Monday, January 9, 2012

Work, energy and power part 2

     Today we'll learn :
  1. Interchange of energy between potential energy and kinetic energy
  2. Internal energy
  3. transformation of energy
  4. Heat energy
  5. The sun as a source of energy

  1. Interchange of energy between potential energy and kinetic energy :

        A swing pendulum bob is an example of a body whose energy can be either kinetic or potential or a mixture of both. It is all potential at the extreme end of the swing and all kinetic when passing through the rest position. At intermediate points it is partly kinetic and partly potential.
        If we turn our attention to a falling body ( free falling ) , it is obvious that at any particular moment, it possesses both potential and kinetic energy. As the body falls its speed increases so that it gains in k.e. at the expense of its p.e. If we ignore the energy the body gives to the air molecules as it pushes them out of its way, then the loss in p.e. of the body is exactly equal to its gain in k.e. This is an example of the law of conservation of energy which states that energy cannot be destroyed : it only changes into a different form of energy. The mechanical energy of a body remains constant ( ME = k.e + p.e.)  As shown above.

  2. Internal energy
          We remarked on the obvious nature of the two kinds of energy possessed by a falling body. What is not quite so obvious is the internal energy of the molecules inside this body. The molecules of a substance are in continual motion. According to the state of the substance, they posses kinetic energy of motion or vibration together with potential energy resulting from the attractions and repulsions they exert on one another. Internal energy increases as the temperature of a body increases.

     Scientists sometimes refer to the k.e. and p.e of a body taken as a whole as being macroscopic ( = visible to the naked eye ). Internal energy may be described as microscopic ( = invisible to the naked eye ). However, it is possible to demonstrate the reality of molecular motion ( see Einstein’s theory for Brownian motion ).
  3. transformation of energy
        The law of conservation of energy states that energy is never destroyed but only changes from one form to another. As an example we'll now discuss the way in which the law applies to a grandfather clock.
        The energy we spend in winding up the weight is derived from chemical changes in our muscles and this is provided from the food we have eaten.
        If the weights have a total mass of 5 kg and are raised through 2 m then a force of 5 × 9.8 N has to be exerted through a distance of 2 m. The work done is stored as potential energy in the weights, so that
        potential energy = work done = force × distance
                                                          = 5 × 9.8 × 2 J
                                                          = 98 J ( to keep up revise, if you did not read ,Work, energy and power)
        As the weights slowly descend, their potential energy is converted into kinetic energy and  potential energy in the moving parts of the clock.
        At this stage we may ask what has become of the original potential energy of the weights when the clock has completely run down.
   The answer is that it has become transformed into internal molecular energy in the various parts of the clock resulting from the work done against friction between the wheel spindles and their bearings, motion of the pendulum against air resistance, and so on. This increase in internal energy raises the temperature of the clock slightly, and as it cools down, the excess internal energy is given out in the form of  heat by conduction, convention and radiation to the surroundings where it is absorbed and becomes internal molecular energy once more.
  4. Heat energy
          In everyday life we sometimes loosely refer to the " heat energy in a body" , instead of using the term: " internal energy".

          In physics, heat is defined as energy which flows from one place to another owing to a temperature difference between them.
      There are three main processes on heat transfer, namely, conduction, convention and radiation.

      In thermodynamics ( the branch in physics concerned with the relation between heat and work ), we have to deal with such things as engines in which internal energy changes take place in a substance both as a result of heat flow and also as a result of work being done by or on the substance. To avoid confusion, special symbols   U ( internal energy),  Q (heat) and W ( work) are used.

      Thus, the internal energy of some steam, for example, is represented by U in joules. This internal energy can be changed by adding or subtracting Q in joules of heat.

The internal energy of steam can be increased by compressing it, i.e., doing W in joules of work on it, or the steam can be allowed to expand and transform some of its internal energy into W in joules of useful work in, say, driving a turbine.

      In a case such as this it will be realized that confusion will certainly follow if we talk of the " heat energy" in a substance being transformed into work when actually we mean the transformation of internal energy into work. I hope that was easy :) . 

5. The sun as a source of energy
        It is interesting to trace back to their origin the series of energy transformations which lead to the release of light energy by pressing an electric switch.

      The light is given out from a thin incandescent tungsten filament inside a glass bulb. The internal energy required to raise the temperature of the tungsten wire is derived from the work done when the electric current moves against the resistance offered to its passage through the wire.

        The electricity itself is generated by a dynamo run by a steam turbine, as shown.

       The turbine, in turn, is driven by expanding high-pressure steam, which we shall suppose has been obtained by heating water in a coal-fired boiler ( polluting but cheaper!).

      Now coal consists mainly of carbon and is formed from the remains of giant forests which flourished millions of years ago. Changes in the earth's crust led to these forests becoming submerged beneath layer of sediment and subjected to pressure. The plants from which the coal is formed derived their growth from the action of the sun-light in which they were bathed.

      Thus we see that electric light and power from a coal-fired generating station comes to us by a series of transformations of energy which was poured out by the sun, millions of years ago, and stored as chemical potential energy in coal.

      I think you, my dear reader, noticed that the last link in the energy chain is the production of internal energy. This is found to be true in all cases. Even the light from the lamp finally turns back into internal energy when it is absorbed by bodies on which it falls. This fact is referred to as the "degradation" of energy into internal energy and has led some physicists to suppose that the end of life, as we know it, will come when all energy in the universe is uniformly distributed as internal molecular energy at the same temperature. Too bad for us!.

See you soon with :  Work, energy and power part 3