Convection heat transfer pdf download

The design of a heat exchanger is an exercise in thermodynamics, which is the science that deals with heat energy flow, temperature, and the relationships to other forms of energy. To understand heat exchanger thermodynamics, a good starting point is to learn about the three ways in which heat can be transferred — conduction, convection, and radiation. In the sections below, a review of each of these heat transfer modes is presented.

Conduction is the passing of thermal energy between materials that are in contact with one another. Temperature is a measure of the average kinetic energy of molecules in a material — warmer objects that are at a higher temperature are exhibiting more molecular motion. When a warmer object is brought in contact with a cooler object one that is at a lower temperature , there is a thermal energy transfer between the two materials, with the cooler object becoming more energized and the warmer object becoming less energized.

This process will continue until thermal equilibrium has been achieved. The rate at which heat energy is transferred in a material by thermal conduction is given by the following expression:. Air and other gases generally have low thermal conductivities, while non-metallic solids exhibit higher values and metallic solids generally showing the highest values.

Convection is the transfer of thermal energy from a surface by way of the motion of a fluid such as air or water that has been heated. Most fluids expand when heated and therefore will become less dense and rise relative to other parts of the fluid that are cooler. So, when the air in a room is heated, it rises to the ceiling because it is warmer and less dense, and transfers heat energy as it collides with the cooler air in the room, then becoming denser and falling again towards the floor. This process creates a natural or free convection current.

Convection can also occur through what is termed forced or assisted convection, such as when heated water is pumped through a pipe such as in a hydronic heating system. The convective heat transfer coefficient h c is a function of the properties of the fluid, similar to the thermal conductivity of the material mentioned earlier regarding conduction. Thermal radiation is a mechanism of heat energy transfer that involves the emission of electromagnetic waves from a heated surface or object.

Unlike conduction and convection, thermal radiation does not require an intermediate medium to carry the wave energy. All objects whose temperature is above absolute zero It is also a function of the temperature of the material.

Thus, heat exchangers function by allowing a fluid of higher temperature F 1 to interact—either directly or indirectly—with a fluid of a lower temperature F 2 , which enables heat to transfer from F 1 to F 2 to move towards equilibrium.

This transfer of heat results in a decrease in temperature for F 1 and an increase in temperature for F 2. Depending on whether the application is aimed towards heating or cooling a fluid, this process and devices that employ it can be used to direct heat towards or away from a system, respectively. As outlined above, all heat exchangers operate under the same basic principles. However, these devices can be classified and categorized in several different ways based on their design characteristics.

The main characteristics by which heat exchangers can be categorized include:. The flow configuration, also referred to as the flow arrangement, of a heat exchanger refers to the direction of movement of the fluids within the heat exchanger in relation to each other.

There are four principal flow configurations employed by heat exchangers:. Cocurrent flow heat exchangers , also referred to as parallel flow heat exchangers, are heat exchanging devices in which the fluids move parallel to and in the same direction as each other.

Although this configuration typically results in lower efficiencies than a counter flow arrangement, it also allows for the greatest thermal uniformity across the walls of the heat exchanger. Countercurrent flow heat exchangers , also known as counter flow heat exchangers, are designed such that the fluids move antiparallel i.

The most commonly employed of the flow configurations, a counter flow arrangement typically exhibits the highest efficiencies as it allows for the greatest amount of heat transference between fluids and, consequently, the greatest change in temperature. In crossflow heat exchangers , fluids flow perpendicularly to one another. The efficiencies of heat exchangers which employ this flow configuration fall between that of countercurrent and cocurrent heat exchangers.

Hybrid flow heat exchangers exhibit some combination of the characteristics of the previously mentioned flow configurations. For example, heat exchanger designs can employ multiple flow passes and arrangements e. These types of heat exchangers are typically used to accommodate the limitations of an application, such as space, budget costs, or temperature and pressure requirements. While in the previous section, heat exchangers were categorized based on the type of flow configuration employed, this section categorizes them based on their construction.

The construction characteristics by which these devices can be classified include:. Heat exchangers can be classified as recuperative heat exchangers and regenerative heat exchangers. The difference between recuperative and regenerative heat exchanger systems is that in recuperative heat exchangers commonly called recuperators , each fluid simultaneously flows through its own channel within the heat exchanger.

