Lecture notes for Lecture #2, September 30
th
, 2010
Definitions
We should now be very familiar with the following expressions and their meanings:
∆E
thermal
= C∆T
–
The change in thermal energy that a substance undergoes due to a
transfer of energy (to the substance or from the substance) in the form of heat.
∆E
bond
=
∆m∆H
–
The change in the substance
’
s bond energy as a phase transition
occurs.
∆E
total
= ∑∆E
i
–
A statement of the law of conservation of energy; that the change in the
total energy of a system is equal to the sum of the changes in all of the energy systems
that makeup or interact within the overall system, which may be open or closed to the
environment.
∆E
total
= ∑∆E
i
=0
This condition describes a closed system.
∆E
total
= ∑∆E
i
= Q + W, This condition describes an open system
–
where energy in the
form of heat and/or work is transferred outside the system to the environment.
Heat Capacity
. This is a property of the substance. It can be calculated according to C =
m x
c,
where
c
is the specific heat.
Specific Heat
. T
he specific heat of a substance is a “look up” value.
Common specific
heats, or at least the ones you will need for this course are given on page 5 (pink page) of
your textbook. It is also commonly referred to as the amount of energy transferred in the
form of heat required to raise the temperature of a gram of a particular substance, one
degree centigrade.
Enthalphy
Also called heat of fusion or heat of vaporization. It
is a “look up” v
alue.
Enthalpies for common substances are given on page 5 (pink page of your textbook).
Units
∆E
total
, ∆E
bond
, ∆E
thermal
, all represent energy changes, and so we expect the units for
these expressions to be in Joules (which is the unit of energy).
Heat capacity
is given by m x
c
, where m is the mass, and
c
is the specific heat. So the
units are [energy/temperature]
Specific heat
is given in units of [energy/masstemp]
Enthalpy
, or heats of fusion (or vaporization) has units of [energy/mass]
Understanding Heat Capacity
We saw in class that the heat capacity of a substance has a graphical interpretation. In
fact, when we closely examine Temperature versus Added Energy graphs, also known as
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View Full DocumentHeating Curves, we can directly see the effect of heat capacity; it is related to the inverse
slope of the parts of the curve that represent thermal energy changes.
In the interval between A and B we see that the slope of the line connecting A and B is
steeper than the slope of the line connecting C and D. Why is that the case? Recall that
for solid H2O (ice), the lookup value for specific heat is 2.05 KJ/kgK.
Therefore, for 1.0 kg of solid H2O we calculate a total heat capacity as follows:
Heat Capacity
(ice)
= mass
(ice)
X specific heat
(ice)
= 1.0 (kg) X 2.05 KJ/kgK
= 2.05 KJ/K
Similarly, recall that for liquid H2O (water), the lookup value for specific heat is 4.18
KJ/kgK. Therefore, for 1.0 kg of liquid H2O, we calculate a total heat capacity as
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 Fall '08
 PARDINI
 Physics, Thermodynamics, Energy, Thermal Energy, energy systems

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