Technology & Engineering
Heat and Work
Heat and work are forms of energy transfer in thermodynamics. Heat is the transfer of energy due to a temperature difference, while work is the transfer of energy due to a force acting over a distance. Both heat and work are important in understanding the behavior of systems and processes in engineering and technology.
Written by Perlego with AI-assistance
Related key terms
1 of 5
11 Key excerpts on "Heat and Work"
eBook - ePubPhysics of Cryogenics
An Ultralow Temperature Phenomenon
- Bahman Zohuri(Author)
- 2017(Publication Date)
- Elsevier(Publisher)
These may be called energy transfer or energy interactions and they bring about changes in the properties of the system. Positive work occurs when the system transfers energy to its surroundings by some mechanical or electrical process. Positive heat transfer occurs when the surroundings transfer thermal energy to the system. Normally a temperature difference is the driving potential that moves thermal energy into or out of a system.4.2. Definition of Work
The formal definition of work is “a force acting through a distance.” When a system undergoes a displacement due to the action of a force, work is taking place and the amount of work is equal to the product of the force and the displacement in the direction of the force. The term work is so common with many meanings in the English language that it is important to be very specific in its thermodynamic definition.4.2.1. Work Is Done by a Force as It Acts Upon a Body Moving in the Direction of the Force
If the force acts, but no movement takes place, no work is done. Work is performed by the expanding exhaust gases after combustion occurs in a cylinder of an automobile engine as shown in Fig. 4.1 . In this case the energy produced by the combustion process can be transferred to the crankshaft by means of the connecting rod, in the form of work. Therefore, the work can be thought of as energy being transferred across the boundary of a system, the system being the gases in the cylinder.A similar concept is the work done in the turbine to generate electricity in a nuclear power plant. The gas pressure rotates the turbine blades producing a torque that turns a generator. Thermal energy is transferred from the reactor core to the steam generator in the first loop. The second loop then uses this steam to drive the turbine. See Fig. 4.2
No longer available |Learn more- Irving Granet, Maurice Bluestein(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
Temperature is a measure of the energy contained in the molecules of a system due to their motion. When the temperature of a system is greater than that of its surroundings, some of that molecular energy is transferred to the surroundings in what we call heat. Thus, tempera-ture is a property of a system in a given state, whereas heat is associated with a change in the state of a system. Because work and heat are both forms of energy in transition, it follows that the units of work should be capable of being expressed as heat units, and vice versa. In the English system of units the conversion factor between work and heat, sometimes called mechanical equivalent of heat , is 778.169 ft.·lb f /Btu and is conventionally given the symbol J . We shall use this symbol to designate 778 ft.·lb f /Btu, because this is sufficiently accurate for engineering applications of thermodynamics. In the SI system, this conversion factor is not necessary, because the joule (N·m) is the basic energy unit. There are two forms of heat transfer: sensible heat and latent heat. Sensible heat transfer occurs when there is a temperature difference between bodies or systems and the amount Solving yields V 2 = 2 × 9.81 × 10 and V = 14.0 m/s Note that this result is independent of the mass of the body. The kinetic energy is readily found from Equation b to be equal to 10 kg × 10 m × 9.81 m/s 2 = 981 N·m. 73 Work, Energy, and Heat of heat transferred is related to the magnitude of that difference. This is the most common form. Latent heat transfer occurs when a body is changing state, such as evaporating from liquid to gas or condensing from gas to liquid. The change of phase occurs at constant temperature. This form of heat transfer is utilized in refrigeration and air conditioning systems (see Chapters 7 and 10). 2.8 Flow Work At this time, let us look at two systems, namely, the nonflow or closed system and the steady-flow or open system .
eBook - PDF- Sam Miller(Author)
- 2015(Publication Date)
- Cambridge University Press(Publisher)
45 3 Work, Heat, and Temperature 3.1. Forms of Energy The early scientists working on thermodynamics problems were interested in under-standing energy in its many forms, as well as how it changes from one to another. This concerned them for many different reasons, most of them practical. For exam-ple, Count Rumford 1 (Benjamin Thompson) was making munitions in Germany, and noticed that drilling a cannon barrel out of a solid column of brass caused the metal to get very hot. It occurred to him that the organized work done by the grinding machine was being converted into random heat. Rumford realized that a rigorous understanding of the relationship between the total amount of energy in a system, the work the system performs, and heat was needed. Eventually, a simple, elegant statement was worked out that described the relation-ship in terms of internal energy . We’ll return to that in Chapter 4 . In this chapter, we’ll develop quantitative ways of thinking about work, heat, and temperature, as well as mention what has sometimes been called “The Zeroth Law” of thermodynamics. 3.2. Work (W) One way of describing the abstract concept of energy in practical terms is to call it “the ability to do work.” Only organized energy can perform work, that is, there must be a difference between the amount of energy in one place and the amount in another that can be exploited. For example, all objects in the Earth’s gravitational field possess potential energy, also known as the energy of position . To use an object’s potential energy to perform work, there must be a way to exploit the potential energy difference between one place and another, such as using the difference in elevation between the top of a hill and the bottom of a valley. If the amount of potential energy is the same everywhere, such as in a flat area (Kansas), there’s no way to do this.
