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


(Part Two)

...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.
      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.

      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.

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      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.
      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.

      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.

      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.

      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.
      It is important to distinguish between the temperature and the quantity of heat.

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      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 is measured by its effect in expanding mercury in a glass tube.

English System:
      Fahrenheit Scale. Water freezes at 32 degrees F.; water boils at 212 degrees F. Difference, 180 degrees F.

Metric System:
      Centigrade Scale. Water freezes at 0 degrees C.; water boils at 100 degrees C. Difference, 100 degrees C.

Relations between Centigrade and Fahrenheit Scales:
      1 degree C. equals 1.8 degree F.; temperature (F.) equals 1.8 x temperature (C.) plus 32.

Quantity of Heat.
      The quantity of heat imparted to a substance is proportional both to the increase of temperature and to the quantity of the substance heated.

English System:
      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.).

Metric System:
      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.

Relations between English and Metric Units:
      One kilogram calorie (kg. cal.) equals 3.968 B.t.u.'s. One B.t.u. equals 0.252 kg. cal.

Heat and Work.
      It has already been shown that work produces heat. How much heat does a given amount of work produce?

English System:
      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.

Metric System:
      4.18 joules of work will produce one gram calorie of heat; 4.18 joules of work would be required to lift a one-

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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.

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