Electric Current Generation

Evaporation

What Is Heat?

When heat, a form of energy, is supplied to a substance, we expect it to produce a rise of temperature. In other words, heat usually causes an increase in the average kinetic energy of the random motion of the molecules of which the substance is made up. However, heat may also produce a change of state without any temperature change.

Today heat is known to be a form of energy. But about a century ago heat was considered to be a kind of a weightless substance, which was neither created, nor destroyed. This substance called "caloric" was believed to pass from a hotter body to a colder one. eventually both of them coming to the same temperature. To explain that phenomenon was easy: a hot body, it was supposed, contains more of the heat fluid, i. e. caloric, than a cold one; and this fluid flows from hot to cold. Again, people knew that it takes more caloric to raise the temperature of a pound of water 10° than a pound of iron. They naturally supposed water to have higher caloric content than the iron. In fact, the caloric theory of heat, as it was called, accounted for almost everything that was known about heat at that time, except one important phenomenon, namely: the production of heat due to friction. However, numerous laboratory experiments demonstrated that each time, when mechanical energy was expended as a result of friction, a corresponding amount of heat was produced.

In spite of that inability to explain the production of heat by friction, the caloric theory of heat seemed to be the only acceptable theory.

Our great scientist and poet Lomonosov was among the first to find and state that heat phenomena were due to the motion of molecules. That statement of his resulted from many carefully performed laboratory experiments, from study and observation.

Lomonosov's theory laid the foundation for the present-day molecular-kinetic theory of heat. As was often the case; he left his contemporaries far behind and his statement was finally proved long after his death. The caloric theory of heat is known to have existed almost up to the middle of the 19th century.

The unit of heat is called a therm or a caloric; the latter term appears to come from the Latin word "calor" which means heat.

A calorie is defined as the amount of heat required, at a pressure of one atmosphere, to raise the temperature of one gram of water one degree Centigrade. (We know the gram to be a metric weight equal to 15.432 grains of the English system of weights).

One should not think that the very amount of heat which will raise the temperature of one gram of water from 0 to 1° C will also raise the temperature of the same mass of water from, say, 60 to 61° C Experiments have shown that the quantities of heat to be required in these two cases are slightly different. Hence, the true calorie is defined as that quantity of heat, which will raise the temperature of 1 gr of water from 19.5 to 20.5° C.

A liquid is known to increase in temperature when heat is applied. This statement is true up to a certain point called the boiling point of the liquid. When the boiling point is reached, however, adding heat to the liquid no longer raises the temperature. The added heat will then cause a change of state since the liquid will be transformed into a gas or a vapor.

Evaporation also called vaporization is the name given to the process, which occurs when some of the molecules of a liquid tear themselves away from the liquid surface and escape into the air. These molecules form the vapor above the liquid. To tear itself away from the liquid, the molecule, which leaves it, should have a large amount of kinetic energy as the molecular attraction which tends to oppose this escape must be overcome by the molecule. Those molecules which escape must have greater energy than the average kinetic energy of the liquid as a whole.

The kinetic energy of the molecules is in a sense a measure of the temperature of the liquid; and if the molecules with the larger amount of energy escape, the average amount of the kinetic energy of the remaining molecules becomes lower. We, therefore, expect the process of evaporation to lower the temperature of the liquid, and observation shows us that such is really the case.

If little ether is poured on to the hand, a sensation of cold is felt as evaporation takes place. This is because heat is absorbed from the hand to transform the liquid into vapor. The change of state from a liquid to a vapor involves absorption of heat, just as does a change from solid to liquid. Different liquids evaporate at different rates because of differences in their molecular attractions and in their molecular speeds. Mercury, for instance, evaporates very slowly, ether vaporizing" rapidly.

The rate of evaporation also depends on the area of the evaporating surface. That is why water will dry up from a large flat vessel much sooner than it will from a tall and narrow vase.

Evaporation takes place at all temperatures. There is another process, however, which takes place at a particular temperature and at which the process of vaporization is hastened by the constant heat application.

In the process of boiling, heat is constantly added to the liquid. The heat to be added increases the kinetic energy of the molecules, which is the same as saying that the temperature of the liquid starts rising. In the process of evaporation described above the phenomenon is a surface effect. The vapor molecules pass from the surface of the liquid into the air surrounding the liquid. Boiling is a similar process except that when liquid boils evaporation takes place throughout the volume of the liquid, small bubbles of vapor forming within the liquid and additional vapor molecules joining each of the bubbles as it rises to the surface.

In certain cases a solid may change directly into a vapor without undergoing liquefaction. The vapor pressure of a solid at any temperature being greater than one atmosphere, the substance will pass directly from the solid to the vaporous condition. By increasing the pressure, however, the substance can be obtained in a liquid state, provided the change from a liquid to a solid is accompanied by an expansion.