eBook - PDFThermodynamics
Concepts and Applications
- Stephen R. Turns(Author)
- 2006(Publication Date)
- Cambridge University Press(Publisher)
Like heat, work is not possessed by a thermodynamic system or a control volume but is just the name of a particular form of energy transfer from a system to the surroundings, or vice versa. For this reason we draw arrows representing work or heat that start or stop at the system or control volume boundary without crossing. In this context, we offer the following formal definition of work: Work is the transfer of energy across a system or control-volume boundary, exclusive of energy carried across the boundary by a flow, and not the result of a temperature gradient at the boundary or a difference in temperature between the system and the surroundings. Before presenting examples of work, it is useful to convert Eq. 4.6 to a form expressing the rate at which work is done. The time rate of doing work W 2 . W 1 ¢E E 2 E 1 . 1 W 2 1 W 2 F ds, dW k ˆ j ˆ i ˆ ds i ˆ dx j ˆ dy k ˆ dz, W F ds; CH. 4 ENERGY AND ENERGY TRANSFER 225 F Fcosθ ds Path 1 2 θ Q out Q in Boundary W out W in is called power, defined as (4.7a) or (4.7b) where we recognize that is the velocity vector V. From this definition, we see that power enters or exits a system or control volume wherever a component of a force is aligned with the velocity at the boundary. Types Some common types of work are listed in Table 4.1. In many situations, the power, or rate of working, is the important quantify; therefore, expressions to evaluate the power are also shown. Expansion (or Compression) Work In systems or control volumes where a boundary moves, work is performed by the system if it expands, whereas work is done on the system if the system is compressed. Concomitantly, work is done on the surroundings by an expanding system, and work is done by the surroundings when the system contracts. As an example of this type of work, consider the expansion of a gas contained in a piston–cylinder assembly as shown in Fig.
eBook - PDFThermodynamics
From Concepts to Applications, Second Edition
- Arthur Shavit, Chaim Gutfinger(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
21 3 Work, Energy, and Heat: First Law of Thermodynamics The terms work, heat, and energy are used in everyday language, sometimes interchangeably. Intuitive understanding of these terms is not sufficient for thermodynamic analysis and may lead occasionally to erroneous results. In this chapter, these terms are carefully defined. It is shown that they represent three discrete noninterchangeable concepts with a distinct relationship among them. The first law of thermodynamics is stated for a closed system and is shown to lead to the law of conservation of energy. 3.1 Work in Mechanical Systems Work in mechanics is defined as the scalar product of a force F and the displacement of its point of application d r . For a differential displacement the work is d d W F r ⋅ (3.1) In thermodynamics, where interactions are regarded from the point of view of a system, this definition may be interpreted as follows: when a system applies a force on its sur-roundings, causing a displacement at the boundary, the scalar product of the force and the boundary displacement is the work of the system. This work obviously causes changes in the environment, for example, the change in the level of a weight in a gravitational field or the stretching of a spring. As far as the system is concerned all these changes are equiva-lent insofar as they are caused by identical changes in the system and at its boundaries. So far, only the modes of work, where the force and the displacement can be easily iden-tified, have been considered. In works associated with electric, magnetic, and other phe-nomena, it may be difficult to identify the force and the displacement. The definition of work in thermodynamics is more general and covers all its possible modes, including the work in mechanics.
