2010 On Line Technocracy Study Course project
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In the preceding lesson we discussed English and metric systems of measurements, and showed what is meant by mass, force, work and power, and how these are related to each other. In the present lesson we want to introduce and discuss the more general concept of energy...
Lesson 3
...Energy is the capacity
to do work, and is therefore measured in units of work---foot-pounds or joules.
Energy
exists in two principal forms, potential energy and kinetic energy.
Potential Energy. A
certain amount of work must be done on a spring to stretch it. This spring on
contracting can be made to lift a weight, and therefore do work. Hence, the
stretched spring must possess energy. A weight at a certain height above the
ground can, while being lowered, be made to lift another weight, and hence to
do work. Thus, the raised weight has potential energy. Gun
powder when it explodes does work in propelling the bullet. Therefore, it must
possess potential energy. In this case the energy is due to chemical
combinations. Page 26 A
storage battery does work by turning an electric motor. In this case chemical
potential energy is converted to electrical energy which finally, by means of
the motor, does work. Kinetic Energy. When
we started, everything was at rest, and the weight had potential energy, due to
its height. When the weight reached the end of the rope, it again came
momentarily to rest. At that instant the flywheel was travelling at its maximum
speed. Hence, the work done by the falling weight served to rotate the
fly-wheel. Next the weight was lifted and the flywheel slowed down. In this
case, the flywheel was doing work in lifting the weight. Since it was thus able
to do work the flywheel, while in motion, must have possessed energy---energy
due to motion. The
energy that a body possesses due to its motion is its kinetic energy, and is
measured by the amount of work that body can perform against an outside force
before it is brought to rest. Any
moving body whatsoever possesses kinetic energy. This is manifested by the wind
in driving sailing vessels or windmills; the destruction caused by impact of
fast-moving objects, automobiles, trains, bullets, etc. Heat. In
all cases there was friction between moving parts, and work was being done to
overcome the friction. Hence work produces heat. In
the case of the automobile mentioned above, in going down hill the car was
losing potential energy, and the brakes were getting hot. Thus potential energy
was being converted into heat. In the second case the car was slowed down,
thereby losing kinetic energy. Hence, kinetic energy was converted into heat. Measurement of
Heat. Page
27 GO Top Lesson 3 Bottom
Imagine
a quart vessel and a gallon vessel, each filled with water at the same
temperature. A blindfolded person could not, by placing his finger successively
in the water in the two vessels, tell which was the quart and which was the
gallon, because temperature is a measure not of quantity of heat, but of its
intensity. While the water in both the gallon and the quart vessels was at the
same temperature the quantity of heat in the gallon vessel was four times as
great as that in the quart vessel, because there was four times as much water. Temperature. English System: Metric System: Relations between
Centigrade and Fahrenheit Scales: Quantity of Heat. English System: Metric System: Relations between English
and Metric Units: Heat and Work. English System: Metric System: Page 28 pound weight to a height
of 37.0 inches. If this weight were then attached to a brake mechanism and
allowed to fall so that it's energy were converted into heat, one gram calorie
of heat would be produced. GO Top Lesson 3 Links to lessons GO preface
(Part Two)
The potential energy of a body is the work
it can do by virtue of its position or of the relative configuration of its
parts.
If we imagine a very heavy flywheel on
frictionless bearings, with a rope wound around its axle and a heavy weight
suspended on the rope, then if the wheel is free to turn, the weight will fall
and the wheel turn faster and faster. Now suppose the rope reaches its end and
starts to wind up on the other side of the axle. From this time on the flywheel
will slow down, and the weight will be lifted. If the wheel turns without
friction it will be found that when it finally comes to rest the weight will be
lifted exactly to the height from which it first began to fall.
If an automobile goes slowly down hill with
its brakes on, the brakes get hot, and if the hill is long enough they may even
burn out. In an exactly similar manner if the brakes are applied on a level
road so as to bring the car to a short stop from a high speed, the brakes are
again heated. Other examples of the same kind are to be found in the heating of
drills and the making of fire by rubbing sticks together.
It is important to distinguish between the temperature
and the quantity of heat.
Temperature is measured by its effect in
expanding mercury in a glass tube.
Fahrenheit Scale. Water freezes at 32 degrees
F.; water boils at 212 degrees F. Difference, 180 degrees F.
Centigrade Scale. Water freezes at 0
degrees C.; water boils at 100 degrees C. Difference, 100 degrees C.
1 degree C. equals 1.8 degree F.;
temperature (F.) equals 1.8 x temperature (C.) plus 32.
The quantity of heat imparted to a
substance is proportional both to the increase of temperature and to the
quantity of the substance heated.
The quantity of heat required to raise the
temperature of one pound of water 1 degree F. is called one British thermal
unit (B.t.u.).
The quantity of heat required to raise the
temperature of one gram of water 1 degrees C. is called a gram calorie.
One kilogram calorie is equal to 1,000 gram calories.
One kilogram calorie (kg. cal.) equals
3.968 B.t.u.'s. One B.t.u. equals 0.252 kg. cal.
It has already been shown that work
produces heat. How much heat does a given amount of work produce?
Imagine a vessel of water insulated against
loss of heat. Fix in this vessel a paddle wheel arranged with a pulley
mechanism so that a suspended weight, upon lowering, will drive the
paddle-wheel. The work done can be measured by the fall of the weight. The heat
generated can be measured by noting the rise in temperature of the water.
Careful measurements of this kind have shown that 778 foot-pounds of work will,
when converted into heat, increase the temperature of one pound of water F. In
other words, 778 foot-pounds of work are equal to one B.t.u. of heat.
4.18 joules of work will produce one gram calorie
of heat; 4.18 joules of work would be required to lift a one-