Energy is
usually defined as "the ability to do work." In this definition, the term
work is not being used like we use it in English. So, in order to understand energy,
you must understand work. Work is defined as "the result of a
force acting on a body and producing motion." If you push a desk across the
floor you are doing work on the desk because you are exerting a force that causes
motion. You use energy to do the work. The amount of work done is equal to the
force used multiplied by the displacement of the object (W=Fxd). Greater amounts of
work require greater amounts of energy. The SI unit of work or energy is the joule
(J). You will learn much more about work in your study of Physics.
The two basic categories of energy are
potential energy and kinetic energy.
Potential Energy
It is probably easiest to think of potential
energy as "stored" energy. It is also defined as "energy of
position."
Gravitational potential energy is "stored"
energy that an object has due to its weight and its position with reference to some other
point. A bowling ball has more gravitational potential energy sitting on a shelf,
than does a ping-pong ball sitting on the same shelf. The same bowling ball would
have even more gravitational potential energy, with reference to the floor, if it
were on a higher shelf.
Chemical energy is stored in foods and fuels, and can be
released when these compounds undergo chemical reactions. You probably remember,
from Biology, how energy is released from glucose during the process of respiration,
as shown below:
C6H12O6 + 6O2
---> 6H20 + 6CO2 + ENERGY
Kinetic Energy
Kinetic Energy is defined as
"the energy of motion." A fast moving car has a great deal of kinetic
energy, based on both its mass and velocity (speed). When a car crashes into the
back of another car, it transfers some of its kinetic energy into the car in front of it.
When a car going 60 mph hits a parked car, the parked car does not move away at 60
mph. In a future lesson, we will discuss where the rest of the kinetic energy goes.
Billiard balls are good models for kinetic energy as well. Before
the break shot, the balls have no kinetic energy with reference to the table.
Energy comes from the moving cue stick to set the balls in motion.
Thermal Energy is defined as the energy that a
substance has due to the chaotic motion of its molecules. Molecules are in constant
motion, and always possess some amount of kinetic energy. In fact, when you measure
the temperature of an object, you are measuring the average kinetic energy of the
molecules of that object. Does that mean when something has a temperature below 0oC
it has negative kinetic energy? Look for the answer to that question in lesson 2-9.
Conservation of Energy
Similar to the law of conservation of
mass, the law of conservation of energy states that "energy is conserved", or
"energy can neither be created nor destroyed." Like the aforementioned
law, this law does not hold true in the case of nuclear reactions, but it does hold true
for the reactions that we encounter in our everyday life.
You know of course that the engine does not
create the energy that powers your car. The engine is a machine that allows the stored
chemical energy of gasoline to be transformed into mechanical energy that drives the
wheels of the car. Energy is not always found in a convenient form, so many of man's
inventions are designed to transform one type of energy to another. Below are a few
examples of what I mean.
1. Different types of stoves are used to transform the
chemical energy of the fuel (gas, coal, wood, etc.) into heat energy.
2. Solar collectors can be used to transform solar energy
into electrical energy.
3. Wind mills make use of the kinetic energy of the air
molecules, transforming it to mechanical or electrical energy.
4. Hydroelectric plants transform the kinetic energy of
falling water into electrical energy.
Now, if energy is always conserved, why does
it seem that energy is sometimes lost? For example, a "break shot" in a
game of billiards causes the balls to bounce around on the table for a period of
time. We transfer kinetic energy from the cue stick - to the cue ball - to the other
balls. Eventually the balls on the table stop moving. If energy were
conserved, wouldn't the balls continue to move? Well, energy is lost, but it is not
destroyed. Some of the kinetic energy is transformed into various types of energy
which the billiard balls can't make use of. A large amount of the kinetic
energy is turned into heat energy because of the friction between each ball and the
surface of the table. That is why more expensive pool tables are made with certain
materials, which will cause less friction
Chemical Reactions
A chemical reaction is a series of changes
that results in the production of one or more new substances. These chemical
changes, which were introduced to you in lesson 1-5, are always
accompanied by a change in energy. That means that either energy is given off during
the reaction, or energy is taken in.
Reactions that release energy are called exothermic.
In this type of reaction, the products have less potential chemical energy than the
reactants, because energy was given off in the form of heat. When you stand next to
a barbecue grill, you feel the heat being released by the combustion reaction that is
taking place around the burners. The reaction of the propane gas found in grills is
shown below:
C3H8 + 5O2
---> 4H2O + 3CO2 + energy
propane + oxygen yields water +
carbon dioxide + energy
Reactions, which take in energy, are called endothermic.
In this type of reaction, the products have more potential chemical energy than the
reactants. Think of the chemical reaction that takes place in
"cold-packs." A seal is broken that separates two containers with the
plastic bag. As the contents from the separate containers begin to react, energy is
absorbed from the surroundings. If you place the cold-pack on your body, your body
begins to supply some of the energy that is required to get the reaction going. What
you experience as "cold" has to do with the temperature of that area of your
body changing as heat flows to the cold-pack.
Some exothermic reactions require
some energy to get them started, but then they release more energy than they originally
took in. Think of the fact that a match requires initial energy, provided by the
friction between it and the sandpaper on the matchbook, to start burning. Once the
match starts burning, it releases more energy than it took in, so the reaction is still
exothermic. The products still have less potential chemical energy than the
reactants. The initial energy that is required to get the reaction to begin is
called activation energy.
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and the online
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