Chapter 18: Heat and the First Law Of Thermodynamics

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Chapter 18Heat and the First LawOf
Thermodynamics
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18-1Heat Capacity and Specific Heat
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Heat Capacity and Specific Heat (1)
The heat energy Q needed to raise the
temperature of a substance of mass m, by an
amount DT, is given by
where C is the heat capacity. The specific heat
is defined by
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Heat Capacity and Specific Heat (2)
The unit of heat energy is the calorie 1 cal
4.184 J
The specific heat of water is
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Example 18 - 1
How much heat is needed to raise the temperature
of 3 kg of copper by 20 oC ?
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Example 18 2 - Calorimetry

• Measure specific heat of lead
• Heat 0.6 kg of lead to 100oC
• Place in an aluminum calorimeter of mass 0.2 kg,
  containing 0.5 kg of water at T 17.3oC

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Example 18 2 - Calorimetry
Final temperature TF 20oC
The heat released by lead equals heat absorbed by
water and container. Heat released by lead
TF 20oC
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Example 18 2 - Calorimetry
Heat absorbed by water
TF 20oC
Heat absorbed by container
Heat released Heat absorbed
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Example 18 2 - Calorimetry
Heat released Heat absorbed
TF 20oC
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18-2Change of Phase and Latent Heat
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If heat is added to ice at 0oC, its
temperature does not change. Instead the
ice melts
Melting is an example of a change of phase
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Change of Phase and Latent Heat
The heat energy required to melt a substance of
mass m is
where Lf is called the latent heat of
fusion. For water, at 1 atm, Lf 333.5 kJ/kg
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Change of Phase and Latent Heat
The heat energy required to vaporize a substance
of mass m is
Lv is called the latent heat of
vaporization. For water, at 1 atm, Lv 2.26
MJ/kg
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Example 18 3 (1)
Compute heat required to change 1.5 kg of ice at
T -20oC and P 1 atm to steam at 100oC
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Example 18 3 (2)

1. Warm ice to 0oC
2. Melt ice at 0oC
3. Warm water to 100oC
4. Vaporize water at 100oC

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18-3Joules Experiment
The First Law of Thermodynamics
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Joules Experiment
The temperature of a system can be increased by
doing work on it. Joule found he could raise the
temperature of 1g of water by 1oC by doing 4.184
J of work on it.
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1st Law of Thermodynamics
Change in internal energy of system heat IN
work done ON system
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18-4Internal Energy of an Ideal Gas
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Internal Energy of Ideal Gas
If the only energy of an ideal gas is kinetic
then its internal energy is given by
Note The internal energy of an ideal gas
depends only on the amount of gas and
its absolute temperature
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18-5Work and the PV Diagram for a Gas
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Work Done By Gas
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Work Done On Gas
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Work Done On Gas
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PV Diagrams (1)
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PV Diagrams (2)
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PV Diagrams (3)
Isobaric compression (constant pressure)
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PV Diagrams (4)
Isothermal compression (constant temperature)
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Work Done On Gas Isothermal Compression
Note For an expansion the work done on the gas
is negative, while for a compression it is
positive.