Announcements. HW#2 is due next Friday, 10/20. The first ‘further activity’ is due next Monday, 10/16 Read chapter 4 for Wednesday, 10/11 - PowerPoint PPT Presentation
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1 Physics 161 Fall 2006 Announcements Announcements HW#2 is due next Friday, 10/20. HW#2 is due next Friday, 10/20. The first ‘further activity’ is due next Monday, 10/16 The first ‘further activity’ is due next Monday, 10/16 Read chapter 4 for Wednesday, 10/11 Read chapter 4 for Wednesday, 10/11 The first quiz is scheduled for Monday, 10/23. This The first quiz is scheduled for Monday, 10/23. This will cover chapters 1-4. We’ll spend the first half will cover chapters 1-4. We’ll spend the first half hour reviewing, if you like, then have the quiz for an hour reviewing, if you like, then have the quiz for an hour, then do something else maybe. hour, then do something else maybe. The Physics Department help room has been set up. The The Physics Department help room has been set up. The schedule can be found at schedule can be found at http://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.ht http://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.ht ml ml
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Physics 161 Fall 2006
AnnouncementsAnnouncements HW#2 is due next Friday, 10/20.HW#2 is due next Friday, 10/20.
The first ‘further activity’ is due next Monday, 10/16The first ‘further activity’ is due next Monday, 10/16
Read chapter 4 for Wednesday, 10/11Read chapter 4 for Wednesday, 10/11
The first quiz is scheduled for Monday, 10/23. This will cover The first quiz is scheduled for Monday, 10/23. This will cover chapters 1-4. We’ll spend the first half hour reviewing, if you chapters 1-4. We’ll spend the first half hour reviewing, if you like, then have the quiz for an hour, then do something else like, then have the quiz for an hour, then do something else maybe.maybe.
The Physics Department help room has been set up. The The Physics Department help room has been set up. The schedule can be found at schedule can be found at http://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.htmlhttp://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.html
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Physics 161 Fall 2006
The Energy of HeatThe Energy of Heat
Hot things have more energy than their cold Hot things have more energy than their cold counterpartscounterparts
Heat is really just kinetic energy on microscopic Heat is really just kinetic energy on microscopic scales: the vibration or otherwise fast motion of scales: the vibration or otherwise fast motion of individual atoms/moleculesindividual atoms/molecules Even though it’s kinetic energy, it’s hard to Even though it’s kinetic energy, it’s hard to
derive the same useful work out of it because the derive the same useful work out of it because the motions are motions are randomrandom
Heat is frequently quantified by calories (or Btu)Heat is frequently quantified by calories (or Btu) One calorie (4.184 J) raises one gram of HOne calorie (4.184 J) raises one gram of H22O 1O 1ºCºC One Calorie (4184 J) raises one kilogram of HOne Calorie (4184 J) raises one kilogram of H22O O
11ºCºC One Btu (1055 J) raises one pound of HOne Btu (1055 J) raises one pound of H22O 1ºFO 1ºF
Answer to the question from the end of lecture 3: In principle, one can convert some forms of energy to others with perfect efficiency, but this is not true when we try to convert heat to mechanical energy. This was first shown by Carnot in the early 1800’s. What factors in society at that time might have motivated Carnot to work on this problem? ‘High tech’ research has changed its nature over the years. . .
Lecture 5
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Physics 161 Fall 2006
Energy of Heat, continuedEnergy of Heat, continued Food Calories are with the “big” C; 1 Cal = 1 Food Calories are with the “big” C; 1 Cal = 1
kilocalorie (kcal)kilocalorie (kcal)
Since water has a density of one gram per cubic Since water has a density of one gram per cubic centimeter, 1 cal heats 1 c.c. of water 1centimeter, 1 cal heats 1 c.c. of water 1ºC, and ºC, and likewise, 1 kcal (Calorie) heats one liter of water 1ºClikewise, 1 kcal (Calorie) heats one liter of water 1ºC these are useful numbers to rememberthese are useful numbers to remember
Example: to heat a 2-liter bottle of Coke from the 5Example: to heat a 2-liter bottle of Coke from the 5ºC ºC refrigerator temperature to 20ºC room temperature refrigerator temperature to 20ºC room temperature requires 30 Calories, or 122.5 kJ. So drink your requires 30 Calories, or 122.5 kJ. So drink your Coke cold – you burn up energy that way. . . Coke cold – you burn up energy that way. . .
