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584 Chapter 14 | Heat and Heat Transfer Methods
change its kinetic or potential energy. However, heat occurs as a spontaneous process, in which thermal energy is transferred from a higher temperature system to a lower temperature system.
Big Idea 1 is supported by examination of the internal structure of systems, which determines the nature of those energy changes and the mechanism of heat transfer. Macroscopic properties, such as heat capacity, latent heat, and thermal conductivity, depend on the arrangements and interactions of the atoms or molecules in a substance. The arrangement of these particles also determines whether thermal energy will be transferred through direct physical contact between systems (conduction), through the motion of fluids with different temperatures (convection), or through emission or absorption of radiation.
Big Idea 1 Objects and systems have properties such as mass and charge. Systems may have internal structure.
Enduring Understanding 1.E Materials have many macroscopic properties that result from the arrangement and interactions of
the atoms and molecules that make up the material.
Essential Knowledge 1.E.3 Matter has a property called thermal conductivity.
Big Idea 4 Interactions between systems can result in changes in those systems.
Enduring Understanding 4.C Interactions with other objects or systems can change the total energy of a system.
Essential Knowledge 4.C.3 Energy is transferred spontaneously from a higher temperature system to a lower temperature system. The process through which energy is transferred between systems at different temperatures is called heat.
Big Idea 5 Changes that occur as a result of interactions are constrained by conservation laws. Enduring Understanding 5.B The energy of a system is conserved.
Essential Knowledge 5.B.6 Energy can be transferred by thermal processes involving differences in temperature; the amount of energy transferred in this process of transfer is called heat.
14.1 Heat
In Work, Energy, and Energy Resources, we defined work as force times distance and learned that work done on an object changes its kinetic energy. We also saw in Temperature, Kinetic Theory, and the 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 if the temperature is higher. If two objects at different temperatures are brought in contact with each other, energy is transferred from the hotter to the colder object until equilibrium is reached and the bodies reach thermal equilibrium (i.e., they are at the same temperature). No work is done by either object, because no force acts through a distance. The transfer of energy is caused by the temperature difference, and ceases once the temperatures are equal. These observations lead to the following definition of heat: Heat is the spontaneous transfer of energy due to a temperature difference.
As noted in Temperature, Kinetic Theory, and the Gas Laws, heat is often confused with temperature. For example, we may say the heat was unbearable, when we actually mean that the temperature was high. Heat is a form of energy, whereas temperature is not. The misconception arises because we are sensitive to the flow of heat, rather than the temperature.
Owing 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 by —specifically, between and , since there is a slight temperature dependence. Perhaps the most common unit of heat is the kilocalorie (kcal), which is the energy needed to change the temperature of 1.00 kg of water by . Since mass is most often specified in kilograms, kilocalorie is commonly used. Food calories (given the notation Cal, and sometimes called “big calorie”) are actually kilocalories (
), a fact not easily determined from package labeling.
Learning Objectives
By the end of this section, you will be able to: • Define heat as transfer of energy.
The information presented in this section supports the following AP® learning objectives and science practices:
• 4.C.3.1 The student is able to make predictions about the direction of energy transfer due to temperature differences based on interactions at the microscopic level. (S.P. 6.1)
This OpenStax book is available for free at http://cnx.org/content/col11844/1.14