Thermodynamics

1
Thermodynamics

• Physics H
• Mr. Padilla

2
Thermodynamics

• The study of heat and its transformation into
  mechanical energy.
• Foundation
• Conservation of energy
• Heat flows from hot to cold
• Until 1840 heat was thought to be an invisible
  fluid called caloric.

3
Q, W, U

• Work can transfer energy to a substance. That
  energy could then leave as heat.
• The reverse is also possible
• A change in internal energy is apparent as a
  change in temperature or phase.
• This change in energy can take place on a
  substance or a combination of substances called a
  system.

4
Work done on/by a gas

• Previously WFd
• When work is done on an enclosed gas
• W?(PV)
• If P is held constant,
• W P?V

• ?V is work is done by the gas (W is )
• ?V is - work is done on the gas (W is -)
• Work is only done if volume changes

5
Heat, Work, and Internal Energy

• If the gas expands, as shown in the figure, DV is
  positive, and the work done by the gas on the
  piston is positive.
• If the gas is compressed, DV is negative, and the
  work done by the gas on the piston is negative.
  (In other words, the piston does work on the
  gas.)

6
Knowledge about Pressure and Volume

• P F/A (force/area)
• ?V Ad (area multiplied by the displacement)
• 1 Pa 1 N/m2

7
(No Transcript)
8
Sample Problem 11A

• An engine cylinder has a cross-sectional area of
  0.010 m2. How much work can be done by a gas in
  the cylinder if the gas exerts a constant
  pressure of 7.5 x 105 Pa on the piston and moves
  the piston a distance of 0.040m?

9
Solution

• Choose the equation
• Plug and Chug
• W(7.5x105 N/m2)(0.010 m2)(0.040 m)
• W 3.0x102 J

10
Thermodynamic Processes

• No Work is done in a constant volume process.
• Any changes to internal energy would be as a
  result of heat
• Called isovolumetric process

• Internal energy is constant in a constant-temp
  process.
• Work can still be done if volume changes
• Called isothermal process

11
Isovolumetric Processes

• A bomb calorimeter is a device where a small
  quantity of a substance undergoes a combustion
  reaction. The reaction causes a change in
  temperature and pressure, but because of the
  thick walls there is no volume change. Energy
  can only be transferred as heat.

12
Isothermal Processes

• Work is done inside by the molecules hitting the
  inside of the balloon (energy is lost as work).
  Energy from the outside is added as heat.

13
Adiabatic Process

• Process in which no heat enters or leaves a
  system.
• Changes of volume can be done rapidly so heat has
  little time to leave or can be insulated
• Changes in air temperature pressure change
• Temp of dry air drops 10C for each 1km increase
  in altitude

14
1st Law of Thermodynamics

• Whenever heat is added to a system, it transforms
  to an equal amount of some other form of energy
• Energy, including heat cannot be created or
  destroyed
• Heat added does 1 or both of 2 things..
• 1) increases the internal energy of the system if
  it remains in the system
• 2) does external work if it leaves the system

15
Energy Conservation

• If friction is taken into account, mechanical
  energy is not conserved.
• Consider the example of a roller coaster
• A steady decrease in the cars total mechanical
  energy occurs because of work being done against
  the friction between the cars axles and its
  bearings and between the cars wheels and the
  coaster track.
• If the internal energy for the roller coaster
  (the system) and the energy dissipated to the
  surrounding air (the environment) are taken into
  account, then the total energy will be constant.

16
Energy Conservation

• Notice that in the presence of friction the
  internal energy (U) of the roller coaster
  increases as KE PE decreases.