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Physics 161 Fall 2006
Heat CapacityHeat Capacity Different materials have different Different materials have different capacitiescapacities for for
heatheat Add the same energy to different materials, Add the same energy to different materials,
and you’ll get different temperature risesand you’ll get different temperature rises Quantified as heat capacity, cQuantified as heat capacity, cpp
Water is exceptional, with cWater is exceptional, with cpp = 4,184 J/kg/ = 4,184 J/kg/ºCºC Most materials are about Most materials are about ccpp = 1,000 J/kg/ºC = 1,000 J/kg/ºC
Example: to add 10Example: to add 10ºC to a room 3 meters on a ºC to a room 3 meters on a side (cubic), how much energy do we need?side (cubic), how much energy do we need?
air density is 1.3 kg/mair density is 1.3 kg/m33, and we have 27 m, and we have 27 m33, so , so 35 kg of air; and we need 1000 J per kg per 35 kg of air; and we need 1000 J per kg per ºC, ºC, so we end up needing 350,000 J (= 83.6 Cal or so we end up needing 350,000 J (= 83.6 Cal or 0.1 kW-Hr)0.1 kW-Hr)
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Physics 161 Fall 2006
And those are the major And those are the major players…players…
We’ve now seen all the major energy players we’ll be discussing in We’ve now seen all the major energy players we’ll be discussing in this class:this class: work as force times distancework as force times distance
kinetic energy (wind, ocean currents)kinetic energy (wind, ocean currents) heat energy (power plants, space heating, OTEC, really random heat energy (power plants, space heating, OTEC, really random
KE)KE)
electromagnetic energy (generators, transformers, etc.)electromagnetic energy (generators, transformers, etc.) radiant energy (solar energy, really the same things as EM)radiant energy (solar energy, really the same things as EM) chemical energy (fossil fuels, batteries, food, biomass, also EM)chemical energy (fossil fuels, batteries, food, biomass, also EM)
gravitational potential energy (hydroelectric, tidal)gravitational potential energy (hydroelectric, tidal)
The Physics 161 Formula ListThe Physics 161 Formula List Lots of forms of energy coming fast and Lots of forms of energy coming fast and
furious, but to put it in perspective, here’s a furious, but to put it in perspective, here’s a list of formulas that you’ll need to use:list of formulas that you’ll need to use:
Relation TypeRelation Type FormulaFormula
Work as force times Work as force times distancedistance
WW = = FFdd
Kinetic EnergyKinetic Energy K.E. = K.E. = 1/21/2mvmv22
(Grav.) Potential Energy(Grav.) Potential Energy E = mghE = mgh
Heat ContentHeat Content E = cE = cppmmTT
PowerPower P = P = E/E/tt
Mass-energyMass-energy E = mcE = mc22
Radiative FluxRadiative Flux F = F = TT44
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Physics 161 Fall 2006
Power, Energy Exchange and Power, Energy Exchange and Conservation of EnergyConservation of Energy
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Physics 161 Fall 2006
Power: Rate of Energy FlowPower: Rate of Energy Flow
Power is simply energy Power is simply energy exchanged per unit time, or how exchanged per unit time, or how fast you get work done (Watts = fast you get work done (Watts = Joules/sec) Joules/sec)
One horsepower = 745 WOne horsepower = 745 W Perform 100 J of work in 1 s, and Perform 100 J of work in 1 s, and
call it 100 Wcall it 100 W Run upstairs, raising your 70 kg Run upstairs, raising your 70 kg
(700 N) mass 3 m (2,100 J) in 3 (700 N) mass 3 m (2,100 J) in 3 seconds seconds 700 W output! 700 W output!
Shuttle puts out a few GW Shuttle puts out a few GW (gigawatts, or 10(gigawatts, or 1099 W) of power! W) of power! Equivalent to a large power Equivalent to a large power plant. . . plant. . .
