Transcript lec05

Physics 161
Fall 2006
Announcements
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HW#2 is due next Friday, 10/20.
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The first ‘further activity’ is due next Monday, 10/16
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Read chapter 4 for Wednesday, 10/11
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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 like, then have the quiz for an
hour, then do something else maybe.
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The Physics Department help room has been set up. The schedule can be
found at http://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.html
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Physics 161
Lecture 5
Fall 2006
The Energy of Heat
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Hot things have more energy than their cold counterparts
Heat is really just kinetic energy on microscopic scales: the
vibration or otherwise fast motion of individual
atoms/molecules
 Even though it’s kinetic energy, it’s hard to derive the same
useful work out of it because the motions are random
Heat is frequently quantified by calories (or Btu)
 One calorie (4.184 J) raises one gram of H2O 1ºC
 One Calorie (4184 J) raises one kilogram of H2O 1ºC
 One Btu (1055 J) raises one pound of H2O 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. . .
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Physics 161
Fall 2006
Energy of Heat, continued
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Food Calories are with the “big” C; 1 Cal = 1 kilocalorie (kcal)
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Since water has a density of one gram per cubic centimeter, 1 cal
heats 1 c.c. of water 1ºC, and likewise, 1 kcal (Calorie) heats one
liter of water 1ºC
 these are useful numbers to remember
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Example: to heat a 2-liter bottle of Coke from the 5ºC
refrigerator temperature to 20ºC room temperature requires 30
Calories, or 122.5 kJ. So drink your Coke cold – you burn up
energy that way. . .
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Physics 161
Fall 2006
Heat Capacity
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Different materials have different capacities for heat
 Add the same energy to different materials, and you’ll get
different temperature rises
 Quantified as heat capacity, cp
 Water is exceptional, with cp = 4,184 J/kg/ºC
 Most materials are about cp = 1,000 J/kg/ºC (including
wood, air, metals)
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Example: to add 10ºC to a room 3 meters on a side (cubic),
how much energy do we need?
air density is 1.3 kg/m3, and we have 27 m3, so 35 kg of
air; and we need 1000 J per kg per ºC, so we end up
needing 350,000 J (= 83.6 Cal or 0.1 kW-Hr)
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Physics 161
Fall 2006
And those are the major players…
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We’ve now seen all the major energy players we’ll be discussing in this class:
 work as force times distance
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kinetic energy (wind, ocean currents)
heat energy (power plants, space heating, OTEC, really random KE)
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electromagnetic energy (generators, transformers, etc.)
radiant energy (solar energy, really the same things as EM)
chemical energy (fossil fuels, batteries, food, biomass, also EM)
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gravitational potential energy (hydroelectric, tidal)
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mass-energy (nuclear sources, sun’s energy)
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Physics 161
Fall 2006
The Physics 161 Formula List
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Lots of forms of energy coming fast and furious, but to put
it in perspective, here’s a list of formulas that you’ll need to
use:
Relation Type
Formula
Work as force times distance
W = Fd
Kinetic Energy
K.E. = 1/2mv2
(Grav.) Potential Energy
E = mgh
Heat Content
E = cpmT
Power
P = E/t
Mass-energy
E = mc2
Radiative Flux
F = T4
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Physics 161
Fall 2006
Power, Energy Exchange and
Conservation of Energy
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Physics 161
Fall 2006
Power: Rate of Energy Flow
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Power is simply energy exchanged per unit
time, or how fast you get work done (Watts
= Joules/sec)
One horsepower = 745 W
Perform 100 J of work in 1 s, and call it 100
W
Run upstairs, raising your 70 kg (700 N)
mass 3 m (2,100 J) in 3 seconds  700 W
output!
Shuttle puts out a few GW (gigawatts, or
109 W) of power! Equivalent to a large
power plant. . .
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Physics 161
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Power Examples
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How much power does it take to lift 10 kg up 2 meters in 2
seconds?
mgh = (10 kg)(10 m/s2)(2 m) = 200J
200 J in 2 seconds  100 Watts
If you want to heat the 3 m cubic room by 10ºC with a 1000 W
space heater, how long will it take?