On the other hand, regenerative heat exchangers , also referred to as capacitive heat exchangers or regenerators, alternately allow warmer and cooler fluids to flow through the same channel. Both recuperators and regenerators can be further separated into different categories of exchangers, such as direct or indirect and static or dynamic, respectively.

Of the two types indicated, recuperative heat exchangers are more commonly employed throughout industry. Recuperative heat exchangers employ either direct contact or indirect contact transfer processes to exchange heat between fluids.

In direct contact heat exchangers , the fluids are not separated within the device and heat transfers from one fluid to another through direct contact. On the other hand, in indirect heat exchangers , the fluids remain separated from one another by thermally conductive components, such as tubes or plates, throughout the heat transfer process. Do the images on this slide give you any hints? Heat is energy that has something to do with temperature and is an important concept used by engineers to design many of the products we use every day.

Slide 2 Open a discussion about what will happen to the temperature of the beverage in each case hot chocolate, iced tea when left unattended for 30 minutes. Why do some things get warmer while other things get colder when they are left out? Given time, both eventually become room temperature. The hot drink releases energy; the cold drink absorbs energy. Slide 3 Remind students about energy and some of its different forms.

Expect them to recall that moving objects have kinetic energy. Slide 4 Conduct a class demonstration to show temperature and kinetic energy using food coloring : Prepare separate transparent cups of hot and cold water ice water is best; remove the ice for the demo.

Into each cup, place a drop of food coloring and direct students to observe what happens. Expect them to notice that the food coloring in the hot water spreads out more quickly than that in the cold water. It is helpful to repeat this experiment after explaining the mechanism. Alternative: If conducting this demo is not possible, show a minute video, "Moving Water Molecules" link also provided in the Additional Multimedia Support section.

Slide 5 Talk about what students observed in the demo. The faster jiggling hot water dispersed the dye more quickly. We can think of the small dots as water molecules, and the yellow dot as a much larger dye molecule being bounced around by the water molecules' thermal jiggling. This was discovered by Scottish botanist Robert Brown, who used a microscope to look at pollen samples in water. He could not see the water molecules, but noticed that pollen in hotter water jiggled around more than in colder water.

The phenomenon was named in his honor: Brownian Motion. Slide 6 Make the point that thermal energy is in everything—even if it is something we consider cold. Slide 7 Explain the definition of heat as flowing thermal energy and clarify the direction of heat flow—from the hotter object to the cooler object. Energy transfers always occur from higher to lower states of energy. Slides Use the provided images of a hot cup of coffee, an ice cream cone and a tea kettle on a burner as examples to talk about the direction of heat flow.

Have students draw arrows to show the direction of heat flow; circulate around the room to verify their understanding. Make sure students realize that 1 heat is a form of energy that is transferred by a difference in temperature; a difference in temperature is needed for heat to flow, 2 heat always flows from hot to cold, or more precisely, heat flows from higher temperature to lower temperature, and 3 the units of heat are Joules, just like kinetic energy.

The three different types of heat transfer the movement of thermal energy are conduction, convection and radiation. The "thought experiments" on slide 13 using the examples of hot soup and snowballs give students practice in using correct terminology and full sentences to explain how heat flows.

Make sure students are able to realize that no heat transfer occurs between objects of the same temperature. Slide 14 Introduce the first type of heat transfer, conduction, which is heat transfer within or between solid objects. With our hands, we experience heat transfer by conduction any time we touch something that is hotter or colder than our skin.

At this point, present a conduction demonstration that you have prepared in advance. Before the activity, use drops of candle wax to "glue" two or three small nails or thumb tacks to a hacksaw blade or metal rod.

Heat the end of the rod with a candle flame. As heat conducts down, the wax holding the nails melts and drops the nails, one by one, in sequence. This shows students the heat traveling down the rod. Then conduct another class demonstration on heat conduction. Give each of five to 10 student volunteers an inflated balloon and have them hold them together, touching, in a line. Start to jiggle one end of the line and observe how this jiggling travels down the line of balloons. Slides Introduce and go over the other two ways heat can move from one object to another: convection and radiation.

Each slide starts with a discussion and examples and then gives a definition that can be used for building students' vocabulary. Slide 20 Introduce the concept of insulation, which is important in heat transfer and necessary background to understand the associated activity Keep It Hot!

