Chapter 17 – Work, Heat, And the First Law of Thermodynamics
Transcript of Chapter 17 – Work, Heat, And the First Law of Thermodynamics
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1Chapter 17 Work, Heat, and the First Law of Thermodynamics
17.1 Its All about Energy17.2 Work in Ideal-Gas Processes17.3 Heat17.4 The First Law of Thermodynamics17.5 Thermal Properties of Matter17.6 Calorimetry17.7 The Specific Heats of Gases
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Internal Energy
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Heat
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Work
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Power
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Energy & Power Units
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Energy & Power Question
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What wattage light bulb gives off about the same amount of heat as a person eats who consumes 2500 Cal/day of food?
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Energy & Power Question
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A student who weighs 70 kg eats a bag of potato chips which have 350 Cal, then decides to work them off by walking up stairs. How many flights of stairs (4 m high) must she climb to use up this number of Calories, assuming the body has an efficiency of 25%?
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Heat Capacity
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Heat Capacity Question
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A potato has C = 2000 J/K, and starts at T = 20C (room temperature). What is its temperature after it absorbs heat at a rate of 700 W (typical microwave oven) for 2 minutes?
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Specific Heat
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Specific Heat Question
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A water heater is operated by solar power. If the solar collector has an area of 6.00 m2 and the power delivered by sunlight is 550 W/m2, how long does it take to increase the temperature of 1.00 m3 of water from 20C to 60C ?
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Specific Heat Question
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Two identical pots are placed on identical stove burners. Initially, one pot has one cup of water at 20C and the other pot has two cups at the same temperature. Both burners are turned on at the same time. At the moment when the water in the first pot boils, approximately what is the temperature of the water in the second pot?
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Latent Heat
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Heat of Fusion (LF)
Heat of Vaporization (LV)
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Latent Heats for Water
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If the same amount of heat necessary to melt 1 kg of ice at 0C were instead applied to warming up 1 kg of liquid water initially at 0C, what would the final temperature of the liquid water be?
What if the heat necessary to boil 1 kg of water at 100C were applied to liquid water initially at 0C?
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Latent Heat Question
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Heat is added at a rate of 200 W to a beaker, which initially contains 200 grams of ice in 300 grams of liquid water. How long will it take for(a) all the ice to melt? (b) then for the water to reach boiling point?(c) then all the water to boil away?
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Latent Heat & Specific Heat
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Heating 0.0010 kg (1.0 cm3) Water
333J 2265J 419J 100
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Latent Heat & Specific Heat Question
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Two identical pots are placed on identical stove burners. Initially, one pot has one cup of water at 20C and the other pot has two cups at the same temperature. Both burners are turned on at the same time. At the moment all of the water in the first pot has boiled away, what can be said about the second pot?
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Latent Heat & Specific Heat Question
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An ice cube (m1 = 10 g at 0C) is added to a glass of water (200 cm3, m2 = 200 g) at room temperature (20C). Assume the system is thermally isolated (no heat enters from the room). Does all the ice melt? What is the final temperature?
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Thermodynamic Systems
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T
- fixed amount of stuff (gas or other)- in equilibrium a thermodynamic system has certain properties (state variables)
Typical State Variables
p pressure force
areaunit ~
Pa
V volume unit ~ m3
T absolute temperature unit ~ K
An equation of state (relevant to a given thermodynamic system) relates p, V, and T.
Ex. (IGL is equation of state for a gas.)
Other thermodynamic systems have different equations of state.
The cylinder is generally NOT part of system.
A gas in a cylinder is a thermodynamic
system.
amount of stuff, identifies the system
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Heat and Work two ways to change the internal energy of a system
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Adiabatic Process
Q: heat flowing into system (E increases)
W: work done to system (E increases)
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Work
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How can we calculate the work done on a thermodynamic system?
before after
What is the work done at a constant pressure?
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Work Questions
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An air-filled balloon is placed in a warm spot. As a result, it expands. What are the signs ( + or ) of the heat absorbed Q and the work done Won the air in the balloon?
A cup of liquid water with ice cubes in it is left on the table. After a long time the ice has melted and everything is at room temperature. Consider the system to be all of the water (liquid & ice). What are the signs ( + or ) of the heat absorbed Q and the work done W on the system?
