Work and heat

Heat and work are both measured in energy units, so they must both represent energy. How do they differ from each other, and from just plain energy itself  

There is another special property of heat that you already know about heat can be transferred from one body (i.e., one system) to another. We often refer to this as a flow of heat, recalling the 18th-century notion 

like energy, can take various forms mechanical, electrical, gravitational, etc. All have in common the fact that they are the product of two factors, an intensity term and a capacity term. For example, the simplest form of mechanical work arises when an object moves a certain distance against an opposing force. Electrical work is done when a body having a certain charge moves through a potential difference. 

Mechanical work is the product of the force exerted on a body and the distance it is moved 1 N-m = 1 J (Illustration from www.benwiens.com/ energyl. html) 

Performance of work involves a transformation of energy thus when a book drops to the floor, gravitational work is done (a mass moves through a gravitational potential difference), and the potential energy the book had before it was dropped is converted into kinetic energy which is ultimately dispersed as heat. 

Although we can use a wide variety of classifications for types of energy, all energy flow is either heat or work. We ll need to understand both forms of energy transfer to assess the energy economy of the world and the role of chemistry in that 

This combustion produces carbon dioxide and water vapor, and those gases do PV-work as they expand against the piston in the cylinder. This PV-work is then transmitted through the drive train to move the car. 

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Temperature becomes a quantity definable either in terms of macroscopic thermodynamic quantities, such as heat and work, or, with equal validity and identical results, in terms of a quantity, which characterized the energy distribution among the particles in a system. With this understanding of the concept of temperature, it is possible to explain how heat (thermal energy) flows from one body to another. 

Click Chemistry Interactive for a self-study module on heat and work. 

The first law of thermodynamics goes a step further. Taking account of the fact that there are two kinds of energy, heat and work, the first law states ... 

Surroundings Heat and work. These quantities are positive... 

Although a typical chemical reaction may appear far removed from the working of an engine, the same fundamental principles of heat and work apply to both. 

That such a mechanical equivalent exists is a consequence of the fact that heat and work are interconvertible. The further fact that it is a fixed constant and independent of the process of conversion was proved by the experiments of Joule (1848-1880), referred to below. In the determination of this constant two measurements are required ... 

Efficiency of a Heat Engine.—The efficiency (N) of a heat engine is measured by the fraction of the quantity of heat received from the source which is converted into work, both heat and work being measured in the same units. 

Heat, like work, is energy in transit and is not a function of the state of a system. Heat and work are interconvertible. A steam engine is an example of a machine designed to convert heat into work.h The turning of a paddle wheel in a tank of water to produce heat from friction represents the reverse process, the conversion of work into heat. 

Consider any reversible cyclic process that involves the exchange of heat and work. Again, the net area enclosed by the cycle on a p-V plot gives the work. This work can be approximated by taking the areas enclosed within a series of Carnot cycles that overlap the area enclosed by the cycle as closely as possible as shown in Figure 2.10b. For each of the Carnot cycles, the sum of the q/T terms... 

In Chapter 2 we used the laws of thermodynamics to write equations that relate internal energy and entropy to heat and work. 

Figure 5.6 An isolated system composed of subsystem A (the one we will eventually designate as the system) and subsystem B (the surroundings containing a heat reservoir). Heat and work will be exchanged until TA = 7b, pA = p%, and equilibrium is established.

In this expression consistent units must be used. In the SI system each of the terms in equation 2.1 is expressed in Joules per kilogram (J/kg). In other systems either heat units (e g. cal/g) or mechanical energy units (e.g. erg/g) may be used, dU is a small change in the internal energy which is a property of the system it is therefore a perfect differential. On the other hand, Sq and SW are small quantities of heat and work they are not properties of the system and their values depend on the manner in which the change is effected they are, therefore, not perfect differentials. For a reversible process, however, both Sq and SW can be expressed in terms of properties of the system. For convenience, reference will be made to systems of unit mass and the effects on the surroundings will be disregarded. 

What Are the Key Ideas Heat and work are equivalent ways of transferring energy between a system and its surroundings. The total energy of an isolated system is constant. The enthalpy change for a process is equal to the heat released at constant pressure. 

Two of the fundamental concepts of thermodynamics are heat and work. People once thought that heat was a separate substance, a fluid called caloric, which flowed from a hot substance to a cooler one. The French engineer Sadi Carnot... 

Fig. 6.1), who helped to lay the foundations of thermodynamics, believed that work resulted from the flow of caloric, just as the flow of water turns a water wheel. Some of Carnot s conclusions survive, but we now know that there is no such substance as caloric. About 25 years after Carnot proposed his ideas in the early nineteenth century, the English physicist James Joule showed that both heat and work are forms of energy (Fig. 6.2). 

Equation is a restatement of the law of conservation of energy. We can show this by substituting into Equation the equalities for heat and work ... 

Energy is a state fianction, but heat and work are path functions. To illustrate this, Figure 6-11 describes two different paths for the combustion of 1 moi of methane. Path 1 represents what happens in an automobile fueled by natural gas As methane bums, the system does work on its surroundings by driving back the piston. At the same... 

A block diagram of two different paths for the reaction of methane and oxygen. Occurring inside an engine (Path 1), the process transfers heat and work. Occurring in a furnace (Path 2), the process does no work and transfers only heat. 

To determine A E using measured values of q, we also must know w. Because heat and work are path functions, however, we proceed differently for constant volume than for constant pressure. To distinguish between these different paths, we use a subscript v for constant-volume calorimetry and a subscript p for constant-pressure calorimetry. This gives different expressions for the two t q)es of calorimeters ... 

Very closely interrelated concepts in thermodynamics are those of energy, work and heat. Energy is generally perceived as the capacity to do work. Mechanical work is performed whenever the point of application of a force is displaced in the direction of the applied force. Heat is a form of energy. Heat and work are interconvertible. The interconversion of heat and work is one of the prime concerns of thermodynamics. 

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