Section 1 Notes: Temperature Scales and Conversions 1. How does a thermometer determine temperature?
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Transcript of Section 1 Notes: Temperature Scales and Conversions 1. How does a thermometer determine temperature?
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Section 1 Notes: Temperature Scales and Conversions
1. How does a thermometer determine temperature?
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Thermodynamics (Unit 1 spring)
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• Thermodynamics- Physics that deals with heat and its conversion into other forms of energy.
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Temperature Variables
• TK= Temperature Kelvin
• TC= Temperature Celsius
• TF= Temperature Fahrenheit
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• Absolute Zero= 0 Kelvin, a temperature where no motion would occur. There is no kinetic energy in the molecules.
• 0 Kelvin= -273.15 ºCelsius
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Conversion Scale
( )
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Example 1
• A healthy person has an oral temperature of 98.6 ºF. What would this reading be on the Celsius scale?
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Example 1
• A healthy person has an oral temperature of 98.6 ºF. What would this reading be on the Celsius scale?
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Example 2
• A time and temperature sign on a bank indicates the outdoor temperature is -20.0 ºC. What is the corresponding temperature on the Fahrenheit scale?
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Example 2
• A time and temperature sign on a blank indicates the outdoor temperature is -20.0 ºC. What is the corresponding temperature on the Fahrenheit scale?
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The Kelvin Temperature Scale• Has scientific significance
due to its absolute zero point.
• Has equal divisions as the Celsius scale
• Not written in degrees• 0º C is 273.15 K
• Therefore the conversion is:
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Intro
1. Convert 50º F into ºC and Kelvin
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Intro
1. Convert 50º F into ºC and Kelvin
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Intro
1. Convert 50º F into ºC and Kelvin
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Section 2 Notes:Kinetic Energy and Temperature
• Kinetic energy (KE)- Energy of movement
• Temperature- A measure proportional to the average kinetic energy of a substance.
– higher temperature = higher kinetic energy
– The more kinetic energy the quicker the molecules are moving around
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Click on the diagram to be taken to the page
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• Draw a picture representing molecular motion of three identical molecules at these two temperatures
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• Draw a picture representing molecular motion of three identical molecules at these two temperatures
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Section 3 Notes: Internal Energy vs. Heat
• Internal energy (U)- Sum of the molecular energy– kinetic energy, potential energy, and all other energies
in the molecules of a substance. – Unit: Joule
• Heat (Q) is energy in transit– energy flows from a hot to a cold substance.– Unit: Joule
• An object never has “heat” or “work” only internal energy (heat is transferred and work is done)
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Heat is energy in transit
• Heat lost by one object equals heat gained by another
• Heat lost = Heat gained
• -QA = QB
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Heat transfers from hot to cold
(a) Holding a hot cup
(b) Holing a cold glass (heat leaving your hand feels cold)
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• The coffee looses 468J of heat. How much heat does Bob gain? (assuming no heat was lost to the surroundings)
• The same: Bob gained 468 J of heat
Example 3
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– Direction: From high temperature to low temperature
– Rate of transfer depends on temperature difference: The greater temperature difference the greater the energy transfer
Twater =
20º C
Tcan =
15º C
Twater =
35º C
Tcan =
5º C
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Example 4
Where would the greater energy transfer take place and which way would the energy transfer?
A.Ice = 0 ºC Juice = 20 ºC
B.Ice = 0 ºC Juice = 25 ºC
B. has a bigger temperature difference and therefore greater energy transfer. Energy transfers from hot to cold: Juice to Ice
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What happens when the temperature inside and out are equal?
Twater =
11º C
Tcan =
11º C
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• Heat is transferred until there is thermal equilibrium
• Thermal Equilibrium- When temperatures are equal and there is an even exchange of heat
Twater =
11º C
Tcan =
11º C
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Section 4 Notes: Heat Transfer
•Types of Heat Transfer:
– Conduction– Convection– Radiation
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• Conduction- Caused by vibrating molecules transferring their energy to nearby molecules. Not an actual flow of molecules.
heat transfer
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• Thermal conductors- rapidly transfer energy as heat
• Thermal insulators- slowly transfer energy as heat
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Challenge
• Put the following in order of most thermally conductive to least.
Copper, Wood, Air, Water, Concrete
12345
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1. Copper
2 Concrete
3. Water
4. Wood
5. Air
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• Convection- process in which heat is carried from place to place by the bulk movement of a fluid (gas or liquid).
• Examples
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• Radiation (electromagnetic radiation) – Reduce internal energy by giving off electromagnetic radiation of particular wavelengths or heated by an absorption of wavelengths.
• Ex. The UV radiation from the
sun making something hot. Absorption
depends on the material.
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Draw your own pictures in the table that represent these three types of heat transfer.
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Draw your own pictures in the table that represent these three types of heat transfer.
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Section 5: Laws of Thermodynamics
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A System
• System- A collection of objects upon which attention is being focused on.
• This system includes the flask, water and steam, balloon, and flame.
• Surroundings- everything else
in the environment
The system and surrounding are
separated by walls of some kind.
System
Surroundings
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Walls between a system and the outside
• Adiabatic walls- perfectly insulating walls. No heat flow between system and surroundings.
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In a system: How can you measure the quantity of heat entering or leaving?
