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Internal
energy

External energy

One liter 250 milliliter
of water of water
FIGURE 4.9 External energy is the kinetic and potential energy at 90° C at 90° C
that you can see. Internal energy is the total kinetic and potential
FIGURE 4.10 Heat and temperature are different concepts,
energy of molecules. When you push a table across the floor, you do
work against friction. Some of the external mechanical energy goes as shown by a liter of water (1,000 mL) and a 250 mL cup of
water, both at the same temperature. You know the liter of water
into internal kinetic and potential energy, and the bottom surfaces
of the legs become warmer. contains more internal energy because it will require more ice
cubes to cool it to, say, 25°C than will be required for the cup of
water. In fact, you will have to remove 48,750 additional calories
energy. External energy is the total potential and kinetic energy to cool the liter of water.
of an everyday-sized object. All the kinetic and potential energy
considerations discussed in chapters 2 and 3 were about the

external energy of an object. losing energy. This measurement procedure will also give us a
Internal energy is the total kinetic and potential energy of working defi nition of heat:

the molecules of an object. The kinetic energy of a molecule can
Heat is a measure of the internal energy that has been
be much more complicated than straight-line velocity might sug-
absorbed or transferred from one body to another.
gest, however, because a molecule can have many diff erent types
of motion at the same time (pulsing, twisting, turning, etc.). Th e process of increasing the internal energy is called heating,
Overall, internal energy is characterized by properties such as and the process of decreasing internal energy is called cooling.
temperature, density, heat, volume, pressure of a gas, and so forth. Th e word process is italicized to emphasize that heat is energy

When you push a table across the floor, the observable in transit, not a material thing you can add or take away. Heat
external kinetic energy of the table is transferred to the internal is understood to be a measure of internal energy that can be

kinetic energy of the molecules between the table legs and the mea sured as energy flows into or out of an object.
floor, resulting in a temperature increase (Figure 4.9). Th e rela-

tionship between external and internal kinetic energy explains Two Heating Methods
why the heating is proportional to the amount of mechanical There are two general ways that heating can occur. Th ese are

energy used. (1) from a temperature difference, with energy moving from the

region of higher temperature, and (2) from an object gaining
HEAT AS ENERGY TRANSFER energy by way of an energy-form conversion.
When a temperature diff erence occurs, energy is transferred
Temperature is a measure of the degree of hotness or coldness of
from a region of higher temperature to a region of lower tem-
a body, a measure that is based on the average molecular kinetic
perature. Energy flows from a hot range element, for example,

energy. Heat, on the other hand, is based on the total internal
to a pot of cold water on a range. It is a natural process for
energy of the molecules of a body. You can see one diff erence in
energy to flow from a region of higher temperature to a region

heat and temperature by considering a cup of water and a large
of a lower temperature just as it is natural for a ball to roll down-
tub of water. If both the small and the large amount of water have
hill. The temperature of an object and the temperature of the sur-

the same temperature, both must have the same average molecu-
roundings determine if heat will be transferred to or from an
lar kinetic energy. Now, suppose you wish to cool both by, say,
object. Th e terms heating and cooling describe the direction of

20°. The large tub of water would take much longer to cool, so it

energy flow, naturally moving from a region of higher energy to
must be that the large amount of water has more internal energy
one of lower energy.
(Figure 4.10). Heat is a measure based on the total internal energy
The internal energy of an object can be increased during

of the molecules of a body, and there is more total energy in a
an energy-form conversion (mechanical, radiant, electrical,
large tub of water than in a cup of water at the same temperature.

etc.), so we say that heating is taking place. The classic experi-
ments by Joule showed an equivalence between mechanical
Heat Defined energy and heating, electrical energy and heating, and other
How can we measure heat? Since it is difficult to see molecules, conversions. On a molecular level, the energy forms are doing

internal energy is difficult to measure directly. Thus, heat is work on the molecules, which can result in an increase of

nearly always measured during the process of a body gaining or internal energy. Thus, heating by energy-form conversion is

4-9 CHAPTER 4 Heat and Temperature 93