Besides the oven mitt and pop can cozy, other examples of insulation include the walls and roof of houses, multi-pane windows, beverage thermos, insulation around car engines to keep passengers cool, inside a jet engine, material on the outside of the space shuttle, plastic casing on wires, a sweater or jacket, and refrigerator and oven walls. Slide 21 Wrap up with a brief review of key terms: heat, conduction, convection, radiation, insulation, and that heat flows from hot or higher temperature to cold or lower temperature.

Heat in Engineering: Heat is the flow of thermal energy that arises from temperature differences. Whenever two things of different temperatures are near one another, thermal energy flows. This flowing energy is called heat. The fans heard whirring in computers are designed to remove heat generated by the electronics. Without these fans, computers would melt or create fires.

On a winter morning, we put on coats to stay warm. Heat and how it flows within and between objects is something we experience every day and a fundamental engineering concern. Thermal Energy and Heat: Every object in the universe has thermal energy stored within it.

Thermal energy is the energy embodied in the vibrations, rotations and translations of atoms and molecules.

This motion is extremely fast, significantly faster than indicated in the animations typically shown, and significantly faster than bulk translation such as the flow of water molecules in a river. Expect the presence of energy in a system of jiggling, bouncing, molecules to be very obvious to students who already understand the concept of kinetic energy; indeed, the underlying physical mechanism is similar.

The energy contained in thermal "jiggling" is a function of many factors such as the mass of the particles and the speed of their motion. However, for a given material, faster molecular movement means more thermal energy is present. Thermal energy is almost impossible to confine to a location.

Rather, it can be causally observed every day. A cup of tea left on the counter cools off. Touching a hot pot lid burns one's hand. Objects that are in thermal contact tend towards thermal equilibrium, that is, they exchange thermal energy until both objects have the same temperature.

When thermal energy moves around, the flowing thermal energy is called heat. This is somewhat confused by the engineering terminology of "heat transfer" the study of just how that heat is moved around , which is somewhat redundant since the word "heat" already conveys the motion of thermal energy. In this document, "heat," "heat flow" and "heat transfer" all mean the flow of thermal energy.

One common example of thermal equilibrium is a cup of hot tea. Thermal energy in hot tea will flow as heat into the air because the tea temperature is higher than the air temperature. Heat leaving the tea causes the tea's temperature to decrease. Heat going into the air causes the air's temperature to increase. This process continues until the temperature of the tea and air is exactly the same, that is, until thermal equilibrium has been reached and no more impetus exists for thermal energy to move as heat.

This is discussed further in the presentation using the analogy of a skier on a hill. The mechanism of heat flow can be understood by remembering thermal "jiggling. We know that heat is flowing from the element to the pot, because the pot's temperature increases. If we had a sufficiently powerful microscope, we could observe the atoms in the element and the pot. All forces between objects arise from a few types of interactions: gravity, electromagnetism, and strong and weak nuclear interactions.

Collisions between objects involve forces between them that can change their motion. Any two objects in contact also exert forces on each other that are electromagnetic in origin. Gravitational, electric, and magnetic forces between a pair of objects do not require that they be in contact. These forces are explained by force fields that contain energy and can transfer energy through space.

These fields can be mapped by their effect on a test object mass, charge, or magnet, respectively. Objects with mass are sources of gravitational fields and are affected by the gravitational fields of all other objects with mass. Gravitational forces are always attractive. For two human-scale objects, these forces are too small to observe without sensitive instrumentation.

Gravitational interactions are nonnegligible, however, when very massive objects are involved. These long-range gravitational interactions govern the evolution and. Electric forces and magnetic forces are different aspects of a single electromagnetic interaction. Such forces can be attractive or repulsive, depending on the relative sign of the electric charges involved, the direction of current flow, and the orientation of magnets. All objects with electrical charge or magnetization are sources of electric or magnetic fields and can be affected by the electric or magnetic fields of other such objects.

Attraction and repulsion of electric charges at the atomic scale explain the structure, properties, and transformations of matter and the contact forces between material objects link to PS1. A and PS1. The strong and weak nuclear interactions are important inside atomic nuclei. These short-range interactions determine nuclear sizes, stability, and rates of radioactive decay see PS1. When objects touch or collide, they push on one another and can change motion or shape.

Objects in contact exert forces on each other friction, elastic pushes and pulls. Electric, magnetic, and gravitational forces between a pair of objects do not require that the objects be in contact—for example, magnets push or pull at a distance.

