Introduction
Energy

As we learned in the book, nothing happens without energy.  The transfer of energy from one form to another is what causes objects to move.  Without it, there would be no motion, and the universe would be a very dull place (although science would be greatly simplified, making this class a breeze to pass).

But what is energy? We are all very familiar with the term, having heard it or used it many times, as in "I just don't have any energy today." Since we are all so familiar with the word energy, one would think that we would all be experts on the subject. However, our use of the word "energy" in our everyday lives is somewhat different from what the word means in a scientific sense. For our purposes, we are going to define energy as "the ability to do work." Of course, this definition is not as useful as it would appear, since the word "work" in this definition also has little to do with our everyday usage. In this definition, work means "the transfer of energy to an object by applying a force through a distance." Putting these two definitions together, we see that energy is the ability to change the motion of objects.

We encounter energy throughout our day in many different varieties.  The gasoline that we put into our cars is a form of chemical energy.  When we ignite it in a piston chamber, it causes the air in the chamber to expand and to push against the piston, which responds by moving and propelling the car forward (or backwards, depending upon the position of our gear shift).  The electricity that we use in our homes and offices is a form of kinetic energy.  As the electrons move through  the wires, they do work by either colliding with other particles or by creating magnetic fields that displace other objects.  Even the food that we put into our bodies is a form of energy.  It fuels our muscles and gives them the ability to contract.

Energy Transfer

In the 1800's, scientists found, empirically, that rules exist that govern how energy can be transferred.  The first of these rules is called the First Law of Thermodynamics.  In most circles, this law is stated as, "Energy can neither be created nor destroyed; it can only be transferred from one form to another."  In mathematical terms, the First Law is normally stated as 

DE = W + Q 

where E is the energy of an object, W is the work done on the object, and Q is the heat added to an object.  In laymen's terms, this means that the only way to change the energy of an object is to either do work on it or add heat to it.  It was not until 1850 that the English scientist James Joule discovered that heat and work are equivalent methods for changing the energy of an object.  In his experimental work, Joule was able to show that he could increase the thermal energy of a pot of water by either placing it over a flame (adding heat), or by stirring it with a paddle (doing work).  For this and other important work in this area, the SI unit of energy is called a joule (1 J = 1 kg m2/sec2). 

The First Law of Thermodynamics tells us that the energy involved in any transfer must be conserved.  This would seem to mean that we should never run out of energy and should pay no heed to anybody talking about an energy crisis.  The problem is that this is not the only law that governs energy transfers.  While the total amount of energy does not change, the Second Law of Thermodynamics puts limits on the amount of usable energy that can be transferred.   One of the consequences of this law is that the total amount of usable energy that comes out of any process will be less than the total amount of energy that went into the process.  The difference between the total amount of energy input and the usable energy output is expended as waste heat.

Next Page