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Power ExamplesPower Examples
How much power does it take to lift 10 kg up 2 How much power does it take to lift 10 kg up 2 meters in 2 seconds?meters in 2 seconds?mghmgh = (10 kg) = (10 kg)(10 m/s(10 m/s22))(2 m) = 200J(2 m) = 200J
200 J in 2 seconds 200 J in 2 seconds 100 Watts 100 Watts If you want to heat the 3 m cubic room by 10If you want to heat the 3 m cubic room by 10ºC with ºC with
a 1000 W space heater, how long will it take?a 1000 W space heater, how long will it take?We know from the last lecture that the room We know from the last lecture that the room needs to have 350,000 J added to it, so at 1000 W needs to have 350,000 J added to it, so at 1000 W = 1000 J/s this will take 350 seconds, or a bit less = 1000 J/s this will take 350 seconds, or a bit less than six minutes.than six minutes.
But: the walls need to be warmed up too, so it will But: the walls need to be warmed up too, so it will actually take longer (and depends on quality of actually take longer (and depends on quality of insulation, etc.)insulation, etc.)
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Physics 161 Fall 2006
If energy = Volts x charge, then what does volts x If energy = Volts x charge, then what does volts x current correspond to?current correspond to?
Recall current = charge/second, so Recall current = charge/second, so
Volts x current = Volts x charge/second = Volts x current = Volts x charge/second = energy/secondenergy/second
Volts x Amperes = power = Watts!Volts x Amperes = power = Watts!
Example: 115 V running at 10 A corresponds to 1150 Example: 115 V running at 10 A corresponds to 1150 W = 1.15 kW of electrical power (sometime also W = 1.15 kW of electrical power (sometime also called 1.15 kV-A).called 1.15 kV-A).
Electrical PowerElectrical Power
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Physics 161 Fall 2006
Electrical Power, continuedElectrical Power, continued
What is the resistance of the filament in a 75W light What is the resistance of the filament in a 75W light bulb?bulb?
What’s the AC line voltage (in the US)?What’s the AC line voltage (in the US)?
How much current to give 75W? (power = voltage x How much current to give 75W? (power = voltage x current)current)
What’s the resistance? (hint: Ohm’s Law)What’s the resistance? (hint: Ohm’s Law)
A A higherhigher power light bulk has a power light bulk has a lowerlower resistance, resistance, since V = constant (supposedly):since V = constant (supposedly):P = I x V = (V/R) x V = VP = I x V = (V/R) x V = V22/R = I/R = I22 x R x R
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Physics 161 Fall 2006
Series and Parallel Circuits
V = 6 V
R=2I = ?
Series circuitParallel circuit
R=2 R=2
V = 6 V
R=2 R=2 R=2
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Physics 161 Fall 2006
Series and Parallel Circuit Questions
Series:
What happens to current in other lamps if one lamp in a series circuit burns out?
What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit?
Parallel:
What happens to the current in other lamps if one of the lamps in a parallel circuit burns out?
What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit?
Which way are the lights in your house wired?
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Physics 161 Fall 2006
Energy Exchange and Conservation of Energy Exchange and Conservation of EnergyEnergy
When we lift a rock, we do work against the force of gravity. When we lift a rock, we do work against the force of gravity. This work is stored as potential energy (PE).This work is stored as potential energy (PE).
If we drop the rock, then gravity does work on the rock, and If we drop the rock, then gravity does work on the rock, and the PE is converted into kinetic energy (KE).the PE is converted into kinetic energy (KE).
In mechanical systems when we can ignore losses to friction In mechanical systems when we can ignore losses to friction and drag (which convert mechanical energy into heat), the and drag (which convert mechanical energy into heat), the energy is conserved and the change in PE+KE is the work energy is conserved and the change in PE+KE is the work done on the system:done on the system:
Energy Exchange and Energy Energy Exchange and Energy Conservation, cont.Conservation, cont.
In general, we cannot ignore friction and drag In general, we cannot ignore friction and drag forces and mechanical energy is ‘degraded’ into forces and mechanical energy is ‘degraded’ into thermal energy (TE). Our energy balance must thermal energy (TE). Our energy balance must take account of this TE:take account of this TE:
W = W = (TE+PE+KE)(TE+PE+KE)
So, if we do work on a system, we can either So, if we do work on a system, we can either change its PE, its KE, or its TE, or some change its PE, its KE, or its TE, or some combination of these three, but the total energy combination of these three, but the total energy will be conserved.will be conserved.