The sizes of the forces in each situation depend on the properties of the objects and their distances apart and, for forces between two magnets, on their orientation relative to each other. Electric and magnetic electromagnetic forces can be attractive or repulsive, and their sizes depend on the magnitudes of the charges, currents, or magnetic strengths involved and on the.

There is a gravitational force between any two masses, but it is very small except when one or both of the objects have large mass—for example, Earth and the sun. Long-range gravitational interactions govern the evolution and maintenance of large-scale systems in space, such as galaxies or the solar system, and determine the patterns of motion within those structures.

Forces that act at a distance gravitational, electric, and magnetic can be explained by force fields that extend through space and can be mapped by their effect on a test object a ball, a charged object, or a magnet, respectively. Forces at a distance are explained by fields permeating space that can transfer energy through space. Magnets or changing electric fields cause magnetic fields; electric charges or changing magnetic fields cause electric fields. Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects.

The strong and weak nuclear interactions are important inside atomic nuclei—for example, they determine the patterns of which nuclear isotopes are stable and what kind of decays occur for unstable ones. Events and processes in a system typically involve multiple interactions occurring simultaneously or in sequence.

A stable system is one in which the internal and external forces are such that any small change results in forces that return the system to its prior state e. A system can be static but unstable, with any small change leading to forces that tend to increase that change e. And a stable system can appear to be unchanging when flows or processes within it are going on at opposite but equal rates e.

Stability and instability in any system depend on the balance of competing effects. A steady state of a complex system can be maintained through a set of feedback mechanisms, but changes in conditions can move the system out of its range of stability e. With no energy inputs, a system starting out in an unstable state will continue to change until it reaches a stable configuration e. Viewed at a given scale, stable systems may appear static or dynamic.

Conditions and properties of the objects within a system affect the rates of energy transfer and thus how fast or slowly a process occurs e. When a system has a great number of component pieces, one may not be able to predict much about its precise future. For such systems e. Whether an object stays still or moves often depends on the effects of multiple pushes and pulls on it e. It is useful to investigate what pushes and pulls keep something in place e. A system can change as it moves in one direction e.

A system can appear to be unchanging when processes within the system are occurring at opposite but equal rates e. Changes can happen very quickly or very slowly and are sometimes hard to see e. Conditions and properties of the objects within a system affect how fast or slowly a process occurs e.

A stable system is one in which any small change results in forces that return the system to its prior state e. A system can be static but unstable e. Many systems, both natural and engineered, rely on feedback mechanisms to maintain stability, but they can function only within a limited range of conditions. Systems often change in predictable ways; understanding the forces that drive the transformations and cycles within a system, as well as the forces imposed on the system from the outside, helps predict its behavior under a variety of conditions.

Systems may evolve in unpredictable ways when the outcome depends sensitively on the starting condition and the starting condition cannot be specified precisely enough to distinguish between different possible outcomes. Interactions of objects can be explained and predicted using the concept of transfer of energy from one object or system of objects to another.

The total energy within a defined system changes only by the transfer of energy into or out of the system. Regardless of the quantities of energy transferred. At the macroscopic scale, energy manifests itself in multiple phenomena, such as motion, light, sound, electrical and magnetic fields, and thermal energy.

Historically, different units were introduced for the energy present in these different phenomena, and it took some time before the relationships among them were recognized. Energy is best understood at the microscopic scale, at which it can be modeled as either motions of particles or as stored in force fields electric, magnetic, gravitational that mediate interactions between particles. This last concept includes electromagnetic radiation, a phenomenon in which energy stored in fields moves across space light, radio waves with no supporting matter medium.

Motion energy is also called kinetic energy; defined in a given reference frame, it is proportional to the mass of the moving object and grows with the square of its speed. Matter at any temperature above absolute zero contains thermal energy. Thermal energy is the random motion of particles whether vibrations in solid matter or molecules or free motion in a gas , this energy is distributed among all the particles in a system through collisions and interactions at a distance.

In contrast, a sound wave is a moving pattern of particle vibrations that transmits energy through a medium. Electric and magnetic fields also contain energy; any change in the relative positions of charged objects or in the positions or orientations of magnets changes the fields between them and thus the amount of energy stored in those fields.

When a particle in a molecule of solid matter vibrates, energy is continually being transformed back and forth between the energy of motion and the energy stored in the electric and magnetic fields within the matter.