How Can We Use Steam?

Today, we wonder how to make use of steam. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

During the seventeenth century, we finally tumbled to the fact that gases and vapors can exert huge forces. People began testing the idea that airy vapor might actually power an engine. Rudimentary steam engines began appearing.

As early as 1606 (or 1601?) an Italian experimenter showed how steam, forced into a closed box of water, could drive the water up a pipe. He had made an embryonic steam pump a century before Thomas Newcomen built the first complete working engine in England, and 160 years before James Watt began making steam engine improvements.

So think for a moment like a seventeenth-century inventor. You realize steam has a huge potential and you want a machine that can harness it. What options do you have?

By now we've settled on two very different means. We can expand steam through a nozzle and let the resulting jet do work for us. Or we can let steam act upon a piston.

First-century Hellenistic engineers used rear-pointing steam jets to power toys -- birds flying on the ends of strings, whirligig devices, vehicles. Another Italian inventor described a complete steam turbine system to power grinding pestles, in 1629. His jet acted upon a turbine wheel, instead of just blowing backward. And, despite all that, practical steam turbines wouldn't appear until just over a hundred years ago.

The piston/cylinder idea is more complex in principle. But it was easier to make. Consider possibilities: You can seal some water in a cylinder, then alternately heat and cool it. Boil and condense -- driving a piston first one way, then the other.

Or you can supply steam from an external boiler. First the steam pushes the piston one direction. Then we condense the steam and outside air pushes it the other way. Or we can exhaust the spent steam and let it condense outside the cylinder. Lots of possibilities there, and all of them were tried.

Denis Papin, a French Huguenot working in Germany, built the first piston engine in 1690. He heated and condensed the same water over and over. It was slow, and he didn't yet have the mechanical wherewithal to make a commercial model. Seventeen years later Papin described another engine. This time he supplied steam from an outside boiler and he exhausted the spent steam. After another five years, Thomas Newcomen built his working engine. He used an external boiler he condensed his steam inside the cylinder, and he gave us the beginning of commercial steam engines.

But so many clever people had brought it into being. By the time working engines could take so much weight off our backs, the variants had all been tossed in the air like wheat and chaff. And one, just right for the time, landed where it could alter life on Planet Earth. So many people; so much fine work! It all makes me want to weep when I'm asked, "Who really invented the first steam engine?" I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work. http://www.uh.edu/engines/epi1953.htm

Let us reconsider a few of the many methods available for generating electric currents. There are at least four principal ones: 1. chemical reaction; 2. thermal or heat action; 3. light action, and last but not least, 4. magnetic action.

In a battery currents produce chemical effects and the process is reversible. In other words, batteries can generate electricity by moans of chemical reactions. To produce a current by chemical reaction, an alkali or an acid is made to react with a metal. The device to be used is called a voltaic or an electric cell, a group of two or more cells connected together forming a battery. The reader is certain to remember that the voltaic cell is so named after Volta, its inventor, who was the first to show that electricity could be generated owing to chemical reactions.

Producing current by thermal action is the next method to be considered. Heat is certain to produce current when applied to two unlike metals soldered together in two points. The apparatus used is called a thermoelectric couple or thermocouple, for short. The word "couple" used in this term should mean that two unlike metals or metals and alloys, say, such as cooper and constantan arc joined together so that they can be properly heated in the point of the joint.

The reason the thermocouple generates current is due to the fact that the heat tears the electrons off of the negatively charged metal at the point of joint, just as the electric cell chemical reaction is expected to tear the electrons off of the zinc electrode. It is these electrons that constitute the current flowing through the circuit.

Our physicists are known to have produced semiconductor thermo elements, which without any machinery convert thermal energy into electric energy as efficiently as small steam installations do.

Falling on a special kind of cell, a light beam can generate an electric current. The appliance using that phenomenon to produce electricity is called a photoelectric cell. Photoelectric cells appear to be used in a great number of common devices. One can't help mentioning lasers, too, since it has been found that laser powerful beams can be turned into electricity with a very high efficiency.

To generate a current by magnetic action, a wire is made to pass through a magnetic field, the latter being set up either by a permanent magnet or an electromagnet. In the case of the wire cutting through the magnetic field of a permanent magnet, the appliance is called a magneto-electric machine or simply "magneto". The wire cutting through the magnetic field of an electromagnet, the apparatus is said to be a dynamo electric machine or a dynamo, for short. Generally speaking, there are several ways by means of which electric currents can be generated by magnetic action, all of them being based on the same principle, namely, on cutting magnetic force lines with a conductor.

You are likely to think that the dynamo is similar to an electric motor. In principle you are right. Any d. c. motor may be used as a generator simply by rotating the armature. Nevertheless, the construction of both types of machines differing in details, a machine designed as a motor does not make a very efficient generator.

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