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actually a transfer of energy by working. This brings us back Btu per hour. A much larger unit is sometimes mentioned in

to the definition that “energy is the ability to do work.” We can news reports and articles about the national energy consump-
mentally note that this includes the ability to do work at the tion. This unit is the quad, which is 1 quadrillion Btu (a million

15
molecular level. billion or 10 Btu).
Heating that takes place because of a temperature diff er- Heat is increased by an energy-form conversion, and the
ence will be considered in greater detail after we consider how equivalence between energy and heating was fi rst measured

heat is measured. by James Joule. He found that the relationship between the
energy form (mechanical, electrical radiant, etc.) and the
resulting heating was always the same. For example, the rela-
MEASURES OF HEAT
tionship between mechanical work done and the resulting
Since heating is a method of energy transfer, a quantity of heat heating is always
can be measured just like any quantity of energy. Th e metric
4.184 J = 1 cal
unit for measuring work, energy, or heat is the joule. However,
the separate historical development of the concepts of heat and or
the concepts of motion resulted in separate units, some based
4,184 J = 1 kcal
on temperature diff erences.

The metric unit of heat is called the calorie (cal). A calorie The establishment of this precise proportionality means that,

is defined as the amount of energy (or heat) needed to increase fundamentally, mechanical work and heat are diff erent forms

the temperature of 1 gram of water 1 degree Celsius. A more of the same thing.
precise defi nition specifies the degree interval from 14.5°C

to 15.5°C because the energy required varies slightly at dif-

ferent temperatures. This precise definition is not needed for EXAMPLE 4.4

a general dis cussion. One kilocalorie (kcal) is the amount of A 1,000.0 kg car is moving at 90.0 km/h (25.0 m/s). How many
energy (or heat) needed to increase the temperature of 1 kilogram kilocalories are generated when the car brakes to a stop?

of water 1 degree Celsius. The measure of the energy released
by the oxidation of food is the kilocalorie, but it is called the SOLUTION
Calorie (with a capital C) by nutritionists (Figure 4.11). Confu-

The kinetic energy of the car is
sion can be avoided by making sure that the scientific calorie is

never capitalized (cal) and the dieter’s Calorie is always capital- 1 _ 2

KE = mv

ized. The best solution would be to call the Calorie what it is, a 2
kilocalorie (kcal). 1 _ 2

= (1,000.0 kg)(25.0 m/s)
The English system’s measure of heating is called the Brit- 2

2
ish thermal unit (Btu). One Btu is the amount of energy (or heat) kg . m
_

= (500.0)(625)
needed to increase the temperature of 1 pound of water 1 degree s
2

Fahrenheit. The Btu is commonly used to measure the heating
= 312,500 J
or cooling rates of furnaces, air conditioners, water heaters, and
so forth. The rate is usually expressed or understood to be in You can convert this to kcal by using the relationship between

mechanical energy and heat:
)
(
1 kcal

(312,500 J) _
Thermometer 4,184 J
312,500 J . kcal
_ _

Wires 4,184 J
74.7 kcal

(Note: The temperature increase from this amount of heating could be
calculated from equation 4.4.)
Water SPECIFIC HEAT
You can observe a relationship between heat and diff erent sub-
Food
stances by doing an experiment in “kitchen physics.” Imagine
that you have a large pot of liquid to boil in preparing a meal.

FIGURE 4.11 The Calorie value of food is determined by Three variables influence how much heat you need:
measuring the heat released from burning the food. If there is

10.0 kg of water and the temperature increased from 10° to 1. The initial temperature of the liquid;
20°C, the food contained 100 Calories (100,000 calories). The 2. How much liquid is in the pot; and,

food illustrated here would release much more energy than this. 3. The nature of the liquid (water or soup?).
94 CHAPTER 4 Heat and Temperature 4-10