12.6

Heat and Internal Energy

An object with a high temperature is said to be hot, and the word “hot” brings to mind the word “heat.” Heat flows from a hotter object to a cooler object when the two are placed in contact. It is for this reason that a cup of hot coffee feels hot to the touch, while a glass of ice water feels cold. When the person in Figure 12-22a touches the coffee cup, heat flows from the hotter cup into the cooler hand. When the person touches the glass in part b of the drawing, heat again flows from hot to cold, in this case from the warmer hand into the colder glass. The response of the nerves in the hand to the arrival or departure of heat prompts the brain to identify the coffee cup as being hot and the glass as being cold.

Figure 12-22 Heat is energy in transit from hot to cold. (a) Heat flows from the hotter coffee cup to the colder hand. (b) Heat flows from the warmer hand to the colder glass of ice water.

But just what is heat? As the following definition indicates, heat is a form of energy, energy in transit from hot to cold.

| Definition of Heat
| Heat is energy that flows from a higher-temperature object to a lower-temperature object because of the difference in temperatures. SI Unit of Heat: joule (J)

Being a kind of energy, heat is measured in the same units used for work, kinetic energy, and potential energy. Thus, the SI unit for heat is the joule.
The heat that flows from hot to cold in Figure 12-22 originates in the internal energy of the hot substance. The internal energy of a substance is the sum of the molecular kinetic energy (due to the random motion of the molecules), the molecular potential energy (due to forces that act between the atoms of a molecule and between molecules), and other kinds of molecular energy. When heat flows in circumstances where the work done is negligible, the internal energy of the hot substance decreases and the internal energy of the cold substance increases. Although heat may originate in the internal energy supply of a substance, it is not correct to say that a substance contains heat. The substance has internal energy, not heat. The word “heat” is used only when referring to the energy actually in transit from hot to cold.

CONCEPTS AT A GLANCE In the next two sections, we will consider some of the effects of heat. For instance, when preparing spaghetti for dinner, the first thing that the cook does is to place a pot of water on the stove. Heat from the stove causes the internal energy of the water to increase, as the Concepts-at-a-Glance chart in Figure 12-23 suggests. Associated with this increase in internal energy is a rise in temperature. After a while, however, the temperature reaches 100 °C and the water begins to boil. During boiling, any heat added to the water goes into producing steam, a process in which water changes from a liquid phase to a vapor phase. The next section investigates how the addition (or removal) of heat causes the temperature of a substance to change (see the upper-right portion of the chart in Figure 12-23). Then, Section 12.8 discusses the relationship between heat and phase changes (see the lower-right part of Figure 12-23), such as that which occurs when water boils.

Figure 12-23 CONCEPTS AT A GLANCE When heat is added to or removed from a substance, its internal energy can change. This change can cause a change in temperature (Section 12.7) or a change in phase (Section 12.8). Because heat has been added to the water in this hot tub, it is obvious that its temperature has risen above that of the cold surroundings. (Ryan McVay/Photodisc Red/Getty Images)

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