Matter in a stable form minimizes the stored energy in the electric and magnetic fields within it; this defines the equilibrium positions and spacing of the atomic nuclei in a molecule or an extended solid and the form of their combined electron charge distributions e. Energy stored in fields within a system can also be described as potential energy.

For any system where the stored energy depends only on the spatial configuration of the system and not on its history, potential energy is a useful concept e. It is defined as a difference in energy compared to some arbitrary reference configuration of a system. For example, lifting an object increases the stored energy in the gravitational field between that object and Earth gravitational potential energy.

When a pendulum swings, some stored energy is transformed into kinetic energy and back again into stored energy during each swing. In both examples energy is transferred out of the system due to collisions with air and for the pendulum also by friction in its support.

Any change in potential energy is accompanied by changes in other forms of energy within the system, or by energy transfers into or out of the system. Electromagnetic radiation such as light and X-rays can be modeled as a wave of changing electric and magnetic fields. At the subatomic scale i. Electromagnetic radiation from the sun is a major source of energy for life on Earth.

The idea that there are different forms of energy, such as thermal energy, mechanical energy, and chemical energy, is misleading, as it implies that the nature of the energy in each of these manifestations is distinct when in fact they all are ultimately, at the atomic scale, some mixture of kinetic energy, stored energy, and radiation. It is likewise misleading to call sound or light a form of energy; they are phenomena that, among their other properties, transfer energy from place to place and between objects.

The faster a given object is moving, the more energy it possesses. Energy can be moved from place to place by moving objects or through sound, light, or electric currents. Boundary: At this grade level, no attempt is made to give a precise or complete definition of energy. Motion energy is properly called kinetic energy; it is proportional to the mass of the moving object and grows with the square of its speed.

A system of objects may also contain stored potential energy, depending on their relative positions. For example, energy is stored—in gravitational interaction with Earth—when an object is raised, and energy is released when the object falls or is lowered. Energy is also stored in the electric fields between charged particles and the magnetic fields between magnets, and it changes when these objects are moved relative to one another.

Stored energy is decreased in some chemical reactions and increased in others. In science, heat is used only for this second meaning; it refers to energy transferred when two objects or systems are at different temperatures. Temperature is a measure of the average kinetic energy of particles of matter.

The relationship between the temperature and the total energy of a system depends on the types, states, and amounts of matter present. Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system.

At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy. Historically, different units and names were used for the energy present in these different phenomena, and it took some time before the relationships between them were recognized.

These relationships are better understood at. This last concept includes radiation, a phenomenon in which energy stored in fields moves across space. What is meant by conservation of energy? How is energy transferred between objects or systems? The total change of energy in any system is always equal to the total energy transferred into or out of the system. This is called conservation of energy. Energy cannot be created or destroyed, but it can be transported from one place to another and transferred between systems.

Many different types of phenomena can be explained in terms of energy transfers. Mathematical expressions, which quantify changes in the forms of energy within a system and transfers of energy into or out of the system, allow the concept of conservation of energy to be used to predict and describe the behavior of a system. When objects collide or otherwise come in contact, the motion energy of one object can be transferred to change the motion or stored energy e.

For macroscopic objects, any such process e. For molecules, collisions can also result in energy transfers through chemical processes, which increase or decrease the total amount of stored energy within a system of atoms; the change in stored energy is always balanced by a change in total kinetic energy—that of the molecules present after the process compared with the kinetic energy of the molecules present before it.

Energy can also be transferred from place to place by electric currents. Heating is another process for transferring energy.

Heat transfer occurs when two objects or systems are at different temperatures. Energy moves out of higher temperature objects and into lower temperature ones, cooling the former and heating the latter.

This transfer happens in three different ways—by conduction within solids, by the flow of liquid or gas convection , and by radiation, which can travel across space. Even when a system is isolated such as Earth in space , energy is continually being transferred into and out of it by radiation. The processes underlying convection and conduction can be understood in terms of models of the possible motions of particles in matter.

Radiation can be emitted or absorbed by matter. Uncontrolled systems always evolve toward more stable states—that is, toward more uniform energy distribution within the system or between the system and its environment e. Any object or system that can degrade with no added energy is unstable. Eventually it will change or fall apart, although in some cases it may remain in the unstable state for a long time before decaying e.