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Physics 161 Fall 2006
Energy Exchange and Energy Energy Exchange and Energy Conservation, cont.Conservation, cont.
In addition to doing work, we can also transfer In addition to doing work, we can also transfer heat (Q) to or from a system, and this must also heat (Q) to or from a system, and this must also be included in our energy balance:be included in our energy balance:
Q+W = Q+W = (TE+PE+KE)(TE+PE+KE)
This is the first law of thermodynamics and it This is the first law of thermodynamics and it simply expresses energy conservation. We would simply expresses energy conservation. We would have to change it a bit to include mass-energy have to change it a bit to include mass-energy (by adding another term on the right).(by adding another term on the right).
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Physics 161 Fall 2006
Conservation of Energy in a Conservation of Energy in a Hydroelectric DamHydroelectric Dam
Water stores PE; we want to convert this to do useful work WWater stores PE; we want to convert this to do useful work W
Water with little KE falls to a turbine, reducing its PE (Water with little KE falls to a turbine, reducing its PE (PE), PE), increasing its KE, and probably slightly increasing its TE due to increasing its KE, and probably slightly increasing its TE due to turbulenceturbulence
The moving water turns the turbine, thereby transferring most of it’s KE, The moving water turns the turbine, thereby transferring most of it’s KE, generating a bit more TE, and leaving a little KE in the water (generating a bit more TE, and leaving a little KE in the water (KE) KE)
The turbine drives a generator which produces electric power that can be The turbine drives a generator which produces electric power that can be transported to do useful work W. transported to do useful work W.
The slight increase in the water’s thermal energy (The slight increase in the water’s thermal energy (TE) is expelled to the TE) is expelled to the environment as heat Q. environment as heat Q.
Overall, these must balance: Q+W = Overall, these must balance: Q+W = (TE+PE+KE)(TE+PE+KE)
The overall efficiency (electric energy out/water PE+KE in) of a The overall efficiency (electric energy out/water PE+KE in) of a hydroelectric plant is quite high, of order 90%, since little energy was hydroelectric plant is quite high, of order 90%, since little energy was converted to heat.converted to heat.
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Physics 161 Fall 2006
Some steps are the same, except Some steps are the same, except PE is stored in fuel, not mghPE is stored in fuel, not mgh
Fuel is burned to produce Fuel is burned to produce heat which boils waterheat which boils water
Steam drives a turbineSteam drives a turbine
KE in and out are smallKE in and out are small
. . . and we need to expel . . . and we need to expel some heat at ambient T, some heat at ambient T, leading to lower overall leading to lower overall efficiency. Why is that?efficiency. Why is that?
Conservation of Energy in a Fossil Fuel Conservation of Energy in a Fossil Fuel Power PlantPower Plant
Q+W = Q+W = (TE+PE+KE): Q is larger than for hydroelectric (why?) so (TE+PE+KE): Q is larger than for hydroelectric (why?) so W must be smallerW must be smaller
and the efficiency must be lowerand the efficiency must be lower
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Physics 161 Fall 2006
EfficiencyEfficiency Generally, we define efficiency in terms of the benefit Generally, we define efficiency in terms of the benefit
derived by a particular process divided by effort expended.derived by a particular process divided by effort expended.
For a power plant:For a power plant:‘‘effort’effort’ is the energy input, either as fossil fuel (chemical is the energy input, either as fossil fuel (chemical PE), water behind a dam (gravitational PE), nuclear fuel PE), water behind a dam (gravitational PE), nuclear fuel (mass PE), solar radiation (light PE), or whatever. (mass PE), solar radiation (light PE), or whatever. ‘‘benefit’benefit’ is the energy output, normally as electric power. is the energy output, normally as electric power.
These definitions will change in other circumstances, e.g, These definitions will change in other circumstances, e.g, for a refrigerator.for a refrigerator.
In all these cases, the really final end result is to convert a In all these cases, the really final end result is to convert a usable energy source to heat – in your toaster, for example.usable energy source to heat – in your toaster, for example.
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Physics 161 Fall 2006
Total EfficiencyTotal Efficiency
Each process degrades some energy to heat.
Improvements in each process are being actively researched to improve energy efficiency, superconducting wire, high T turbines, fluorescent lamps, etc.
