Chapter 24 Thermodynamics

1
Chapter 24 Thermodynamics
2

• The study of heat and its transformation into
  mechanical work.

3

• If we increase the thermal motion of atoms what
  happens?
• Temperature increases.

4

• There seems to be no upper limit of temperature.
• However, there is a lower limit.

5
Absolute Zero

• The point at which all molecular motion stops.

6

• No more energy can be extracted from a substance
  and the temperature can not be lowered any
  further.

7

• Kelvin 0 K
• Celsius -273 oC
• Fahrenheit -459 oF

8
(No Transcript)
9
First Law of Thermodynamics

• The internal energy of an isolated system is
  constant.

10

• The increase in the internal energy of a system
  is equal to the amount of energy added by heating
  the system, minus the amount lost as a result of
  the work done by the system on its surroundings.

11
Signs (/-) will tell you if energy is entering
or leaving a system.

• indicates energy enters a system
• .- indicates energy leaves a system.

12
First Law of Thermodynamics
                                          thermo     heat       dynamics  force------------------------------------------------------Work done on water by paddle-wheelis converted to thermal energy.------------------------------------------------------First LawHeat added Work done                   or Heat added Thermal energy increase                                                         Work done by system
13

• Heat can be used to do work.
• Heat and work are energy transferred to or from a
  system.

14

• Work done on or by a gas is the pressure times
  the change in volume.
• W PDV

15
Work is the transfer of energy that takes place
when an object is moved against an opposing force
i.e. a system does work when it expands against
an external pressure
Car engine petrol burns produces gases which
push out pistons in the engine and transfer
energy to the wheels of car

• Work stimulates uniform motion
• Heat and work can be considered as energy in
  transit

16

• Ex An engine cylinder compresses a volume of gas
  by 0.04 m3. How much work is done by the cylinder
  if it exerts a constant pressure of 7.5 x 105 Pa?

17

• G P 7.5 x 105 Pa, DV 4 x 10-2 m3
• U W ?
• E W PDV
• S W (7.5 x 105)(4 x 10-2)
• S W 30,000 J

18
DU Q W

• Q quantity of heat that enters or leaves a
  system.

19

• DU the change in internal energy.
• W work done on () or by the system (-).

20
Ex When 5000 J of heat is added to an engine,
the engine does 1250 J of work. What is the
change in the internal energy of engine?
21

• Q 5000 J
• W 1250 J
• DU ?

22
DU Q W

• DU Q W
• DU 5000 J1250 J
• DU 3750 J

23
Adiabatic

• The process of compression or expansion of a gas
  so that no heat enters or leaves a system.

24

• Read Ch 24.3 in CP book.
• Examples combustion engine, bike pump, weather
  conditions.

25
Starting position, intake stroke, and compression
stroke.
Ignition of fuel, power stroke, and exhaust
stroke.
26

• intake/induction stroke
• 2. compression stroke
• 3. power stroke
• 4. exhaust stroke

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
2nd Law of Thermodynamics

• Heat flows spontaneously from a body of a higher
  temperature to a one at a lower temperature.

35
Heat Engine

• Any device that changes internal energy into
  mechanical work.

36
(No Transcript)
37

• To reverse the flow work must be done.

38

• Ex steam engine, internal combustion engine, jet
  engine, etc.

39

• The basic idea of a heat engine is that
  mechanical work can be obtained only when heat
  flows from hot to cold temperatures.

40

• Not all heat is transformed into work.

41

• "Typical engines are only 15 to 20 percent
  efficient in consuming oil, as they lose a lot of
  energy when they emit hot exhaust gas into the
  air."

42
Every heat engine will

1. Absorb internal energy from a reservoir of higher
   temperature.

43

• 2. Convert some of this energy into mechanical
  work.
• 3. Expel the remaining energy to some lower
  temperature reservoir, usually called a sink.

44

• When undesirable heat is expelled, its called
  thermal pollution.

45
(No Transcript)
46
Power plants and industrial factories are among
the major contributors to the problem of thermal
pollution. These facilities draw water from
nearby lakes and streams, which they use to cool
their machinery and steam-driven equipment.
Although many such facilities now take care not
to contaminate the water with chemical
pollutants, few return the heated water to its
original temperature before dumping it back into
the lakes and streams from which it came. The
heated water warms local bodies of water by as
much as 10 C (18 F), making the water
uninhabitable for fish and other organisms.
47
Carnot Efficiency

• The maximum amount of energy that can be
  converted to work in a heat engine.

48

• This gives us the ideal efficiency.

49

• This gives us the ideal efficiency.

50
(No Transcript)
51
(No Transcript)
52

• Qh energy added as heat
• Qc energy removed as heat

53
3rd Law of Thermodynamics

• There is no process by which a body may be cooled
  to a temperature of 0 K (Absolute Zero).

54
Entropy

• The measure of the amount of disorder.