thermal2n.ppt

Thermal Physics II
Thermodynamics
Thermodynamics is the branch of physics that is built
upon the fundamental laws that heat and work obey.

Terminology
The collection of objects on which
attention is being focused is called
the system, while everything else in
the environment is called the
surroundings.
To understand thermodynamics, it is
necessary to describe the state of a
system in terms of temperature,
pressure, and volume.

Terminology
Walls that permit heat flow are called
diathermal walls, while walls that do
not permit heat flow are called
adiabatic walls.
Two systems are said to be in thermal
equilibrium if there is no heat flow
between then when they are brought
into contact.
Temperature is the indicator of thermal
equilibrium in the sense that there is
no net flow of heat between two
systems in thermal contact that have
the same temperature.

The Zeroth Law of Thermodynamics
Two systems individually in thermal equilibrium
with a third system are in thermal equilibrium
with each other.

The First Law of Thermodynamics
Suppose that a system gains heat Q and that is the only
action that is occurring.
Consistent with the law of conservation of energy, the
internal energy of the system changes:
f i
U U U Q
   
Heat is positive when the system gains heat and
negative when the system loses heat.

If a system does work W on its surroundings and there is
no heat flow, conservation of energy indicates that the
internal energy of the system will decrease:
f i
U U U W
    
Work is positive when it is done by the system and
negative when it is done on the system.

The internal energy of a system changes due to heat
and work:
f i
U U U Q W
    
Work is positive when it is done by the system and
negative when it is done on the system.
Heat is positive when the system gains heat and
negative when the system loses heat.

Positive and Negative
Work
In part a of figure, the system gains
1500 J of heat and 2200 J of work is
done by the system on its
surroundings.
In part b, the system also gains 1500 J
of heat, but 2200 J of work is done on
the system.
In each case, determine the change in
internal energy of the system.

(a)
(b)
   
1500 J 2200 J 700 J
U Q W
  
     
   
1500 J 2200 J 3700 J
U Q W
  
     
The system gains 1500 J of heat and 2200
J of work is done by the system on its
surroundings.
The system also gains 1500 J of heat, but
2200 J of work is done on the system

When one gallon of gasoline is burned in a car engine, 1.19 x 108 J of
internal energy is released. Suppose that 1.00 x 108 J of this energy
flows directly into the surroundings (engine block and exhaust
system) in the form of heat. If 6.0 x 105 J of work is required to make
the car go one mile, how many miles can the car travel on one gallon
of gas?
According to the first law of thermodynamics, the work that is
done when one gallon of gasoline is burned in the engine is
Since 6.0 x 105 J of work is required to make the car go one mile,
the car can travel

The Second Law of Thermodynamics
THE SECOND LAW OF
THERMODYNAMICS:
THE HEAT FLOW STATEMENT
Heat flows spontaneously from a
substance at a higher temperature to
a substance at a lower temperature
and does not flow spontaneously in
the reverse direction.

The Second Law of
Thermodynamics
A heat engine is any device that uses
heat to perform work. It has three
essential features.
1. Heat QH is supplied to the engine at a
relatively high temperature from a place
called the hot reservoir.
2. Part of the input heat is used to perform
work W by the working substance of the
engine (such as the gasoline-air mixture
in car engines).
3. The remainder of the input heat is
rejected as QC to a place called the cold
reservoir.

The efficiency of a heat engine is defined as
the ratio of the work done to the input heat:
H
Q
W
e 
If there are no other losses, then
C
H Q
W
Q 

H
C
Q
Q
e 
1

An automobile engine has an efficiency of 22.0% and
produces 2510 J of work. How much heat is rejected by
the engine?
H
W
e
Q

H
W
Q
e

C
H Q
W
Q 

 
1
1
1
2510 J 1 8900 J
0 220
.
C
W
Q W W
e e
 
   
 
 
 
  
 
 
W
Q
Q H
C 


Carnot’s Principle and the Carnot Engine
A reversible process is one in which both the
system and the environment can be returned to
exactly the states they were in before the process
occurred.
Nicolas Léonard Sadi
Carnot (June 1, 1796 -
August 24, 1832) was
a French physicist and
military engineer
CARNOT’S PRINCIPLE:
AN ALTERNATIVE STATEMENT OF THE SECOND
LAW OF THERMODYNAMICS
No irreversible engine operating between two reservoirs at constant
temperatures can have a greater efficiency than a reversible engine
operating between the same temperatures. Furthermore, all reversible
engines operating between the same temperatures have the same
efficiency.

Carnot Engine
The Carnot engine is useful as an idealized
model.
All of the heat input originates from a source at a
single temperature, and all the rejected heat
goes into a cold reservoir at a single temperature.
Since the efficiency can only depend on
the reservoir temperatures, the ratio of
heats can only depend on those temperatures.
Carnot 1 1
C C
H H
Q T
e
Q T
   

A heat engine operates between a hot reservoir at 1500 K and a cold reservoir
at 500 K. During each cycle, 1.0 × 105 J of heat is removed from the hot
reservoir and 5.0 × 104 J of work is performed.
(a) Determine the Carnot efficiency of this engine.
Carnot
500 K
1 1 0 67
1500 K
.
C
H
T
e
T
    
(b) What is the actual efficiency of this engine?
4
5
5.0 10 J
0 50
1.0 10 J
.
H
W
e
Q

  


It is claimed that a heat engine has been built that takes in 102 kJ and
produces 36 kJ of useful work. It supposedly operates between 335.4 K and
258.2 K. Is such a device physically possible?
Determine the Carnot efficiency of this engine.
Carnot
258 2 K
1 1 0 23
335 4 K
.
.
.
C
H
T
e
T
    
What is the actual efficiency of this engine?
36 kJ
0 30
120 kJ
.
H
W
e
Q
  
No, it is not possible.

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