LECTURE 6
HEAT
Lecture Instructor: Kazumi Tolich
Lecture 6
2
! 
Reading chapter 16-4 to 16-5.
!  Heat
and mechanical work
!  Specific heats
!  Calorimetry
Heat
3
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
! 
! 
! 
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
! 
In nutrition, the Calorie (C) is used instead of
calorie.
1 kcal = 1 C
! 
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
! 
Drill and Dowel
!  Demonstration
of frictional work producing heat
Demo: 2
7
! 
Cork popper
!  Demonstration
of friction producing heat
Example: 1
8
! 
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!
9
! 
Have you ever noticed that some foods remain
hotter much longer than others?
!  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
10
! 
The heat required (Q) for a given increase in temperature
(ΔT) of an object is given by the heat capacity (C) of a
substance. The heat capacity is a positive constant for a
given object.
Q
C=
ΔT
! 
! 
! 
Q = CΔT
Q > 0 if ΔT > 0. Heat is added to a system.
Q < 0 if ΔT < 0. Heat is removed from a system.
We can think of heat capacity as thermal inertia.
Specific heat
11
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
12
Heat capacity depends on both what kind of
substance an object is made of and the mass of the
object.
!  Analogy:
! 
!  Heat
capacity of an object is like mass of an object.
!  Specific heat of a substance is like density of a
substance
Example: 2
13
! 
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
14
! 
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
15
People have walked barefoot on hot coals.
!  Coals have low specific heat.
!  Sweat on their feet and blood in their feet have high
specific heat.
!  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: 3
16
! 
Balloon heat capacity
!  Demonstration
of heat capacity
Clicker question: 1
17
Calorimetry
18
! 
With a calorimeter (a lightweight
thermally insulated container), we can
determine the specific heat of a substance
with known mass.
1. 
Heat the sample to a known temperature.
2. 
Placing it in a known mass of a substance with
a known specific heat (often water).
3. 
Measure the change in temperature of that
substance.
4. 
Apply conservation of energy: the heat flow
from the sample is equal to the heat flow to
the water.
Demo: 4
19
! 
Calorimetry and specific heat
!  Measuring
specific heat for aluminum, steel, and lead
Qm + Qw = 0
mm cm (T − Tm ) + mwcw (T − Tw ) = 0
mwcw (T − Tw )
cm =
mm (Tm − T )
Example: 3
20
! 
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.