Heat and Thermodynamics

1
Heat and Thermodynamics

• Physics(P)

2
State Standards

• 3. Energy cannot be created or destroyed,
  although in many processes energy is transferred
  to the environment as heat. As a basis for
  understanding this concept
• a. Students know heat flow and work are two forms
  of energy transfer between systems.
• b. Students know that the work done by a heat
  engine that is working in a cycle is the
  difference between the heat flow into the engine
  at high temperature and the heat flow out at a
  lower temperature (first law of thermodynamics)
  and that this is an example of the law of
  conservation of energy.
• c. Students know the internal energy of an object
  includes the energy of random motion of the
  object's atoms and molecules, often referred to
  as thermal energy. The greater the temperature of
  the object, the greater the energy of motion of
  the atoms and molecules that make up the object.

3
Additional Standards

• d. Students know that most processes tend to
  decrease the order of a system over time and that
  energy levels are eventually distributed
  uniformly.
• e. Students know that entropy is a quantity that
  measures the order or disorder of a system and
  that this quantity is larger for a more
  disordered system.
• f. Students know the statement "Entropy tends to
  increase" is a law of statistical probability
  that governs all closed systems (second law of
  thermodynamics).
• g. Students know how to solve problems involving
  heat flow, work, and efficiency in a heat engine
  and know that all real engines lose some heat to
  their surroundings

4
What is the Difference Between Heat and
Temperature?

• Both are related to energy but theres a big
  difference

5
Temperature

• Measure of how hot or cold an object is
• Measured by thermometers
• Work by expansion of a liquid
• Other types use bimetallic strip

6
Digital Thermometers
Use thermistors - temperature dependant
semiconductor resistors
7
Temperature Scales

• Fahrenheit T(0F) 9/5T(0C) 32
• Celsius (centigrade) T(0C ) 5/9T(0F) 32
• (degree is 9/5 that of Fahrenheit)
• Kelvin (Celsius 273)

8
Examples

• Zero degrees Celsius is what Kelvin?
• Answer 273o
• What is the boiling point of water in degrees
  Kelvin?
• Answer 373o
• 200 degrees Celsius is what in Kelvin?
• Answer 473o

9
Absolute Zero

• 0 degrees Kelvin -273 Celsius
• Lowest possible temperature
• Molecular motion ceases

Courtesy Michigan State University
10
Kinetic Theory of Heat

• All matter is made of tiny atoms and molecules,
  constantly in motion
• Faster is hotter

solid
gas
11
Temperature and Kinetic Energy

• In ideal gas temperature is proportional to
  average kinetic energy per molecule
• Closely related in liquids and gases

12
Temperature does not depend on the amount
13
Heat does depend on the amount
There is twice as much kinetic energy of moving
molecules in two liters of water as in one liter.
Analogy Heat is like the total height of
students in this room, temperature is like their
average height.
14
Which has more heat?

• A swimming pool full of ice water?
• A cup full of boiling water?

Answer the swimming pool, because it has so
much more water.
15
Heat Energy Transferred

• Definition Energy that transfers because of
  temperature difference
• Heat flows governed by average molecular kinetic
  energy difference

Heat flows from faster molecules to slower ones
16
Thermal Equilibrium

• Objects at same temperature are at thermal
  equilibrium no heat flows.

17
Internal Energy

• Total of all forms of energy inside something
  (thermal)
• Includes
• Translational kinetic
• Rotational kinetic
• Potential

18
Summary

• The greater the temperature of an object, the
  greater the energy of motion of the atoms and
  molecules that make up that object.
• Internal energy of an object includes the energy
  of random motions of atoms and molecules in the
  object

19
Measuring Heat

• One calorie is the amount of heat needed to raise
  the temperature of one gram of water by one
  degree Celsius.
• Kilocalorie raises the temperature of one kg of
  water by 10 C (also called Calorie or food
  calorie)
• One calorie 4.186 joules
• One kilocalorie 4186 joules

20
Density of Water

• Density is mass per unit volume
• D M/V
• One gram per cubic centimeter
• One kilogram per liter
• One thousand kg per cubic meter

21
Calorie Questions

• How many calories are needed to raise the
  temperature of 10 grams of water by 10C?
• 10
• How many calories are needed to raise the
  temperature of one gram of water by 10 0C?
• 10

22

• A certain amount of heat raises the temperature
  of 1 liter of water by 20C. What would be the
  temperature increase if the same amount of heat
  were added to 2 liters of water?
• Answer 10C

23
Example

• A person consumes and expends about 2000 Calories
  per day. What is their thermal power output?
• 2000cal/d x 1 d/24h x 1h/3600s x 4184J/C
• 96.8J/s 96.8 Watts

24
Specific Heat Capacity

• Different materials change their temperature by
  different amounts when they absorb the same
  amount of heat.
• Some have more ways of storing energy than others
• Water has very high specific heat (capacity)
• Metals have much less

25
Q mcDT

• Q mcDT expresses how heat absorption works. C
  is specific heat
• Question A certain rock has a specific heat of
  0.25 (water is 1.0) How much heat will be
  required to heat 5.0 kg rock from 20 to 800C?
• Q 5.0kg x 1000g/kg x 0.25 cal/g 0C x 60 0C
• Q 75,000 c 7.5 x 104 calories

26
Thermal Expansion

• Most materials expand when heated
• Only exception is water between 00C and 40C
• Expansion joints in bridges, cracks in sidewalks
  allow for expansion

27
Bimetallic Strip

• How your thermostat works

28
Dont Let Your Cars Engine Overheat

• Aluminum expands more than iron
• Pistons made of aluminum
• Cylinder made of iron

29
Mechanical Equivalent of Heat

• Discovered by James Joule
• Falling weight makes
• paddle turn
• 4.186 x 103 J 1 kcal
• Interpretation
• HEAT IS ENERGY
• TRANSFER

Courtesy W. Bauer http//lecture.lite.msu.edu/mmp
/kap11/cd295.htm
30
Joules Apparatus
Link to Joules original article
31
Example

• When digested a slice of bread yields 100 kcal.
  How high a hill would a 60 kg student need to
  climb to work off this slice of bread?

