Chapter 26
26.2 Measuring Changes in Heat
You know that energy can take many forms such as kinetic energy, gravitational potential energy,
electrical potential energy, and chemical potential energy. In this section, we explore thermal energy
and how electrical energy becomes thermal energy. You will see how thermal energy increases the
temperature of an object. You will also develop a relationship between the rate of increase in
temperature of water and the increase in thermal energy of water.
Temperature, thermal energy, and heat
Changing Nature never creates or destroys energy; energy only gets converted from one
temperature form to another. Changes in temperature involve changes in energy. When you
changes energy heat a pot of soup with an electric hot plate, electrical energy is converted into
thermal energy. What, exactly, is thermal energy?
What is thermal Thermal energy and temperature are not the same. Temperature, as you have
energy? learned, measures the average kinetic energy of the molecules inside an object.
Thermal energy is the sum of all the kinetic and potential energies of the
molecules of a substance. The amount of thermal energy stored in an object
depends on three things: the mass, the temperature, and the amount of energy that
a particular material stores per degree.
Figure 26.6: What energy
changes are involved in this scenario?
Figure 26.7: Where does the
energy come from? Where does it go?
Suppose you are asked to heat up a single cup of soup or a huge pot of soup. Both
have to get to the same temperature. Which takes more energy? Heating up the
huge pot takes more energy because it is like heating up 100 individual cups!
Because it took more energy to heat the pot of soup, it now contains more thermal
energy than the cup of soup, even though they are at the same temperature.
Heat and thermal What happens when you hold an ice cream cone on a hot day? Thermal energy
energy flows from your hand and the surrounding air to melt the ice cream. We call this
flow of thermal energy heat. In the scientific sense, heat occurs only when there is
a difference in temperature. Heat always flows from the warmer object to the
cooler one.
Figure 26.8: On a hot day, the
flow of thermal energy can turn your
ice cream cone into a puddle if you
don’t eat it fast!
26.2 Measuring Changes in Heat
447
Chapter 26
Measuring heat
Understanding The flow of thermal energy (which we call heat) is happening around us all the
heat is important time. When you cook a pot of soup, thermal energy flows from the burner to the
soup and to the surrounding air. You might have a fan above the stove to remove
the hot air from the kitchen. A lot of the energy we consume goes toward heating
or cooling our homes, businesses, and schools.
The calorie Because heat is such an important concept in our lives there are three units of
energy that relate directly to heat. The metric unit used to measure heat or energy
is the calorie. The calorie is defined as the quantity of heat needed to increase the
temperature of 1 gram of water by 1 degree Celsius. Both calories and joules are
units of energy: 1 calorie = 4.184 joules.
The British Still another unit of heat you may have heard of is the British thermal unit, or Btu.
thermal unit The Btu is often used to describe heat produced by heating systems or heat
removed by air-conditioning systems. A Btu is the quantity of heat it takes to
increase the temperature of 1 pound of water by 1 degree Fahrenheit.
James Prescott Joule
In the 1840s,
James
Prescott
Joule proved that
the
law
of
conservation of
energy also applied to heat.
Until that time, physicists
only considered the law of
conservation of mechanical
energy, which did not include
heat. Joule showed that when
he
converted
electrical
energy and kinetic energy
into thermal energy, energy
was still conserved. The unit
for heat and energy is named
in his honor.
Unit
How do you
If you add heat to an object, how much will its temperature increase? Based on our
quantify amounts definition of the calorie we can say that the amount of heat that flows into an
of heat? object is proportional to the mass of the object times the change in temperature.
This statement is known as the heat-temperature rule. Simply stated, the more
heat you add to an object, the greater the increase in temperature. If you have
twice as much mass of object to heat, you need twice as much energy to increase
the temperature by the same amount.
448
Equals
1 calorie
4.186 joules
1 Calorie
1000 calories
1 Btu
1055 joules
1 Btu
252 calories
Figure 26.9: Conversion table for
units of heat
Chapter 26
Differences in The amount of temperature increase also depends on what you are heating. It takes
materials more energy to raise the temperature of some materials than others. For example,
suppose you put 100 calories of heat into a beaker with 100 grams of water. The
temperature goes up one degree. If you apply the same amount of heat to 100
grams of iron, the temperature goes up 20 degrees!
What is specific As water and iron illustrate, substances vary greatly in their ability to store thermal
heat? energy. The specific heat is a property of a substance that tells us how much the
temperature goes up when a given amount of heat is added. A large specific heat
means you have to put in a lot of energy for each degree increase in temperature.
The heat equation The whole story is told by the heat equation below. The equation tells you how
much heat (Q) it takes to change the temperature (∆ T) of a mass (m) of a
substance with specific heat (c).
Figure 26.10: The temperature
gains in iron and water for equal
amounts of heat.
Specific heats of materials
substance
Example:
How much heat is needed to raise the temperature of 250 grams of water from 20 oC to 40 oC?
1. Identify the variables in the equation.
Heat equation: Q = mc∆T
Q = trying to determine
m = mass of water = 250 grams
c = specific heat of water = 1 calorie/g oC
∆T = 40 oC - 20 oC = 20 oC
2. Plug the variables into the equation and solve.
Q = (250 g) × (1 calorie/g oC) × (20 oC)
Q = 5000 calories
water
ice
benzene
methanol
ethanol
aluminum
carbon
silver
gold
26.2 Measuring Changes in Heat
specific
heat
calorie
-------------------gram°C
1.00
0.493
0.416
0.586
0.584
0.215
0.170
0.052
0.031
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