Energy Conversions
When energy is changed from one form to another form, this is an energy conversion.
Mechanical energy conversions:
The mechanical energy of a system is the sum of the kinetic and potential energy. ME = KE + PE
A bouncing ball. The falling ball has kinetic energy. When the ball strikes the ground the bottom of the ball
stops, the top of the ball keeps moving. The ball stretches as it becomes flatter. Potential energy is stored in the
stretched ball. The stretched ball
expands back to round, sending
the ball up. The ball reaches its
highest point, stops and starts to
fall—the process repeats, until the
ball stops bouncing.
Each time the ball stretches, some of the KE is converted into thermal energy, so there is less PE than KE. This
is why the ball bounces lower and lower and finally stops. When the ball finally stops, all of the ME has been
converted into thermal energy.
Pendulums are another example of KE to PE and back to KE
KE = 0
conversions.
PE Maximum
As the weight starts to swing, it begins to speed up. PE is
converted into KE. At the bottom of the swing, the PE is 0 and
the KE is at the maximum amount. On the way back up the
KE is converted back into PE. At the very top of the swing,
KE
the KE is 0 and PE is at the maximum amount
increasing
PE
The pendulum eventually stops because some of the energy is
decreasing
KE = 0
being converted into thermal energy by friction. There is
PE Maximum
friction in the string and friction against the air. Because some
of the KE energy is converted into thermal energy, the PE is
always less than the KE, so the pendulum can never get back to as high as when it started.
KE = 0
PE Maximum
KE
decreasing
PE
increasing
Chemical energy conversions.
Food:
Food is a good example of chemical energy conversions. Nuclear energy in the sun is created when matter is
converted into energy. The nuclear energy makes thermal energy (the sun is hot), the hot sun gives off light
energy, the light energy makes it to the Earth where plants use photosynthesis to convert the light energy into
chemical energy. Photosynthesis allows plants to make sugar from water and carbon dioxide. Plants and
animals change sugar into other molecules, which we eat when we eat food. In our bodies digestion breaks the
food down back into sugar. In our cells sugar is combined with oxygen to make water and carbon dioxide. The
energy from “burning” sugar makes a chemical called ATP. ADP + P = ATP. ATP is the chemical which
powers all living things. When your muscles move, they break apart ATP and convert the chemical energy into
kinetic energy.
Thermal energy—all of these energy conversions also convert some of the energy into thermal energy (heat).
Your body has a temperature of 37° C (98.6° F) because of this thermal energy. When you exercise, your
muscles change more ATP into kinetic energy, which makes more waste heat. Your body must get rid of this
extra heat. Sweat is your body’s attempt to get rid of extra thermal energy.
Car and trucks:
Gas and Diesel powered engines use chemical energy stored in petroleum (oil). The oil has chemical energy
which started as sugar made by plankton in the ocean. When the plankton die, they fall to the bottom along with
the oil trapped in their shells. The original source of the energy in oil was the light energy from the sun. When
gas or diesel is burned the chemical energy is converted into thermal energy. The thermal energy makes the
gasses in the engine cylinder expand, which moves the crankshaft, converting thermal energy into kinetic
energy. The crankshaft moves the wheels and the car or truck moves. Almost all of the energy in gas or diesel is
eventually converted into thermal energy. Some of the thermal energy causes the engine to be hot and is gotten
rid of using the radiator. The thermal energy which is converted into kinetic energy is eventually changed back
into thermal energy by friction—either air or rolling, or by the brakes.
Electrical energy conversions.
Making electrical energy:
Modern society uses electrical energy as one form of energy we use the most.
Most electrical energy is made by generators. (The exceptions are solar cells and thermocouples). A generator
makes electrons move in a wire when a magnet moves near the wire. Most generators spin. Any form of energy
which will spin the generator can be used to make electrical energy. In the Oregon most of our electrical energy
comes from dams on the Columbia River system. Water running from the bottom of the dam spins a turbine
(kinetic energy) which turns the generator to make electrical energy. Wind energy can also be used to turn
turbines.
Other forms of energy can be used to make electricity. Nuclear, geothermal , solar, or chemical (coal, oil, or
natural gas) are converted into thermal energy and make water boil. Boiling water makes steam, which turns a
turbine converting thermal energy into kinetic energy. The turbine turns a generator which converts kinetic
energy into electrical energy.
Using electrical energy:
At your home, the electrical energy can be converted back into other forms. If light is desired, electrical energy
can be used to make electrons give off light. Incandescent and fluorescent lights cause electrons to give off light
by making atoms hot. Some devices make light directly without heat like LEDs and LCDs. Electric motors are
like a reverse of generators—electrical energy makes a magnet spin near a wire, so electrical energy can be
converted into kinetic energy. Sound energy (music) is made when electricity causes a speaker coil to move
near an magnet.