The findings of Joule and others led Rudolf Clausius, a German physicist, to state in 1850: "In any process, energy can be changed from one form to another (including heat and work), but it is never created or destroyed." This is now known as the first law of thermodynamics. An adequate mathematical statement of this first law is delta E = q - w, where delta E is the change (delta) in internal energy (E) of the system, q is the heat added to the system (a negative value if heat is taken away), and w is work done by the system. In thermodynamic terms, a system is defined as a part of the total universe that is isolated from the rest of the universe by definite boundaries, such as the coffee in a covered Styrofoam cup; a closed room; a cylinder in an engine; or the human body. The internal energy, E, of such a system is called a state function. This means that E is dependent only on the state of the system at a given time, not on how the state was achieved.
If the system considered is a chemical system of fixed volume--for example, a substance in a sealed bulb--the system cannot do work (w) in the traditional sense, as could a piston expanding against an external pressure. If no other type of work--such as electrical work--is done on or by the system, then an increase in internal energy is equal to the amount of heat absorbed at constant volume, the subscript v indicating that the volume of the system remains constant throughout the process.
If the heat is absorbed at constant pressure instead of constant volume, which can occur to any unenclosed system, the increase in the energy of the system is instead represented by the state function H, which is closely related to the internal energy. Changes in H (heat content) are called changes in enthalpy.
In 1840, before Joule had made his determinations of the mechanical equivalent of heat, Swiss chemist Germain Henri Hess reported the results of experiments that indicated that the heat evolved or absorbed in a given chemical reaction (delta H) is independent of the particular manner in which the reaction takes place or the path the reaction follows. This generalization is now one of the basic postulates of thermochemistry.
The Third Law of Thermodynamics Entropy, as a measure of disorder, is a function of temperature. Increasing temperature results in an increase in entropy (positive delta S). The third law of thermodynamics considers perfect order. It states that the entropy of a perfect crystal is zero only at absolute zero. This reference point allows absolute entropy values to be expressed for compounds at temperatures above absolute zero, which is impossible to achieve.
Heat/Overview Heat is a form of energy. According to the Kinetic Theory of Matter, heat is the result of the continuous motion and vibration of the atoms and molecules that constitute all matter. The transfer of heat between objects of different temperatures by thermal flow processes involves a reduction in the average motion of the particles of the hotter object and an increase in the average motion of the particles of the cooler object. The branch of physics comprising the comprehensive study of the transfer of heat, and of the conversion of heat into WORK and work into heat in physical and chemical processes, is known as Thermodynamics.
Development of the Concept of Heat Until the growth of classical physics in the 18th century, scientists did not comprehend the true nature of heat. Even though earlier investigators such as English scientist Robert Boyle had considered heat to be some form of manifestation of the movement of small particles in objects, throughout most of the 18th century heat was still mainly thought of in terms dating back to the days of ancient Greek philosophy. That is, heat was considered to be a kind of basic "element," a form of actual substance. This substance, while all-pervasive, was conceived of as an invisible, weightless material--a fluid called caloric by French chemist Antoine Lavoisier. If a hot object were to be placed in contact with a cooler object, for example, this invisible fluid would enter the cooler object to make it hotter. The notion was sometimes expanded to a two-fluid form--the other, cold fluid being called frigoric.
By the end of the 18th century the notion of heat as a fluid, while still prevalent among scientists, was on the way out. It was helped along in particular by the work of Count Rumford and English chemist Humphry Davy. Experiments that they conducted gave strong support to Boyle's concept of heat as a result of motions. After that, in the course of the 19th century, the basic concepts of thermodynamics were worked out by a number of noted physicists, including British scientists James Prescott Joule and Lord Kelvin. From their work arose the modern understanding of heat as a form of energy in transit.
Active Systems Active solar heating systems commonly consist of several hundred square meters of solar collector panels, plus a storage medium to hold the heat collected during the day, and a set of automatic controls that monitor and regulate both heat collection and delivery between the storage medium and the living space. Active systems use either a liquid (of which the most popular is a mixture of water and an antifreeze, such as propylene glycol) or air as the heat-transfer medium. Insulated pipes or ducts carry the heat-transfer medium, called the working fluid, to the collector panels--where the fluid absorbs heat--and then back to the storage, which in liquid-based (hydronic) systems is an insulated tank or in air systems is an insulated bin of fist-sized rocks. (Alternatively, phase-change materials may be used to store heat.) The absorbed heat is transferred to the storage medium, and the cooled working fluid is then returned to the collectors to pick up more heat. Heat is removed from storage and delivered to the living space as needed. Most types of active systems require an auxiliary heating system to provide extra heat during extended periods of cloudiness or extreme cold. A typical active heating system might cost between $2,000 and $5,000 per thousand square feet of living space in the northeastern United States--depending on the type of system, its efficiency, and so forth. A large portion of a building's annual domestic hot water (DHW) needs can be supplied by a relatively inexpensive (between $2,000 and $2,500) active hydronic system using about 9 sq m (100 sq ft) of collectors for a typical residence. A heat exchanger, usually in the hot-water tank, keeps the working fluid separate from the potable water supply. Such systems, although they require a backup energy source, may pay for themselves in energy savings in less than 10 years.
