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Work and Heat Are Two Equivalent Forms of Energy

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As mentioned in the chapter Temperature, kinetic theory and laws of gases, heat is often confused with temperature. For example, we can say that the heat was unbearable when we actually think the temperature was high. Heat is a form of energy, while temperature is not. The misunderstanding comes from the fact that we are sensitive to heat flow rather than temperature. We think of this equation as the conversion between two different units of energy. Why can`t electrical, chemical or nuclear energy or such energies be a form of energy transmission? It is also possible to change the temperature of a substance through work. Work can transfer energy inside or outside a system. This realization helped establish the fact that heat is a form of energy. James Prescott Joule (1818-1889) conducted numerous experiments to establish the mechanical equivalent of heat – the work required to produce the same effects as heat transfer. In terms of units used for these two terms, the best modern value for this equivalence is 1,000 kcal = 4186 J.

Conclusion: electrical energy, chemical energy, nuclear energy, etc. are all forms of energy. Heat and work, on the other hand, are not forms of energy. Things don`t “contain” heat or don`t work. Strictly speaking, they are means of energy transmission (with mass transfer). As defined in thermodynamics, heat is energy that exceeds a boundary of a system without mass transfer solely due to a temperature difference between the system and its environment, and work is energy that exceeds a limit of a system without mass transfer due to an intense property difference other than temperature between the system and its environment. Work therefore includes all uncontrolled energy transfers across the boundaries of a system, with the exception of mass transfer, such as electrical work in addition to mechanical work (force-causing movement). Note that neither heat nor work involves mass transfer.

Heat and work, on the other hand, are mechanisms of energy transfer, but not forms of energy themselves. In the case of heat, it is a transfer of energy that is solely due to the difference in temperature. In the exothermic chemical reaction discussed above, the rise in temperature during the reaction can result in a transfer of energy in the form of heat to the colder environment. Due to the fact that heat is a form of energy, it has the SI unit of joule (J). The calorie (cal) is a common unit of energy, defined as the energy needed to change the temperature of 1.00 g of water from 1.00 ° C – especially between 14.5 ° C and 15.5 ° C, since there is a slight dependence on temperature. Perhaps the most common unit of heat is the kilocalorie (kcal), the energy required to change the temperature of 1.00 kg of water by 1.00 °C. Since mass is most often expressed in kilograms, kilocalorie is often used. Food calories (with Cal notation and sometimes called “big calories”) are actually kilocalories (1 kilocalorie = 1000 calories), a fact that is not easy to determine from the labeling of the package. For a given form of energy, there may also be transformations between potential energy and kinetic energy. For example, in an exothermic chemical reaction, the chemical molecular potential energy is converted into chemical molecular kinetic energy, resulting in an increase in temperature. At the risk of circularity, we can define heat as the energy entering or leaving a system due to a temperature difference between the system and the environment. The very different mechanisms of conduction and radiation, which are controlled by body temperatures, can thus lead to a transfer of energy by heat.

The classic case of energy transfer through work is actually a force that travels over a distance, as when a piston is moved in a gas cylinder. If the system contains a heating coil and we transmit a current from an external battery, we deliver the work to the system again – electrical work. [There`s a catch, though. The work is irreversible. It could just as easily be heat and must be treated as such in the application of the second law.] Figure 1. In Figure (a), soft drink and ice have different temperatures, T1 and T2, and are not in thermal equilibrium. In Figure (b), when soft drink and ice cream are allowed to interact, energy is transferred until they reach the same temperature T′, thus reaching equilibrium. Heat transfer occurs due to the temperature difference.

Since both soft drink and ice are in contact with the ambient air and the bench, the equilibrium temperature is the same for both. Figure 2. Schematic representation of the Joule experiment that determined the equivalence of heat and work. Mechanical equivalent of heat: the work required to produce the same effects as heat transfer Nuclear energy is a form of internal energy that is a means of storing energy (not transferring energy). Chemical energy is almost always associated with potential energy functions, which is also another form of energy storage. What are the other possibilities for energy transfer outside of heat and work? Further examples and explanations of this form of energy transfer can be found in John W. Jewett “Energy and the Confused Student: A Global Approach to Energy”, especially in this one, on the second page Two samples (A and B) of the same substance are stored in a laboratory. Someone adds 10 kilojoules (kJ) of heat to one sample while 10 kJ of work is done on the other sample. How can you know which sample the heat was added to? Both heat and work change the internal energy of the substance. However, the properties of the sample depend only on the internal energy, so it is impossible to tell whether sample A or B was supplied with heat.

In the chapter Work, Energy and Energy Resources, we defined work as force multiplied by distance and learned that working on an object changes its kinetic energy. We have also seen in temperature, kinetic theory and gas laws that temperature is proportional to the (average) kinetic energy of atoms and molecules. We say that a thermal system has a certain internal energy: its internal energy is higher when the temperature is higher. When two objects of different temperatures are brought into contact with each other, energy is transferred from the hotter object to the cooler object until equilibrium is reached and the bodies reach thermal equilibrium (i.e. they have the same temperature). No work is done by either object because no force acts remotely. Energy transfer is caused by the temperature difference and stops as soon as the temperatures are equal. These observations lead to the following definition of heat: heat is the spontaneous transfer of energy due to a difference in temperature. A closed system is defined in thermodynamics as a system in which no transfer of heat, work or mass occurs. In a closed system, exothermic chemical or nuclear reactions can increase the pressure/temperature in the system and are in this sense energy transfers.

Classical thermodynamics deals with these internal energy sources using the heat of formation or the heat of reaction. Thanks to Einstein, we now know that such exothermic reactions convert idle mass into kinetic energy. (That is, the resting masses of the products of the exothermic reaction are smaller than the rest masses of the reactants, and this difference is an increase in the kinetic energy of the products relative to the reactants.) As for the chemical and nuclear energies to which you refer, it is difficult to consider them as energy transfers. How are they supposed to cross the boundaries of a system? It is understandable that if they are already in the system, for example, the system could initially consist of a mixture of hydrogen and oxygen.

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