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ENERGY CONSERVATION individual to another, energy flow from one object to another
Energy can be transferred from one object to another, and it can requires a mode of transfer. In energy matters, the mode of
be converted from one form to another form. If you make a de- transfer is working or heating. Any time you see working or
tailed accounting of all forms of energy before and after a transfer heating occurring, you know that an energy transfer is taking
or conversion, the total energy will be constant. Consider your bi- place. The next time you see heating, think about what energy
cycle coasting along over level ground when you apply the brakes. form is being converted to what new energy form. (The final
What happened to the kinetic mechanical energy of the bicycle? It form is usually radiant energy.) Heating is the topic of chapter 4,
went into heating the rim and brakes of your bicycle, then eventu- where you will consider the role of heat in energy matters.
ally radiated to space as infrared radiation. All radiant energy that
reaches Earth is eventually radiated back to space (Figure 3.18).
Thus, throughout all the form conversions and energy transfers
that take place, the total sum of energy remains constant. Myths, Mistakes, & Misunderstandings
The total energy is constant in every situation that has been
measured. This consistency leads to another one of the conser- Leave the Computer On?
vation laws of science, the law of conservation of energy:
It is a myth that leaving your computer on all the time uses less
Energy is never created or destroyed. Energy can be energy and makes it last longer. There is a very small surge when
converted from one form to another, but the total energy the computer is first turned on, but this is insignificant com-
remains constant. pared to the energy wasted by a computer that is running when
it is not being used. In the past, cycling a computer on and off
You may be wondering about the source of nuclear energy. may have reduced its lifetime, but this is not true of modern
Does a nuclear reaction create energy? Albert Einstein answered computers.
this question back in the early 1900s, when he formulated his
2
now-famous relationship between mass and energy, E = mc .
This relationship will be discussed in detail in chapter 13. Basi-
cally, the relationship states that mass is a form of energy, and
this has been experimentally verified many times. 3.4 ENERGY SOURCES TODAY
Prometheus, according to ancient Greek mythology, stole fire
ENERGY TRANSFER from heaven and gave it to humankind. Fire has propelled hu-
Earlier it was stated that when you do work on something, you man advancement ever since. All that was needed was some-
give it energy. The result of work could be increased kinetic thing to burn—fuel for Prometheus’s fire.
mechanical energy, increased gravitational potential energy, Any substance that burns can be used to fuel a fire, and
or an increase in the temperature of an object. You could sum- various fuels have been used over the centuries as humans
marize this by stating that either working or heating is always advanced. First, wood was used as a primary source for heating.
involved any time energy is transformed. This is not unlike Then coal fueled the Industrial Revolution. Eventually, human-
your financial situation. To increase or decrease your financial kind roared into the twentieth century burning pe troleum.
status, you need some mode of transfer, such as cash or checks, According to a 2005 report on primary energy consumed in the
as a means of conveying assets. Just as with cash flow from one United States, petroleum was the most widely used source of
energy (Figure 3.19). It provided about 40 percent of the total
energy used, and natural gas contributed about 23 percent of
the total. The use of coal provided about 23 percent of the to-
tal. Biomass, which is any material formed by photosynthesis,
contributed about 3 percent of the total. Note that petroleum,
Nuclear
coal, biomass, and natural gas are all chemical sources of energy,
sources that are mostly burned for their energy. These chemical
Radiant
Radiant sources supplied about 89 percent of the total energy consumed.
About one-third of this was burned for heating, and the rest was
burned to drive engines or generators.
Nuclear energy and hydropower are the nonchemical
sources of energy. These sources are used to generate electri-
cal energy. The alternative sources of energy, such as solar
and geo thermal, provided about 1 percent of the total energy
consumed.
Chemical Chemical Mechanical Heating The energy-source mix has changed from past years, and
it will change in the future. Wood supplied 90 percent of the
FIGURE 3.18 Energy arrives from the Sun, goes through a
number of conversions, then radiates back into space. The total energy until the 1850s, when the use of coal increased. Then,
sum leaving eventually equals the original amount that arrived. by 1910, coal was supplying about 75 percent of the total energy

74 CHAPTER 3 Energy 3-14