Battery Active solar heating systems commonly consist of several hundred square meters of solar collector panels, plus a storage medium to hold the heat collected during the day, and a set of automatic controls that monitor and regulate both heat collection and delivery between the storage medium and the living space. Active systems use either a liquid (of which the most popular is a mixture of water and an antifreeze, such as propylene glycol) or air as the heat-transfer medium. Insulated pipes or ducts carry the heat-transfer medium, called the working fluid, to the collector panels--where the fluid absorbs heat--and then back to the storage, which in liquid-based (hydronic) systems is an insulated tank or in air systems is an insulated bin of fist-sized rocks. (Alternatively, phase-change materials may be used to store heat.) The absorbed heat is transferred to the storage medium, and the cooled working fluid is then returned to the collectors to pick up more heat. Heat is removed from storage and delivered to the living space as needed. Most types of active systems require an auxiliary heating system to provide extra heat during extended periods of cloudiness or extreme cold. A typical active heating system might cost between $2,000 and $5,000 per thousand square feet of living space in the northeastern United States--depending on the type of system, its efficiency, and so forth. A large portion of a building's annual domestic hot water (DHW) needs can be supplied by a relatively inexpensive (between $2,000 and $2,500) active hydronic system using about 9 sq m (100 sq ft) of collectors for a typical residence. A heat exchanger, usually in the hot-water tank, keeps the working fluid separate from the potable water supply. Such systems, although they require a backup energy source, may pay for themselves in energy savings in less than 10 years.
Capacitor Another device capable of electrical work is the capacitor, a descendant of the Leyden jar, which is used to store charge. If a charge Q is placed on the metal plates the voltage rises to amount V. The measure of a capacitor's ability to store charge is the capacitance C, where C = Q/V. Charge flows from a capacitor just as it flows from a battery, but with one significant difference. When the charge leaves a capacitor's plates, no more can be obtained without recharging. This happens because the electrical force is conservative. The energy released cannot exceed the energy stored. This ability to do work is called electric potential.
This type of conservation of energy is also associated with emf. The electrical energy obtainable from a battery is limited by the energy stored in chemical molecular bonds. Both emf and electric potential are measured in volts, and, unfortunately, the terms voltage, potential, and emf are used rather loosely. For example, the term battery potential is often used instead of emf.
This type of conservation of energy is also associated with emf. The electrical energy obtainable from a battery is limited by the energy stored in chemical molecular bonds. Both emf and electric potential are measured in volts, and, unfortunately, the terms voltage, potential, and emf are used rather loosely. For example, the term battery potential is often used instead of emf.
Heat Capacity /Specific Heat Heat can be measured quantitatively. The units of measurement are typically either the calorie or the British thermal unit (Btu). Both of these units were at one time defined in terms of the amount of heat energy required to raise a given amount of liquid water by a given thermometric degree (within a certain degree range). Now the units are defined in mechanical terms--by the amount of work they can do, which can be expressed in electrical-unit equivalents, as well. temperature is a measurement of the degree of heat of an object, not the total quantity of heat energy that the object possesses. Thus an object that is at a higher temperature than another does not necessarily have a greater total heat content than the object at a lower temperature. The sizes and types of material of the objects, as well as their temperatures, determine the total quantities of heat energy the contain.
In thermodynamics, the heat capacity of an object or substance is the amount of heat energy required to raise the temperature of the object or substance by one degree Celsius. Specific heat is a closely related concept. It is the amount of heat necessary to raise a unit mass of matter by one degree. Common units of heat, such as the calorie and British thermal unit, have been defined in terms of the specific heat of water at a standard temperature. The specific heat of copper is 0.093 cal/(g) at room temperature, and the heat capacity of a 100-g copper bar is 9.3 cal/C degree. The specific heats of most materials remain essentially constant over the common range of temperatures. At extremely low temperatures, however, specific heats become considerably smaller.
A volume of gas will accept more heat energy per degree of temperature rise if it is allowed to expand freely than if it is confined. Thus a gas has two distinct values of specific heat: one value at constant pressure, and another, smaller value at constant volume. The ratio of these values is different for different gases and is of importance in describing the behavior of a gas undergoing a thermodynamic process.
Combustion Turbines More recently, combustion turbine generators have become popular as peaking units, not only because of their quick-start and intermittent-operation capabilities, but often because of the short time they require for installation. In recent years many U.S. utilities have found themselves deficient in generating capacity when the installation of new facilities has been delayed by problems in procurement, licensing, or construction. Lengthy delays have occurred in many planned nuclear, fossil-fueled, and hydroelectric facilities. Procurement and construction of a large steam-electric station takes from 5 to 10 years even after advance procedural requirements have been met. Thus a combustion turbine unit of 30,000 kW or more, which can be installed and operated within a year or two after procurement, is an attractive alternative. Such units can also provide emergency service during power outages are often valuable as sources of start-up power for conventional generating plants following a blackout. Capital costs of combustion turbines are lower than those of conventional steam units, but fuel efficiency is usually not as high and maintenance is more expensive.
Evaporation Evaporation is the conversion of a liquid substance into the gaseous state. If the liquid is in an open container, eventually it will evaporate completely. If a liquid is placed in a closed container of larger volume, some molecules leave the liquid and go into the excess space. This process continues until an equilibrium is reached, in which the molecules of vapor return to the liquid at the same rate as they evaporate. The pressure exerted by the vapor in equilibrium with its liquid is called the vapor pressure; it is a characteristic property of each substance at a given temperature, and it increases as temperature increases.
Natural Gas Evaporation is the conversion of a liquid substance into the gaseous state. If the liquid is in an open container, eventually it will evaporate completely. If a liquid is placed in a closed container of larger volume, some molecules leave the liquid and go into the excess space. This process continues until an equilibrium is reached, in which the molecules of vapor return to the liquid at the same rate as they evaporate. The pressure exerted by the vapor in equilibrium with its liquid is called the vapor pressure; it is a characteristic property of each substance at a given temperature, and it increases as temperature increases.