We know from the last lecture that the room 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 than six minutes.
But: the walls need to be warmed up too, so it will actually take
longer (and depends on quality of insulation, etc.)
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Physics 161
Fall 2006
Electrical Power
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If energy = Volts x charge, then what does volts x current
correspond to?
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Recall current = charge/second, so
Volts x current = Volts x charge/second = energy/second
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Volts x Amperes = power = Watts!
Example: 115 V running at 10 A corresponds to 1150 W = 1.15
kW of electrical power (sometime also called 1.15 kV-A).
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Physics 161
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Electrical Power, continued
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What is the resistance of the filament in a 75W light bulb?
What’s the AC line voltage (in the US)?
How much current to give 75W? (power = voltage x current)
What’s the resistance? (hint: Ohm’s Law)
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A higher power light bulk has a lower resistance, since V =
constant (supposedly):
P = I x V = (V/R) x V = V2/R = I2 x R
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Physics 161
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Series and Parallel Circuits
R=2W
R=2W
R=2W
I=?
R=2W
R=2W
R=2W
V=6V
Series circuit
V=6V
Parallel circuit
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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
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When we lift a rock, we do work against the force of gravity. This work is
stored as potential energy (PE).
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If we drop the rock, then gravity does work on the rock, and the PE is
converted into kinetic energy (KE).
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In mechanical systems when we can ignore losses to friction and drag (which
convert mechanical energy into heat), the energy is conserved and the change
in PE+KE is the work done on the system:
W = (PE+KE)
Demos: roller coaster, bowling ball pendulum
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Physics 161
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Energy Exchange and Energy Conservation, cont.
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In general, we cannot ignore friction and drag forces and
mechanical energy is ‘degraded’ into thermal energy (TE). Our
energy balance must take account of this TE:
W = (TE+PE+KE)
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So, if we do work on a system, we can either change its PE, its
KE, or its TE, or some combination of these three, but the total
energy will be conserved.
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Physics 161
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Energy Exchange and Energy Conservation, cont.
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In addition to doing work, we can also transfer heat (Q) to or
from a system, and this must also be included in our energy
balance:
Q+W = (TE+PE+KE)
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This is the first law of thermodynamics and it simply expresses
energy conservation. We would have to change it a bit to include
mass-energy (by adding another term on the right).
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Physics 161
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Conservation of Energy in a Hydroelectric Dam
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Water stores PE; we want to convert this to do useful work W
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Water with little KE falls to a turbine, reducing its PE (PE),
increasing its KE, and probably slightly increasing its TE due to
turbulence
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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 (KE)
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The turbine drives a generator which produces electric power that can be transported to do useful
work W.
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The slight increase in the water’s thermal energy (TE) is expelled to the environment as heat Q.
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Overall, these must balance: Q+W = (TE+PE+KE)
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The overall efficiency (electric energy out/water PE+KE in) of a hydroelectric plant is quite high, of
order 90%, since little energy was converted to heat.
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Physics 161
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Conservation of Energy in a Fossil Fuel Power Plant
Some steps are the same, except
 PE is stored in fuel, not mgh
Q+W = (TE+PE+KE):
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Fuel is burned to produce heat which
boils water
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Steam drives a turbine
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KE in and out are small
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. . . and we need to expel some heat at
ambient T, leading to lower overall
efficiency. Why is that?
Q is larger than for hydroelectric (why?) so W must be smaller
and the efficiency must be lower
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Physics 161
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Efficiency
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Generally, we define efficiency in terms of the benefit derived by a particular
process divided by effort expended.
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For a power plant:
‘effort’ is the energy input, either as fossil fuel (chemical PE), water behind a
dam (gravitational PE), nuclear fuel (mass PE), solar radiation (light PE), or
whatever.
‘benefit’ is the energy output, normally as electric power.
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These definitions will change in other circumstances, e.g, for a refrigerator.
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In all these cases, the really final end result is to convert a usable energy
source to heat – in your toaster, for example.
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Physics 161
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Total 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.
Figures from the text, Hinrichs and Kleinbach
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