eBook - ePub- Irving Granet, Jorge Alvarado, Maurice Bluestein(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
We shall use the convention that heat to a system from its surroundings is positive and that heat out of a system is negative . To learn these conventions, it is convenient to consider the typical situation in which heat is transferred to a system to obtain useful work from the system. This sets the convention that heat into a system is positive and work out of the system is also positive. Positive in this sense means either desirable or conventional from the viewpoint of conventional power cycles. For refrigeration cycles, the opposite of this convention will be more useful.It is important to recognize the difference between heat and temperature. Temperature is a measure of the energy contained in the molecules of a system due to their motion. When the temperature of a system is greater than that of its surroundings, some of that molecular energy is transferred to the surroundings in what we call heat. Thus, temperature is a property of a system in a given state, whereas heat is associated with a change in the state of a system.Because work and heat are both forms of energy in transition, it follows that the units of work should be capable of being expressed as heat units, and vice versa. In the English system of units the conversion factor between work and heat, sometimes called mechanical equivalent of heat , is 778.169ft.·lbf /Btu and is conventionally given the symbol J . We shall use this symbol to designate 778ft.·lbf /Btu, because this is sufficiently accurate for engineering applications of thermodynamics. In the SI system, this conversion factor is not necessary, because the joule (N·m) is the basic energy unit.There are two forms of heat transfer: sensible heat and latent heat. Sensible heat transfer occurs when there is a temperature difference between bodies or systems and the amount of heat transferred is related to the magnitude of that difference. This is the most common form. Latent heat transfer occurs when a body is changing state, such as evaporating from liquid to gas or condensing from gas to liquid. The change of phase occurs at constant temperature. This form of heat transfer is utilized in refrigeration and air conditioning systems (see Chapters 7 and 10
eBook - PDF- Mike Tooley, Lloyd Dingle(Authors)
- 2010(Publication Date)
- Routledge(Publisher)
The heat rejected within the system is often referred to as the heat sink. We know from the second law that for a complete cycle, the net heat supplied is equal to the net work done, then from Figure 5.84 using the symbols: Q Q W in out net We also know from the second law that the total heat supplied (heat in) has to be greater than the net work done, that is, Q in W. Now the thermal efficiency ( η) of a heat engine is given by: Thermal efficiency ( net work done total heat supp net η ) ( ) W lied in ( ) Q or Thermal efficiency in out in ( ) η Q Q Q So, for example, from the above formula if the heat supplied to a thermal system is 50 MJ and the net work output of the system in this case is 30 MJ, then the efficiency of the system η 30 50 60% . Note : The ‘net work’ is W net Q in Q out 30 MJ in this case. There are many examples of the heat engine, designed to minimize thermal losses, predicted by the second law. These include among others: the steam turbine, refrigerators and air-conditioning units. The internal combustion engine is not strictly a heat engine because the heat source is mixed directly with the working fluid, but because it is used as the basis for so many thermodynamic engineering systems it has been mentioned here. TYK 5.21 1. Define heat and explain the difference between heat energy and heat flow. 2. Define a thermodynamic system using your own words. 3. What elements are necessary for a thermodynamic system? 4. Explain the difference between intrinsic and extensive properties of the working fluid. 5. ‘A closed system is one in which no mass transfer takes place and one that must have fixed system boundaries.’ State whether this statement is true or false and give a reason to qualify your answer. T e s t y o u r k n o w l e d g e TYK Mechanical Principles and Applications 431 UNIT 5 6. What is the essential difference between a closed system and an open system? 7.
eBook - PDFStatistical and Thermal Physics
Fundamentals and Applications
- M.D. Sturge(Author)
- 2018(Publication Date)
- A K Peters/CRC Press(Publisher)
On the other hand, we might have put the co ee into a metal cup (a diathermal container) and immersed it in a bowl of hot water, with exactly the same e ect on the co ee. Because U is the same, but W = 0, we have to suppose that the hot water supplied this energy. We call energy transferred from one body to another through a diathermal wall or by mixing, without any mechanical (or electrical) energy being involved, heat Q . That this is a valid procedure is shown by the empirical fact that if the hot water is isolated from the surroundings, with no external source of energy, it is found to lose the same amount of internal energy that the co ee gains, so that in the special case of a thermally isolated system in which no work is done, Q = 0, the nized centuries earlier by the monk Walter of Evesham ( circa 1280—1330) [ Dictionary of Scienti h c Biography , ed. C. C. Gillispie (Scribner, 1970—1980)]. However, confusion of the two persists in some places to this day (for some examples, see the article by Zemansky referred to in Footnote 13). 2.3. Work and Heat; the First Law of Thermodynamics 13 sum being over all the di erent bodies in the isolated system. Heat is the energy which is transferred from one object to another without mechanical or electrical means. Note that heat is de h ned only in the context of such a process of energy transfer; for this reason, some authors insist that the word “heat” should be used only as a verb, never as a noun. 12 This insistence leads to some awkward circumlocutions 13 and will not be adhered to in this book, but the reader must remember that heat is not a substance and that it makes no sense, for example, to talk of the “quantity of heat” in a body. For a general process in which work is done and energy is transferred as heat, the change in internal energy is equal to the sum of both contri-butions: U = W + Q. (2.3) Equation (2.3) is the most elementary form of the h rst law of thermody-namics.
