heat & thermodynamics physics(p). state standards 3. energy cannot be created or destroyed,...
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Heat & Thermodynamics
Physics(P)
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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.
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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
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What is the Difference Between Heat and Temperature?
Both are related to energy but there’s a big difference
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Temperature
Measure of how hot or cold an object is
Measured by thermometers
Work by expansion of a liquid
Other types use bimetallic strip
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Digital Thermometers
Use “thermistors” - temperature dependant semiconductor resistors
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Temperature Scales
Fahrenheit T(0F) = 9/5T(0C) + 32
Celsius (centigrade) T(0C ) = 5/9[T(0F) –32]
(degree is 9/5 that of Fahrenheit)
Kelvin (Celsius + 273)
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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
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Absolute Zero
0 degrees Kelvin = -273 Celsius
Lowest possible temperature
Molecular motion ceases
Courtesy Michigan State University
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Kinetic Theory of Heat
All matter is made of tiny atoms and molecules, constantly in motion
Faster is hotter
gas
solid
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Temperature and Kinetic Energy
In ideal gas temperature is proportional to average kinetic energy per molecule
Closely related in liquids and gases
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Temperature does not depend on the amount
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Heat does depend on the amount
Analogy: Heat is like the total height of students in this room, temperature is like their average height.
There is twice as much kinetic energy of moving molecules in two liters of water as in one liter.
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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.
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Heat – Energy Transferred
Definition: Energy that transfers because of temperature difference
Heat flows governed by average molecular kinetic energy difference
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Thermal Equilibrium
Objects at same temperature are at thermal equilibrium – no heat flows.
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Internal Energy
Total of all forms of energy inside something
Includes– Translational kinetic– Rotational kinetic– Potential
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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
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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 joulesOne kilocalorie = 4186 joules
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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
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Calorie Questions
How many calories are needed to raise the temperature of 10 grams of water by 10C?
10How many calories are needed to raise the temperature of one gram of water by 10 0C?
10
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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
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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
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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 othersWater has very high specific heat (capacity)Metals have much less
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Q = mcT
Q = mcT 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
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Thermal Expansion
Most materials expand when heated
Only exception is water between 00C and 400C
Expansion joints in bridges, cracks in sidewalks allow for expansion
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Bimetallic Strip
How your thermostat works
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Don’t Let Your Car’s Engine Overheat
Aluminum expands more than iron
Pistons made of aluminum
Cylinder made of iron
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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
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Joule’s Apparatus
Link to Joule’s original article
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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?
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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
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Thermodynamics
Study of heat and its transformation into mechanical energy
Based on conservation of energy
Explains how engines like car motors work
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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
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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
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Questions
20 J of heat is added to a system that does no work. What is the change in internal energy?
Answer +20 J20 J of heat is added to a system that does 10 J of work. What is the change in internal energy?
Answer +10 J
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20 J of heat is added to a system that does 30 J of work. What is the change of internal energy?
Answer -10 J20 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
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Bicycle Pump
What do you think happens when you operate the pump. Where does the work you do go?It goes to heat, some through friction, some to adiabatic compression of the air inside the pumpWhat does “adiabatic” mean?Answer: No heat enters or leaves Q=0
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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
A process can also be adiabatic if it happens inside a well insulated conatiner.
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Courtesy “How Stuff Works”
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Courtesy Shell Canada
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Adiabatic Expansion
Produces cooling
Example: blow on your hand first with wide open mouth, then with puckered lips
How do you explain the results?
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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
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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
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Second Law Applied to Engines
It is impossible to build a heat engine that changes heat completely into work.
Courtesy University of Oregon
Such an engine would be 100% efficient!
Allowed by 1st law, forbidden by 2nd law
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Heat Engine
Some heat is converted to useful workThe 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
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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
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Steam Turbine
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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?
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Courtesy University of Colorado
Can operate at 3000 degrees without cooling, is light and doesn’t need much cooling, but…
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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)
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Order and Disorder
Useful energy tends to degenerate and become less usefulAlternate statement of 2nd Law: Natural systems tend toward disorderQuestion: Could all the air molecules in this room spontaneously concentrate at the top of the room (more orderly system)?
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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
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Mess to Neat?
Will this mess become neat all by itself?
No way!
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Will this dish reassemble all by itself?
No Way, the Second Law of Thermodynamics prohibits it
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Entropy Summary
Entropy is a quantity that measures the order or disorder of a systemThis quantity is larger for a more disordered systemThe Second Law of Thermodynamics says that entropy tends to increaseAll real engines lose heat to their surroundings
Courtesy California Science Standards in Physics
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Courtesy University of Oregon
Global Warming