Energy: Home Energy Audit
Energy Transfers and the First Law of Thermodynamics
In the 1800's, scientists found, empirically, that rules exist that determine how energy can be transferred.
The first of these rules is called the First Law of Thermodynamics.
This law is usually stated as, "Energy
can neither be created nor destroyed; it can only be transferred from one form to another."
leads to the re-titling of this law as the Conservation of Energy Principle since it says that energy must be
This statement of the First Law does not say anything about how energy can be transferred, though.
turns out that there are only two ways.
This was discovered in 1850 by the English scientist James Joule,
who found that heat and work are equivalent methods for changing the energy of an object.
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 m
Using this, we can
re-write the First Law mathematically as
E = W + Q
E is the change in 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 exchange either work or heat with it.
The discovery of the laws of thermodynamics was extremely important, as our need to understand energy
is fueled by the overwhelming use of energy in human society.
From the earliest days, humankind has
recognized the need to use energy to condition the environment around it.
Wood was needed to heat
homes and to cook food.
Beasts of burden were needed to plow fields and to provide transportation.
When either of these commodities became scarce, hardship prevailed, and solutions were sought.
ancient Rome, for example, the lack of available firewood led to the passing of laws that made it illegal to
build a house or structure that would block another person's home from getting sunlight, as this was the
primary method of heating homes without fire.
Fig. 1: U.S. Oil Consumption (Source: DOE)
In the 20th century, fossil fuels (oil in particular)
reigned supreme as the energy of choice.
ubiquitous nature created historically low prices for
This led to a substantial increase in the
number of mechanized tools used by the average
By the year 2000, the U.S. had a population
of about 283 million people that were driving over
200 million passenger vehicles.
Almost every home
in America has a television, some type of range or
stove, and a refrigerator.
About 3/4 of all
households have their own washer, dryer, and air
Of course, this cheap price does not
come without some political and economic
Energy, and oil in particular, have