Energy is present whenever there are moving objects, sound, light, or heat. When objects collide, energy can be transferred from one object to another, thereby changing their motion.

In such collisions, some energy is typically also transferred to the surrounding air; as a result, the air gets heated and sound is produced. Light also transfers energy from place to place. For example, energy radiated from the sun is transferred to Earth by light. Energy can also be transferred from place to place by electric currents, which can then be used locally to produce motion, sound, heat, or light.

The currents may have been produced to begin with by transforming the energy of motion into electrical energy e. When the motion energy of an object changes, there is inevitably some other change in energy at the same time. For example, the friction that causes a moving object to stop also results in an increase in the thermal energy in both surfaces; eventually heat energy is transferred to the surrounding environment as the surfaces cool.

Similarly, to make an object start moving or to keep it moving when friction forces transfer energy away from it,. The amount of energy transfer needed to change the temperature of a matter sample by a given amount depends on the nature of the matter, the size of the sample, and the environment. Energy is transferred out of hotter regions or objects and into colder ones by the processes of conduction, convection, and radiation. Conservation of energy means that the total change of energy in any system is always equal to the total energy transferred into or out of the system.

Mathematical expressions, which quantify how the stored energy in a system depends on its configuration e. The availability of energy limits what can occur in any system. Uncontrolled systems always evolve toward more stable states—that is, toward more uniform energy distribution e. Eventually it will do so, but if the energy releases throughout the transition are small, the process duration can be very long e.

When two objects interact, each one exerts a force on the other. These forces can transfer energy between the objects. Forces between two objects at a distance are explained by force fields gravitational, electric, or magnetic between them. Contact forces between colliding objects can be modeled at the microscopic level as due to electromagnetic force fields between the surface particles. When two objects interacting via a force field change their relative position, the energy in the.

For any such pair of objects the force on each object acts in the direction such that motion of that object in that direction would reduce the energy in the force field between the two objects. However, prior motion and other forces also affect the actual direction of motion.

Patterns of motion, such as a weight bobbing on a spring or a swinging pendulum, can be understood in terms of forces at each instant or in terms of transformation of energy between the motion and one or more forms of stored energy. Elastic collisions between two objects can be modeled at the macroscopic scale using conservation of energy without having to examine the detailed microscopic forces.

A bigger push or pull makes things go faster. Faster speeds during a collision can cause a bigger change in shape of the colliding objects. Magnets can exert forces on other magnets or on magnetizable materials, causing energy transfer between them e. When two objects interact, each one exerts a force on the other that can cause energy to be transferred to or from the object. For example, when energy is transferred to an Earth-object system as an object is raised, the gravitational field energy of the system increases.

This energy is released as the object falls; the mechanism of this release is the gravitational force. Likewise, two magnetic and electrically charged objects interacting at a distance exert forces on each other that can transfer energy between the interacting objects.

Force fields gravitational, electric, and magnetic contain energy and can transmit energy across space from one object to another. When two objects interacting through a force field change relative position, the energy stored in the force field is changed. Each force between the two interacting objects acts in the direction such that motion in that direction would reduce the energy in the force field between the objects.

How do food and fuel provide energy? If energy is conserved, why do people say it is produced or used? This refers to the fact that energy in concentrated form is useful for generating electricity, moving or heating objects, and producing light, whereas diffuse energy in the environment is not readily captured for practical use.

Therefore, to produce energy typically means to convert some stored energy into a desired form—for example, the stored energy of water behind a dam is released as the water flows downhill and drives a turbine generator to produce electricity, which is then delivered to users through distribution systems. Food, fuel, and batteries are especially convenient energy resources because they can be moved from place to place to provide processes that release energy where needed. A system does not destroy energy when carrying out any process.

However, the process cannot occur without energy being available. The energy is also not destroyed by the end of the process. Most often some or all of it has been transferred to heat the surrounding environment; in the same sense that paper is not destroyed when it is written on, it still exists but is not readily available for further use.

Naturally occurring food and fuel contain complex carbon-based molecules, chiefly derived from plant matter that has been formed by photosynthesis. The chemical reaction of these molecules with oxygen releases energy; such reactions provide energy for most animal life and for residential, commercial, and industrial activities.

Electric power generation is based on fossil fuels i. Transportation today chiefly depends on fossil fuels, but the use of electric and alternative fuel e.