| Energy and Work |
Concept Pages:
 Physical
Science "Key Concepts" Home Page |
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Section
Review questions
Chapter review questions |
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| Energy |
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Exists in a confusing
variety of forms:
- Radiant
- Electrical
- Chemical
- Thermal
- Nuclear
- and others.
Energy causes change, but is hard to define.
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Kinetic Energy |
is the name given to energy that involves
motion. It is
related to both mass and velocity, which you know as momentum
when considered together:
Increase either mass or velocity, and the kinetic energy is increased, too. |
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Potential
Energy |
is stored energy. In fact, it is a description of the
future.
An object's potential energy is related to the effect of the release of energy by that
object in certain situations.An object on the floor has a certain amount of potential
energy.
Place that object on a table, and it has more potential energy.
Why?
Because it can fall farther than it could on the floor, which would have a greater
effect on something, the floor, the object, or both.
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Which has more potential energy: A
small economy car, or a Ferrari?
A cigarette lighter, or a welding torch?
A 6-inch fan, or a movie-set wind machine 6-foot fan?
A firecracker, or a stick of dynamite? |
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| Work |
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Work is the transfer of energy through motion. Work is
directly related to energy by calculation: |
Calculating Work |
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Work = force X distance
W = F x dThe application of a force over a distance is measured in joules.
One joule equals one Newton-meter (N·m)
(Remember, a Newton
is 1kg • m/s2)
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Example: |
A suitcase weighs 35 N. You lift it to a shelf 1.25
meters from the floor. The work done is: W = F x d
W = 35 N x 1.25 m
W = 43.75 j
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One must keep in mind that work calculated in this way can
only be for a force applied in the same direction as the motion that results from that
force. |
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Conservation
of Energy |
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Mechanical
Energy |
is the total of the potential and kinetic energy in a system. |
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The Law of Conservation of Energy refers
to "ordinary" conditions, those that are to be found on a day-to-day basis in
our world, and in a closed system. If energy can leave the system, it is an
open one.
It states that energy may change form, but
cannot be created nor destroyed.
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Calories, |
as in the food Calorie, is a unit used to measure
energy we can derive from food. A Calorie (capital "C") actually equals a
kilocalorie, which is equal to about 4,180 joules.
A gram of fat supplies about 9 Calories of energy.
A gram of protein or carbohydrate supplies about 4 Calories.
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Temperature and Heat |
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| Temperature |
Temperature is actually a measure of the average kinetic
energy of the molecules, atoms, and subatomic particles in a sample of matter. That
means that temperature is a measure of motion. Since we call what we are measuring
in this way heat, heat must be motion. |
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Thermal
Energy |
is the total energy of the particles in a sample of matter,
potential and kinetic. It is, of course, directly related to mass, too.
The more mass, the more atoms,
the more atoms, the more motion,
the more motion, the more heat. |
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Heat |
is the thermal energy that is transferred from an
object with higher temperature to one with lower temperature. |
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| Measuring Thermal Energy |
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| Specific Heat |
Specific Heat (C)is the amount of energy it takes to raise the
temperature of 1 kg of a particular material 1º Kelvin. C is measured in joules
per kilogram per Kelvin
J/(kg ·K) |
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Heat
Absorption, |
then, is related to the specific heat of the object or
material you are concerned with. Water feels cooler than you often expect it to, because
it has a high specific heat. It requires so much more energy than air to heat up,
that it tries to absorb your body heat much more quickly than air can. |
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Calculating Thermal Changes |
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Change in
thermal energy |
= |
mass X |
Change in
temperature |
X specific heat |
Written in formula form as: |
Q = m x DT x
C |
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Remember delta (D)
from the calculation of acceleration ? Updated
10/14/07 |
