute zero you would need something that is at a lower temperature than -273 degrees Celsius. Such a mechanism is impossible.

“You can't get out of the game!!!!”