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Energy Transfer
Energy can be transferred from one location to another,
as in the sun's energy travels through space to Earth.
The two ways that energy can be transferred are by doing
work and heat transfer.
| Doing Work |
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Energy can be transferred from one object to another by
doing work.
When work is done on an object, it results in a
change in the object's motion (more specifically, a
change in the object's kinetic energy).
Energy is often defined as the ability to do work. Work
equals force multiplied by distance. To learn more about
work please visit
What is Energy?
- Section C. Measuring and Quantifying
Energy if you have not already.
An illustration of how
doing work is an example of energy transfer
Suppose that a person exerts a force on the wheelbarrow
that is initially at rest, causing it to move over a
certain distance. Recall that the work done on the
wheelbarrow by the person is equal to the product of
the person's force multiplied by the distance
traveled by the wheelbarrow. Notice that when the
force is exerted on the wheelbarrow, there's a
change in its motion. Its kinetic energy increases.
But where did the wheelbarrow get its kinetic
energy? It came from the person exerting the force,
who used chemical energy stored in the food they ate
to move the wheelbarrow. In other words, when the
person did work on the wheelbarrow, they transferred
a certain amount of chemical energy stored in the
person was transferred to the wheelbarrow, causing
its kinetic energy to increase. As a result, the
person's store of chemical energy decreases and the
wheelbarrow's kinetic energy increases.
Wherever you look, you can see examples of energy
transfers. When you turn on a light, you see result of
energy being transferred from the sun to the plants to
the coal to electricity and finally to light you see.
During each of these transfers, energy changes form.
There are two main forms of energy, kinetic energy
(motion) and potential energy (position). To further
classify energy, these forms are sometimes further
described as thermal (heat), elastic, electromagnetic
(light, electrical, magnetic), gravitational, chemical
(food), and nuclear energy. See the
What is Energy?
- Section B. Two Main Forms of Energy for
more information on kinetic and potential energy.
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Heat Transfer |
Heat is given off when an object's thermal energy is
transferred. Thermal energy (see below) can be
transferred in three ways: by conduction, by
convection, and by radiation.
| 1. Conduction |
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Conduction is the transfer of energy from one
molecule to another. This transfer occurs
when molecules hit against each other, similar to a
game of pool where one moving ball strikes another,
causing the second to move. Conduction takes place
in solids, liquids, and gases, but works best in
materials that have simple molecules that are
located close to each other. For example, metal is a
better conductor than wood or plastic.
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2. Convection |
Convection is the movement of heat by a
liquid such as water or a gas such as air.
The liquid or gas moves from one location to
another, carrying heat along with it. This movement
of a mass of heated water or air is called a
current.
| 3. Radiation |
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Heat travels from the sun by a process
called radiation.
Radiation is the transfer of heat by electromagnetic
waves. When infrared rays strike a material,
the molecules in that material move faster. In
addition to the sun, light bulbs, irons, and
toasters radiate heat. When we feel heat around
these items, however, we are feeling convection heat
(warmed air molecules) rather than radiated heat
since the heat waves strike and energize surrounding
air molecules.
More about
Thermal (Heat) Energy
Heat is given off whenever energy is being used. You
can tell if a television has been on by feeling if
it is warm. When you run up a flight of stairs you
feel warm because you are burning food energy.
What exactly is heat? Heat is the transfer or
flow of energy from a hot object to one that is
cooler. When you feel a warm object, you are
actually feeling thermal energy, which is the
movement of molecules that make up the object. An
object has more thermal energy when it is warm than
when it is cool.
| The more thermal energy an
object has, the faster its molecules move. These
faster moving molecules bump into each other
more frequently and spread out as they require
more space (decreasing the density of the
molecules). Think of people standing in an |
| elevator. If they started moving
around, they would start bumping into each other
and need more space. This is essentially what
happens when molecules get more energy and start
moving around; they spread out. |
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| For the most
part, the volume of an object increases as
the amount of thermal energy it receives
increases. In other words, the molecules in
warmer objects are less densely packed than
the molecules in cooler objects. (NOTE:
Temperature is a measurement of how fast
molecules move.) |
You can't see thermal energy, but you can
detect evidence of heat transfer. You might see
the air shimmering over a radiator (convection),
put your hand on a warm spoon that's been
sitting in a hot bowl of soup (conduction),
or notice that the sun shine feels warm on your
skin (radiation). If you need evidence of
thermal energy or heat in your life, just
feel your arm. Your body generates heat 24 hours
a day! (Taken from KEEP Activity Guide
"Exploring Heat").
A note about thermal energy and heat. In strict
scientific terms, there is a distinct difference
between heat and thermal energy. A
way to think about this distinction is objects
possess thermal energy, while heat is
transferred from one object to another.
Wherever possible, we have tried to remain true
to these distinctions. However, since heat is
the more familiar term we often use that to
facilitate understanding.
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