- John Reisel(Author)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 2 The Nature of Energy 56 is to produce work, and in order to get this work, heat is added to the gases in the cylinder through the combustion process. So for an engine, the processes can be interpreted as energy being added to the gases through heat (heat in is positive) and the gases in the engine cylinders then expanding and doing work (work out is positive). From a simple analysis, heat into the system produced work out of the system—and with the sign convention used here, both terms are positive quantities. There is no right or wrong sign convention to be used as long as the same convention is used consistently to develop equations that use the derived quantities appropriately. Keep this idea in mind in Chapter 4 when we develop equations for the First Law of Thermodynamics. FIGURE 2.24 The total work experienced by a system is the sum of the contributions from all possible work modes. Total Work Rotating Shaft Work Moving Boundary Work Other Work Modes Spring Work Electrical Work Magnetic Work Chemical Work FIGURE 2.25 The sign convention for work is that work into a system is negative and work out of a system is positive. System System W = +10 kJ W = –10 kJ QUESTION FOR THOUGHT/DISCUSSION While the use of electricity is properly considered work, the energy transfer involved with electrical work may be thought of as a heat input. When might a heat transfer interpretation of the energy transfer associated with electrical work be reasonable? Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
eBook - PDF- Donald Olander(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
109 4 Heat Engines, Power Cycles, and the Thermodynamics of Open Systems 4.1 HEAT ENGINES In Section 1.9, it was noted that the first law regards Heat and Work as completely interchangeable; if a certain number of Joules of heat added to a system increases the internal energy of a body by, say, Δ U , the same number of Joules of work performed on the body would produce the same Δ U . In addition, work can be completely converted to heat, as everyday experience with friction attests. However, the reverse is not true; heat cannot be completely transformed into work. This limitation, which is a consequence of the second law, is best demonstrated by studying the properties of heat engines . A heat engine is a system operating in a cycle that receives heat from a high-temperature source (called a thermal reservoir) and produces useful work. However, since the efficiency of conversion must be less than 100%, some of the input heat is rejected to a cold reservoir. Figure 4.1 shows a schematic of a heat engine/heat pump and their associated thermal reservoirs. The reservoirs supply or receive heat without alteration of their temperatures. Heat flows in the reservoirs are reversible whether or not the engine is. FIGURE 4.1 A schematic of a heat engine or heat pump. The heat pump is a heat engine running in reverse. Hot Reservoir T H Cold Reservoir T L Heat Pump Heat Engine Cold Reservoir T L Hot Reservoir T H W W Q H Q L Q L Q H 110 General Thermodynamics The circle with the arrows in Figure 4.1 is a shorthand representation of the heat engine. It is intended to signify that the working substance (a fluid such as an ideal gas or water) moves through many thermodynamic states in a never-ending cyclic process. The detailed structure of the heat engine can vary greatly, but the simplest version contains the following four steps: 1. One in which heat is absorbed isothermally from the high-temperature reservoir. 2. The next, in which work is produced adiabatically.
eBook - PDFEntropy Generation Minimization
The Method of Thermodynamic Optimization of Finite-Size Systems and Finite-Time Processes
- Adrian Bejan(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
For any process between equilibrium states 1 and 2, the first law of thermody-namics can be written as or simply n)Q-f20W= £2 -Et Jl Jl '--v--' '------v-------energy change energy transfer (1.1) (1.2) * For historical reasons this convention can be called the heat engine sign convention (Bejan, 1988). SYSTEM E 2 -E 1 (energy change) 5 2 -5 1 (entropy change) 6 Q (entropy transfer) T ENVIRONMENT (work transfer) Figure 1.2 Closed thermodynamic system and its interactions with the environment. 3 The energy transfer interactions between the system of interest and its environ-ment, Q and W, are indicated in Figure 1.2 using arrows of different complexion. In this treatment wherever possible we use the arrow convention f-(work transfer) and <= (heat transfer) to punctuate the fundamental difference between work and heat as forms of energy transfer. The first law of thermodynamics, as stated in Equation (1.1), is a reminder that heat transfer and work transfer are quantities that depend on the path of the process. Unlike Q 1 , 2 and W 1 ,2, the energy change E 2 -£ 1 is path independent, its value being determined directly from the end equilibrium states 1 and 2. It is said that energy change (such as entropy change, pressure, temperature, etc.) is a system property. Heat transfer and work transfer interactions, whose magnitudes depend on the path of integration 1 ~ 2, are not thermodynamic properties. The second law of thermodynamics, applied to the system of Figure 1.2, is r2 sQ J1 T '--v--' :rz ~ s2 -s~ ...__,__... entropy change (1.3) The entropy transfer 8Q/T is shown in Figure 1.2, where the link among entropy transfer, heat transfer, and boundary temperature can be recognized. The entropy transfer interaction between the closed system and its environment (8Q/n is effected via heat transfer (8Q) at a frontier point of temperature T. Entropy transfer, as a concept, makes the distinction between heat transfer and work transfer as parallel forms of energy transfer.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