Q = Δ U or Q = Uf – U0
• Q: The quantity of heat that enters or leaves a system• U0: Initial internal energy in system• Uf: Final internal energy in system
• If Q is positive then energy entered the system• If Q is negative then energy left the system
• This is directly related to temperature. – If the system gets colder then heat left– If the system gets warmer then heat entered
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Example 5
• The internal energy of the substance is 50 J before
• The internal energy of the substance is 145 J after
a) How much heat was transferred in this system? b) Did it enter or leave?
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• First Law of Thermodynamics:
– Conservation of energy applied to thermal systems.
– Energy can neither be created nor destroyed. It can only change forms
– Stated in an equation
ΔU = Q + W
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First Law of Thermodynamics: Conservation of Energy
ΔU = Q + W
– Internal Energy (U) • (positive if internal energy is gained)
– Heat (Q) • (positive if heat is transferred in)
– Work (W)
• (positive if work is done on the system)
– The unit for all of these is the Joule (J)
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Example 6 & 7
6. A system gains 1500 J of heat from its surroundings, and 2200 J of work is done by the system on the surroundings. What is the change in internal energy?
7. A system gains 1500 of heat, but 2200 J of work is done on the system by the surroundings. What is the change in internal energy?
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6. A system gains 1500 J of heat from its surroundings, and 2200 J of work is done by the system on the surroundings. What is the change in internal energy?
7. A system gains 1500 of heat, but 2200 J of work is done on the system by the surroundings. What is the change in internal energy?
1500 - 2200
1500 + 2200
Example 6 & 7
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Now how can you tell if work is done by or on a system?
Is work done on or by the system ?a) nail/wood system b) Bunsen burner,
flask, balloon system
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• Work is done by the man causing frictional forces between the nail and the wood fiber.
• Work increases the internal energy of the wood and nail.
Work done on a system:Work to Internal Energy
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Work done by a system:Internal Energy to Work
• The balloon expands doing work on its surroundings
• The expanding balloon pushes the air away
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Work done on or by a gas
• Volume must change or no work is done.
• On a gas- Volume decreases (work must be done to force molecules into a smaller area)
• By a gas- Volume increases (the pressure of the gas causes the volume to increase)
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Section 5 Notes
4 Common Thermal Processes
• Isobaric Process
• Isochoric process (isovolumetric)
• Isothermal process
• Adiabatic process
• Each will have their own assumptions
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4 Thermal Processes
• Isobaric Process – occurs at constant pressure
• ΔP = 0
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4 Thermal Processes
• Isochoric process (Isovolumetric) – one that occurs at constant volume.
• ΔV = 0 and therefore W = 0
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Thermal Processes
• Isothermal process – one that occurs at constant temperature
• T (temperature) directly relates to U (internal energy)
• ΔU = 0
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Thermal Processes
• Adiabatic process – on that occurs with no transfer of heat
• ΔQ = 0
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Example 8
• How much heat has entered or left the system when 500J of work has been done on the system in an isothermal process?
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Example 8
• How much heat has entered or left the system when 500J of work has been done on the system in an isothermal process?
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Example 9
• How much work is done on or by the system when internal energy increases by 55J in n adiabatic process?
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Example 9
• How much work is done on or by the system when internal energy increases by 55J in n adiabatic process?
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Section 6: Three Laws of Thermodynamics
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First Law of Thermodynamics
• Energy Conservation: Conservation of energy applied to thermal systems.
• Energy can neither be created nor destroyed. It can only change forms
• When heat is added to a system, it transforms to an equal amount of some other form of energy.
• Equation:• ΔU = Q + W (work is done on a system)
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Second Law of Thermodynamics
• (Second Law) Law of Entropy– Heat goes from hot to cold.– No cyclic process is 100% efficient
• it can’t convert heat entirely into work • Some energy will always be transferred out to
surroundings as heat.
– Energy systems have a tendency to increase their entropy or disorder.
• Entropy- Measure of randomness or disorder in a system
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Third Law of Thermodynamics
• As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.
• A theoretical impossibility– If it occurred everything would stop and there
would be no more entropy
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Section 7: Transformation of energy in a heat engine
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The Heat Engine– a device that used a difference in temperature of two
substances to do mechanical work – It transferring energy from a high-temperature
substance (the boiler) to a lower temperature substance
– For each complete cycle: Wnet = Qh - Qc
What the variables stand for here:
Qh = Heat from high temperature substance
Qc = Heat to low temperature substance
W or work equals the difference of Qh and Qc
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The Heat EngineHow it works: main points
There will be an area of high temperature (boiler) and an area of low temperature
• Heat wants to go from a high temperature to a low temperature.
• Work is done by capturing energy in the transfer and using it to do work
• The work done by the engine equals the difference in heat transferred from the hot to cold substance.
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The Heat Engine
– For each complete cycle: Work = Energy transferred as heat from the high temperature substance to the colder temperature substance
What the variables stand for here:
Qh = Heat from high temperature substance
Qc = Heat to low temperature substance
W or work equals the difference of Qh and Qc
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Example 10
• A heat engine is working at 50% efficiency. How much work is done between a 670J and 200J reservoir?
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Example 10
• A heat engine is working at 50% efficiency. How much work is done between a 670J and 200J reservoir?