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Work Question
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0.200 moles of an ideal gas is enclosed in a cylinder with a movable piston on top. The piston has a mass of 8000 g and an area of 5.00 cm2and is free to slide up and down, keeping the pressure of the gas constant. How much work is done on the gas as the temperature of the gas is raised from 20.0C to 300C?
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Work
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What is the sign (+ or ) of the work done on the systems below?
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Work Question
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Below are three processes in which a given sample of gas goes from the same original state to the same final state? How does the work done on the system compare?
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Work Question
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How much work is done on the system in case (a) below?
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Work Question
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How much work is done on the system in case (b) below?
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Work Question (assume IGL holds)
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How much work is done on the system in case (c) below if the temperature T is constant?
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Work in a Cyclic Process
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V
p- system starts/stops in same
thermodynamic state
- ex. fridge, AC, engines
- in practical applications often interested in work done by system. Note: This is , not .
Sign Convention for Cyclic Processes
clockwise thermodynamic system does work 0 [like moving a car]
counterclockwise work done to thermodynamic system 0 [refrigerator]
enclosed signed area represents work done by system.
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First Law of Thermodynamics
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Q is heat energy going into system.
W is work energy going into system.
E is internal energy of system. (total KE + PE of system)
Conservation of Energy:
1st Law of Thermodynamics
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First Law of Thermodynamics Question
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A sample of stuff is compressed, which causes it to warm up above room temperature. While keeping it at the new, small volume, it is allowed to cool back to room temperature (by thermal conduction). What relationship does the net change in energy E satisfies?
What if the sample above were an ideal gas?
p
V
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First Law of Thermodynamics Question
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How much work is done by 1.00 mol of water at 100C when it boils and becomes 1.00 mol of steam at 100C and 1.00 atm pressure? Assuming the steam to be an ideal gas, determine the change in internal energy of the steam as it vaporizes.
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Adiabatic Process
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Isovolumetric (Isochoric) Process
- no heat enters/leaves system
- volume constant
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Cyclic Process
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Isothermal Process
- system ends up in same thermodynamic state (p, V, T, E)
- process that occurs at constant temperature 0
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Mechanisms of Thermal Energy Transfer Q (or rate P)
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Conduction
Convection
Radiation
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Thermal Conduction
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xSample Thermal Conductivities
Material k (W/m-K)
rateofheattransferwatts temp. diff. K distancebetweenhot&coldsidesm crosssectionalaream thermalconductivity
(depends on material)
graphene ~5000diamond 1000
steel 50water (20C) 0.6
brick 0.6air (0C) 0.024
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Thermal Conduction Question
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Consider two aluminum rods, with the same cross section, but one rod twice as long as the other one. One end of each rod is maintained at 400C by a flame, while the other ends are maintained at 0C by an ice bath. The rate of heat conduction through the shorter rod is 50 W. What is the rate of conduction through the longer rod?
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Convection
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- Time required for a temperature change to spread by conduction is proportional to distance squared (relatively slow). Its more efficient for the material carrying the heat to do the moving (this is called convection).
- Often differences due to thermal expansion drive convection currents. Most commonly, hot fluid is less dense and so rises (natural convection).
Ex. Weather patterns
- Other times there is a circulator.Ex. Refrigerator or forced heating
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Thermal Radiation (Black-Body Radiation)
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Objects at 0K continuously emit EM radiation (in a spectrum of light)
surfaceareaofobjectm
absolutetemperatureofobjectK
StefanBoltzmannconstant 5.67 10
emissivity, unitlessnumberbetween0and1(depends on object)
Perfect absorber/emitter (black-body) has e = 1.Perfect mirror (reflects all light) has e = 0.
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Thermal Radiation Question
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How many Calories per day must a person (surface area 2 m2, emissivity 0.5) eat to make up for the power he radiates thermally?(a) Compute the answer ignoring absorbed radiation.(b) Compute the answer assuming radiation is also absorbed from surroundings at 20C
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Thermal Radiation Question
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An object is at room temperature. By approximately what percent does the thermal radiation emitted by the object increase if the object is warmed by 3C?