Perpetual Motion Machines A Complete Analysis

The First Law


Statement of the First Law of Thermodynamics:

Energy cannot be created nor destroyed; it can only change forms. Energy is always conserved.


 

What is energy?

Ask any physicist what energy is and the most likely response will be something along the lines of:

"Energy is the ability to do work."

I believe this to be an eloquent way of describing what energy is with one exception,the word work is still not quite clear. Work is certainly something with which we are more familiar. Most of us have jobs and work something like 5 days a week! Is this what we mean by work? Well actually yes! In way it is! We are constantly expending energy in order to accomplish tasks within our daily lives. We lift things, we push things, we pull things and even the very act of moving ourselves takes work. So what allows us to do these things? Energy of course!

Where do we get our energy from? Well that is in reality a very complex process however simply put we get our energy from the chemical bonds that hold our foods together. When eat food our body begins a process of breaking down the food. These chemical reactions that take place are able to release energy into our bodies which we can then use to perform our daily tasks. It is actually quite amazing! So we that we have just discovered one source of energy, chemical energy! However is that the only source of energy? Well no, there are actually several sources of energy but we can categorize energy into two general forms. These two forms are energy are call kinetic energy and potential energy. Although there are many sources of energy, all of these sources can be described in terms of these two forms of energy. To clarify this point lets consider a simple pendulum. A pendulum is simply a device that swings back and forth and it gets its energy from gravity, which is another source of energy. The image below illustrates a simple pendulum for those of you who have not seen such a contraption before.

The most simple case would be when the pendulum is at equilibrium; which would be the bob is not moving and is simply in the center position. There is no kinetic energy or potential energy association with this state. When someone pulls the bob to one side they are effectively doing work on the bob and therefore are adding energy to the system. This work that is being done is a kind of lifting work. In order to move the bob we have to work against gravity which is always trying to pull the bob into its stationary equilibrium position. As the bob is held to one side and has not yet been released, the energy that is has is called potential energy. Potential energy is simply a form of stored energy. The potential to do work exists but it is not yet doing work. Once the bob is released it will swing from side to side. The reason the bob is able to move is because we put potential energy into it, however as soon as the bob is released that potential energy is converted into kinetic energy. Kinetic energy is energy due to motion. Recall if you will the statement at the top of the page, energy cannot be created nor destroyed it can only change forms! As the bob leaves the stationary position it begins to gain speed and as a result the potential energy is being converted into the kinetic energy of the bob. When the bob reaches the absolute bottom of the swing it has 100% kinetic energy and no potential energy. As the bob continues its swing however it will start to slow down until it reaches the end of its swing at which point it will be momentarily stationary once again. At this point all of the kinetic energy was transferred back into potential energy. Now the bob will switch direction and repeat the process of converting potential energy into kinetic energy. This process is illustrated in the animation below.

Now that we have an understanding of the two general forms of energy we can start to think about the different sources of energy.

Mathematical Description