History Weblecture for Unit 46
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We are almost through with constructing the Newtonian, deterministic world view of the 19th century. There two pieces left: the definition of energy, the topic of this week's unit, and the discovery of the wave nature of light.
The study of energy in the 19th century is a study of heat and how heat can be used to do work. This wasn't an entirely new idea. In the first century A.D., Hero of Alexandria created a small steam "engine", but it was never used to do serious work. The question became important again, however, in the seventeenth century, when the mining engineers and factory owners of Europe were looking for a way to pump water out of mine shafts and power the newly-invented thread spinning machines.
The English ironmonger, Thomas Newcomen, created the first practical steam engine to pump water out of flooded tin mines. He built on the work of Thomas Savery, who had created akind of thermic siphon, a way of using a partial vacuum created by injecting steam into a cylindar and then condensing it, as a pump. Newcomen used steam instead to drive a piston that could push air out, then suck water in.
A Scottish chemist, Joseph Black, made the distinction between the intensity of heat (measured by the temperature of a substance) and the quantity of heat necessary to produce a change in temperature. Black determined that different substances required specific amounts of heat for their temperatures to change one degree. He created a new unit, the calorie, and defined it as the amount of heat required to change the temperature of a cubic centimeter of water (a centiliter) by one degree centigrade. Once this unit had been defined, he could determine the specific heats of other substances in calories, effectively comparing them directly to the specific heat of water.
Black also discovered that when a substance is undergoing a phase change, its temperature doesn't change. This latent heat energy is used to effect the change, breaking bonds between molecules in a solid so that they are free to move in liquid form, and between liquid molecules so that the individual molecules can behave as gasses.
Basing his ideas on Black's work, Lavoisier envisioned heat as a fundamental element called caloric found in a liquid form (similar to phlogiston and the contemporary concept of electricity) that could flow from one object to another— and always from the hotter to the colder object. During the eighteenth century, this conceptualization proved inadequate to meet two needs: the new theoretical understanding of energy as some characteritic separate from the type of matter in which it occurs, and the practical application of energy to do work in machines as the industrial revolution began. The concept of efficiency, or work done as a result of energy input, becomes a key standard for determining the effectiveness of the new machines.
To pump water and power the machines of the textile industry, tool-makers turned to the medieval standby, falling water, but they soon realized that steam under pressure could not only push harder than falling water, but could be more easily directed and controlled. New methods of refining coal created fuel capable of generating higher temperatures. The higher temperatures also made it possible to refine iron and create new alloys steel (carbon and iron melted together). With the new alloys, industrialists could use iron for structures as well as decoration, and create steel able to withstand higher pressures and temperatures. Both characteristics were necessary to create machines such as the steam engine, capable of generating and harnessing greater amounts of heat energy.
James Watt worked on improving the efficiency of the steam engine. This was a problem of very great importance to the industrial revolution. The ability to increase work done by machines without increasing the expense of supplying the machines with fuel meant that more could be produced and sold at cheaper prices, making many products available to the general public for the first time in history. Before the engineers could make significant advances, however, they had to understand how heat, energy, and work were related.
The work on temperature and energy culminated in the 19th century with the law of the conservation of energy and the laws of thermodynamics. These concepts still govern our understanding of how energy changes forms. Both rely on the idea that heat is a form of kinetic energy, the energy of motion. Molecules in solids, liquids, and gases vibrate in place or move and collide with each other. The more energy a sample of a substance has, the more its molecules vibrate or the faster they move. To study the sample as a whole, we need to use statistical methods that sum up the possible behaviors of individual particles to create a probable total behavior. So the laws of thermodynamics predict the total behavior of systems, not the behavior of any individual particle.
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