Chapter 26
26.3 Specific Heat
Energy is always conserved and can take many forms. Energy can be converted from kinetic energy
into potential energy. Electrical energy can be converted into thermal energy. In this section, you will
learn about the transfer of thermal energy from one body to another body.
Flow of heat and equilibrium
Energy can When heat flows from a warm mug of hot chocolate to your hand, there is an
change from one exchange of energy for both objects. The warm mug loses energy and cools down.
object to another Your hand gains energy and warms up. When you touch the mug, your hand is in
thermal contact with it.
Energy loss is Suppose you were able to take some objects or substances in thermal contact with
equal to energy each other and place them in a container that would not allow any energy to leave
gain the system. For example, you place a cup of hot coffee mixed with a cup of ice
into your container. Because the mixed drink is isolated from the outside, the
energy that the hot coffee loses must equal the energy that the ice gains. This is an
example of the law of conservation of energy. When we are talking about heat, this
law is also known as the first law of thermodynamics. Both laws state that the
energy in an isolated system is conserved.
Heat flow and the
origin of the word calorie
We now understand that the
flow of heat is due to the
transfer of energy. However,
until the 1840s scientists
thought that heat traveled by
an invisible fluid called
caloric, which comes from
the Latin word for heat. We
still use the word calorie
even though we no longer
believe in a fluid called
caloric.
What is thermal Have you ever filled your kitchen sink with hot water to wash dishes? If the water
equilibrium? was too hot, you may have added cold water to cool down the hot water. The
temperature of the water in the sink eventually reaches a balance where everything
is evenly warm. Whenever you have a hot object or substance in thermal contact
with a cold one, heat will flow from the hot object to the cold object until they are
at the same temperature, which means they are in thermal equilibrium.
When do objects Objects reach thermal equilibrium when they reach the same temperature. When
reach thermal you submerge a thermometer in water to measure its temperature, you need to wait
equilibrium? for a few seconds until you see the mercury or alcohol level of the thermometer
stops rising. At that point, the thermometer and the water will have reached the
same temperature. Both objects transfer energy (heat) until they reach thermal
equilibrium. Thus, the reading you get is the thermometer’s own temperature.
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Figure 26.11: When you hold a
mug of hot chocolate, your hand is in
thermal contact with the cup.
Chapter 26
Specific heat
What does You learned in the last section that when heat flows it raises the temperature of
specific heat some substances more than others. We refer to this property of a substance as its
mean?
calorie
-.
specific heat. For example, the specific heat for water is equal to 1 ----------------gram°C
Water has a high It is important to point out that other substances have very different specific heat
specific heat values. Water has a very high specific heat, which is why it is used as a coolant. It
takes more energy to increase the temperature of water than for other substances in
nature.
Metals have low Figure 26.12 shows the specific heats for different substances. For example,
caloriespecific heats aluminum has a specific heat 0.215 ----------------. Like most metals, aluminum has a
gram°C
specific heat that is five times smaller than the specific heat of water. If we add
1 calorie of energy to one gram of water, the temperature of the water will increase
1°C. However, if we add 1 calorie of energy to 1 gram of aluminum, the
temperature will increase almost 5°C!
Although the temperature of water did not go up by very much, the water still
absorbed the heat that flowed into it. Therefore, water at a certain temperature will
transfer more heat to a cooler object than most other substances. For example, 10
grams of water at 90°C will raise the temperature of a cup of cold water much
higher than would 10 grams of aluminum at 90°C.
substance
specific
heat
calorie
-------------------gram°C
water
1.00
ice
0.493
benzene
0.416
methanol
0.586
ethanol
0.584
aluminum
0.215
carbon
0.170
silver
0.052
gold
0.031
Figure 26.12: The specific heats
of some common substances.
26.3 Specific Heat
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Chapter 26
Joseph Black
Scottish chemist Joseph Black developed the theory of specific heat in 1760.
Black realized the difference between heat that increases the temperature of a
substance, and heat that melts or boils a substance. For instance, if we add heat
to water, initially the temperature of the water increases. Black called this heat
sensible heat because it could be sensed with a thermometer. Once the
temperature of water reaches 100°C, it boils. Any heat added to boiling water causes water
to vaporize, but does not raise the temperature. Black called the heat for melting and boiling
latent heat because it could not be sensed with a thermometer. Latent means “hidden.”
More about specific heat: the law of Dulong and Petit
The specific heat Every substance has a different specific heat, but what physical principle
of an element gives rise to these differences? The answer was discovered in 1819 by the
French physicists Pierre Louis Dulong and Alexis Therese Petit. They
showed that the specific heat of an element depends on how many atoms
there are per gram. One gram of a heavy element, like silver, will have
fewer atoms than 1 gram of aluminum.
How energy is Suppose you add one calorie of heat to two samples (aluminum and
divided silver) of equal mass. The temperature of the silver goes up by 18°C
while the aluminum only goes up by 5°C (figure 26.13). The silver
sample has fewer atoms than the aluminum sample because silver atoms
are heavier. When you add heat, each atom gets an equal share of the
energy. If there are more atoms, each atom gets less energy. Because the
energy added to the two samples is the same, each silver atom gets more
energy that each aluminum atom.
More atoms per
gram means
higher specific
heat
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The temperature depends on the amount of energy per atom. If each atom
gets more energy, the temperature change will be greater. This explains
why the specific heat of aluminum is greater than the specific heat of
silver. Aluminum has more atoms per gram, therefore it takes more
energy per gram to raise the temperature.
Figure 26.13: The specific heat of aluminum
is greater than that of silver because aluminum
has more atoms per gram than silver.