17
Heat added increase in internal energy
external work done by system

• If 10J of energy is added to a system that does
  no external work, by how much will the internal
  energy of that system be raised?
• 10J

• If 10J of energy is added to a system that does
  4J of external work, by how much will the
  internal energy of that system be raised?
• 10J 4J 6J

18
Energy Conservation

• The principle of energy conservation that takes
  into account a systems internal energy as well
  as work and heat is called the first law of
  thermodynamics.
• The first law of thermodynamics can be expressed
  mathematically as follows
• DU Q W
• Change in systems internal energy energy
  transferred to or from system as heat energy
  transferred to or from system as work

19
Signs of Q and W for a system
20
1St Law of Thermodynamics for Special Processes
21
Sample Problem 11B

• A total of 135 J of work is done on a gaseous
  refrigerant as it undergoes compression. If the
  internal energy of the gas increases by 114 J
  during the process, what is the total amount of
  energy transferred as heat? Has energy been added
  to or removed from the refrigerant as heat?

22
Solution

• Choose your equation
• DU Q W
• Rearrange the equation
• Q DU W

23
Solution

• Plug and Chug
• Q 114 J (135 J)
• Q 21 J
• The sign for the value of Q is negative. This
  indicates that energy is transferred as heat from
  the refrigerant.

24
Absolute Temperature

• Absolute Zero Lowest possible temperature, 273
  degrees C below zero
• Other absolute temperatures

25
Cyclic Process

• A thermodynamic process in which a system returns
  to the same conditions under which it started.
• The change in internal energy of a system is zero
  in a cyclic process
• DUnet 0 and Qnet Wnet

26
The Steps of a Gasoline Engine Cycle
27
The Steps of a Refrigeration Cycle
28
Heat Engines use Heat to do Work

• A heat engine is able to do work by transferring
  energy from a high-temperature substance at Th to
  a substance at a lower temperature (the air
  surrounding the engine) at Tc.
• Wnet Qh Qc
• The larger the difference between the amount of
  energy transferred as heat into the engine and
  out of the engine, the more work the engine can
  do.

29
2nd Law of Thermodynamics

• Heat will never of itself flow from a cold object
  to a hot object.
• Heat can be made to flow the other way, but only
  by external effort.
• Ex Air Conditioner, Refrigerator

30
Heat Engines

• Changing heat completely into work can never be
  done.
• It is possible to change some.
• A heat engine is any device that changes external
  energy into mechanical work.
• Amount of work done
• WnetQh-Qc

31
Heat Engines

• Every heat engine will
• 1) absorb heat from a reservoir of higher temp.
  increasing its internal energy
• 2) convert some of this energy into mechanical
  work
• 3) expel the remaining energy as heat to some
  lower-temp. reservoir, usually called a sink
• There is always heat exhaust

32
Thermodynamic Efficiency

• The ideal efficiency, or Carnot Efficiency, of a
  heat engine can be found using the formula
• Ideal efficiency Wnet/Qhot
• Ideal Efficiency Thot - Tcold
• Thot
• Heat converted to useful work depends on the temp
  difference between the hot reservoir and the cold
  sink.

33
Carnot Efficiency

• What is the ideal efficiency of an engine having
  a hot reservoir at 300K and a cold reservoir at
  150K?
• What is the ideal efficiency of an engine having
  a hot reservoir at 400K and a cold reservoir at
  0K?

• (300K-150K)/300K .5
• 50
• (400K-0K)/400K 1
• 100

34
Sample Problem 11C

• Find the efficiency of a gasoline engine that,
  during one cycle, receives 204 J of energy from
  combustion and loses 153 J as heat to the exhaust.

35
Disorder

• Natural systems tend to proceed toward a state of
  greater disorder.
• This idea is called entropy
• Ex Gas molecules in a bottle. What will happen
  when the bottle is opened?
• Entropy normally increases in natural systems.
• When work is input, entropy can decrease

36
(No Transcript)
37
Entropy

• Greater disorder means there is less energy to do
  work.
• If all gas particles moved toward the piston, all
  of the internal energy could be used to do work.
  This extremely well ordered system is highly
  improbable.

38
Entropy

• Because of the connection between a systems
  entropy, its ability to do work, and the
  direction of energy transfer, the second law of
  thermodynamics can also be expressed in terms of
  entropy change
• The entropy of the universe increases in all
  natural processes.
• Entropy can decrease for parts of systems,
  provided this decrease is offset by a greater
  increase in entropy elsewhere in the universe.