LECTURE 7
HEAT
Lecture Instructor: Kazumi Tolich
Lecture 7
2
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Reading chapter 14-1 to 14-2.
¤  Heat
and mechanical work
¤  Specific heats
¤  Calorimetry
Heat
3
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Heat (Q) is the energy transferred from one object to another
due to temperature difference.
Objects do not contain heat.
Temperature and heat are not the same quantities. The word
“hot” does not mean “high heat” but rather “high temperature.”
Heat, like work, is a kind of energy transfer, so it needs to be
taken into account when applying conservation of energy.
Heat and mechanical work
4
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As the weights fall, they turn a paddle wheel,
which does work on the water.
Joule found that it takes about 4186 J of work
to increase the temperature of 1kg of water
by 1°C.
Heat was measured in calories before his
work, and this experiment showed the
mechanical equivalent of heat.
1 cal = 4.186 J
Insulating walls
Units for heat
5
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In nutrition, the Calorie (C) is used instead of calorie.
1 kcal = 1 C
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Another common unit for measuring heat is the British thermal
unit (Btu). A Btu is the energy required to heat 1 lb of water
from 63 °F to 64 °F.
1 Btu = 0.252 kcal = 1055 J
Demo: 1
6
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Drill and Dowel
¤  Demonstration
¨ 
of frictional work producing heat
Cork popper
¤  Demonstration
of friction producing heat
Example: 1
7
¨ 
During a workout, a person
repeatedly lifts a barbell
with a weight w = 53 N
through a height of
h = 0.46 m. How many
“reps” of this lift are
required to burn off 120 C?
Don’t get burnt!
8
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Have you ever noticed that some foods remain hotter much
longer than others?
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The filling of hot apple pie can burn your tongue while crust will not.
You can touch with your bare hand the aluminum foil covering a hot
dinner out of the oven, but not the dinner.
You can touch a toast straight out of a toaster, but not soup from a stove
as hot as the toaster.
Heat capacity
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The heat required (Q) for a given increase in temperature (ΔT) of an object
is given by the heat capacity (C) of the object. The heat capacity is a
positive constant for a given object.
Q
C=
ΔT
Q = CΔT
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Q > 0 if ΔT > 0. Heat is added to a system.
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Q < 0 if ΔT < 0. Heat is removed from a system.
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We can think of heat capacity as thermal inertia.
Specific heat
10
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Different substances have
different capacities for storing
thermal energy.
The specific heat (c) of a
substance is defined as
Q
c=
mΔT
m is the mass of an object.
at Pat
Heat capacity vs. specific heat
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Heat capacity depends on both what kind of
substance an object is made of and the mass of the
object.
¨  Analogy:
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¤  Heat
capacity of an object is like mass of an object.
¤  Specific heat of a substance is like density of a
substance
Example: 2
12
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Suppose Q = 63.0 J of heat
is added to a piece of
aluminum with a mass of
m = 128 g at a temperature
of T0 = 25.0 °C. What is the
final temperature of the
aluminum?
Specific heat capacity of water
13
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Water has a much higher specific heat capacity
than most common materials.
¤  The
climate in many places are influenced by ocean
currents.
¤  Islands and peninsulas do not have extreme
temperatures (hot and cold) that are observed in the
interior of a continent.
Hot coal walking
14
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People have walked barefoot on hot coals.
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Coals have low specific heat.
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Sweat on their feet and blood in their feet have high specific heat.
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As heat transfers from the coals to the feet, coals’ temperature decreases a
lot, but feet’s temperature increases only by a little amount.
Demo: 2
15
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Balloon heat capacity
¤  Demonstration
of heat capacity
Clicker question: 1
16
Calorimetry
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With a calorimeter (a lightweight thermally
insulated container), we can determine the specific
heat of a substance with known mass.
1. 
2. 
3. 
4. 
Heat the sample to a known temperature.
Placing it in a known mass of a substance with a
known specific heat (often water).
Measure the change in temperature of that
substance.
Apply conservation of energy: the heat flow from
the sample is equal to the heat flow to the water.
Demo: 3
18
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Calorimetry and specific heat
¤ 
Measuring specific heat for aluminum (900 J/kgŸ°C), steel (450 J/kgŸ°C), and
lead (128 J/kgŸ°C)
Qm + Qw = 0
mm cm (T − Tm ) + mwcw (T − Tw ) = 0
"
"
J %
J %
0.080 kg) $ 4186
' (T − Tw ) $ 2791
' (T − Tw )
(
m c (T − Tw )
kgK &
kgK &
#
#
cm = w w
=
=
mm (Tm − T )
(0.120 kg) (Tm − T )
(Tm − T )
Example: 3
19
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A blacksmith drops an iron horseshoe
with a mass of mh = 0.50 kg into a
bucket of water with a mass of
mw = 25 kg. If the initial temperature
of the horse shoe is Thi = 450 °C, and
the initial temperature of the water is
Twi = 23 °C, what is the equilibrium
temperature of the system, T? Assume
no heat is exchanged with the
surroundings.