100 kcal x 4.186 x 103 J/kcal 4.2 x 105 J
W mgh
h W/mg 4.2 x 105 / (60 kg)(9.80 m/s2) 714m
7.1 x 102 m
If the body is only 20 percent efficient in
transforming the bread, how high need they climb?
32
Bullet in Block

• When a 10 g bullet traveling 500 m/s is stopped
  inside a 1kg wood block nearly all its KE is
  transformed to heat. How many kcal are released?

KE ½ mv2 0.5 x 0.010 kg x (500)2 1250 J
1250 J x 1 kcal/4186 J 0.30 kcal
33
Thermodynamics

• Study of heat and its transformation into
  mechanical energy
• Based on conservation of energy
• Explains how engines like car motors work

34
First Law of Thermodynamics

• Generally, when you add heat to a system it
  changes into an equal amount of some other form
  of energy
• Heat added increase in internal energy
  external work done by the system

35
Work Done On and By

• Compressing a gas by pushing down on a piston
  work done on
• A gas expands by pushing a piston up work done
  by

36
Questions

• 20 J of heat is added to a system that does no
  work. What is the change in internal energy?
• Answer 20 J
• 20 J of heat is added to a system that does 10 J
  of work. What is the change in internal energy?
• Answer 10 J

37

• 20 J of heat is added to a system that does 30 J
  of work. What is the change of internal energy?
• Answer -10 J
• 20 J of heat is added to a system that has 10 J
  of work done on it. What is the change of
  internal energy?
• Answer 30 J

38
Adiabatic Processes

• Compression or expansion of a gas so that no heat
  enters or leaves
• Example gas in cylinder of car or diesel engine
• Why adiabatic? Because it happens too fast for
  much heat to enter or leave.
• In adiabatic compression, temperature rises.
• In diesel engine, enough to ignite gas without
  spark plug

39
Courtesy How Stuff Works
40
Courtesy Shell Canada
41
(No Transcript)
42
Adiabatic Expansion

• Produces cooling
• Example blow on your hand first with wide open
  mouth, then with puckered lips
• How do you explain the results?

43
The Chinook

• What would you expect to happen if cold air moves
  down the slopes of mountains
• Hint it will be compressed by atmosphere into
  smaller volume
• Chinook wind is warm
• Common in Rocky mountains

44
Second Law of Thermodynamics

• Heat flows from hot to cold. By itself it will
  never flow from cold to hot.
• Question Would it violate the First Law of
  Thermodynamics (energy conservation) if heat
  flowed from a cold object to a warm object
  touching it?
• Answer No

45
Second Law Applied to Engines

• It is impossible to build a heat engine that
  changes heat completely into work.

Courtesy University of Oregon
46
Heat Engine

• Some heat is converted to useful work
• The rest is exhausted on at a lower temperature
  (cause of thermal pollution)
• Efficiency useful work / heat input
• About 20-25 for gasoline engine
• About 35-40 for diesel engine
• The energy exhausted is waste, cannot be recovered

47
Ideal (Carnot) Engine

• Ideal (Maximum possible efficiency)
• (Thot Tcold)/Thot (Kelvin
  temperatures)
• What is the efficiency of a steam turbine
  (assumed ideal) operating between 400K (1270C)
  and 300K (270C)?
• (400 300)/400 ¼ or 25
• What would be the efficiency if the turbine could
  operate at 600K?
• What would the exhaust temperature need to be for
  an engine to be 100 efficient?

1/2
0 K
48
(No Transcript)
49
(No Transcript)
50
Limits to Technology

• What factors limit the efficiency of an engine?
• Friction
• Temperature at which parts melt
• Carnot efficiency
• What would be the advantages of a ceramic engine?
  Disadvantages?

51
Can operate at 3000 degrees without cooling, is
light and doesnt need much cooling, but
Courtesy University of Colorado
52
Heat Engine Summary

• Work done is difference between heat flow in at
  high temperature and the heat flow out at a lower
  temperature (conservation of energy)

53
Order and Disorder

• Useful energy tends to degenerate and become less
  useful
• Alternate statement of 2nd Law Natural systems
  tend toward disorder
• Question Could all the air molecules in this
  room spontaneously concentrate at the top of the
  room (more orderly system)?

54
Entropy

• A measure of how much change occurs when energy
  spreads out according to the second law.
• More generally (and less accurately) a measure of
  disorder
• When disorder increases, entropy increases

55
Mess to Neat?

• Will this mess become neat all by itself?

No way!
56
Will this dish reassemble all by itself?
No Way, the Second Law of Thermodynamics
prohibits it
57
Entropy Summary

• Entropy is a quantity that measures the order or
  disorder of a system
• This quantity is larger for a more disordered
  system
• The Second Law of Thermodynamics says that
  entropy tend to increase
• All real engines lose heat to their surroundings

Courtesy California Science Standards in Physics
58
Global Warming
Courtesy University of Oregon