Announcements - PowerPoint PPT Presentation

1 / 27
About This Presentation
Title:

Announcements

Description:

HW#2 has been posted on Webassign and is due on ... at barely detectable level in all energy transformations, but effect is tiny! ... ( read Angels and Demons) ... – PowerPoint PPT presentation

Number of Views:160
Avg rating:3.0/5.0
Slides: 28
Provided by: physics3
Category:

less

Transcript and Presenter's Notes

Title: Announcements


1
Announcements
  • HW1 Is due this Friday, October 6 at 500pm (on
    Webassign).
  • HW2 has been posted on Webassign and is due on
    Friday, 10/20.
  • Read chapter 3 in the text for class on Monday,
    10/9 and chapter 4 for class on Wednesday, 10/11.
  • I will be out of town all next week (10/9-10/13)
    class will meet as usual with lectures by our
    demo guy Stan Micklavzina. Be nice.

2
Physical Forms of Energy
Lecture 4
  • Kinetic
  • Potential
  • Gravitational
  • Electrical and Magnetic
  • Chemical
  • Radiant
  • Mass-energy
  • Heat (sort of a random kinetic energy)

3
Electrical Energy Some Definitions
  • Voltage is a measure of electric potential, units
    Volts (V). Was there someone named Volt?
  • Charge, measured in Coulombs (C), is a measure of
    certain types of matter that have charge (e.g.,
    electrons or protons) a flowing charge is a
    current, measured in Amperes or Amps (A), which
    is the same as Coulombs/second. An electron has
    a charge of -1.6x10-19 C. Who was Coulomb?
  • When voltage is multiplied by charge, a potential
    energy results, measured by physicists (as usual)
    in Joules (J) 1 C at an electric potential of 1V
    means a potential energy of 1 J.
  • By itself, voltage will not hurt you can grab a
    high voltage line and not be hurt as long as you
    dont grab anything else at the same time.
    Current running through you can ruin your day.

4
Electrical Energy, continued
  • There is normally a proportionality between
    voltage and current with proportionality constant
    given by the resistance R, measured in Ohms (W),
    and expressed by Ohms law
  • V I x R
  • For example, the (dry) human body has a
    resistance of 2000 W. With V 120V, I V/R
    0.06A 60 mA. A wet human has a substantially
    lower resistance. . .
  • Did you know that women really are more sensitive
    than men? Which sex has a lower resistance?

5
Electric Shock
A van der Graaf operates at very high voltage
(100 kV), but does not store much charge and so
cannot hurt you (very much). Its current and
the current x time charge that can kill.
BODILY EFFECT DIRECT CURRENT (DC) 60 Hz AC 10 kHz
AC Slight sensation felt at hand(s) Men
1.0 mA 0.4 mA 7 mA Women 0.6 mA
0.3 mA 5 mA Threshold of perception Men
5.2 mA 1.1 mA 12 mA Women 3.5 mA 0.7
mA 8 mA Painful, but voluntary muscle control
maintained Men 62 mA 9 mA 55 mA Women
41 mA 6 mA 37 mA Painful, unable to let
go of wires Men 76 mA 16 mA 75 mA
Women 51 mA 10.5 mA 50 mA Severe pain,
difficulty breathing Men 90 mA 23 mA 94
mA Women 60 mA 15 mA 63 mA Possible
heart fibrillation after 3 seconds Men 500
mA 100 mA Women 500 mA 100 mA
hmmm. . . women really are more sensitive than men
6
Chemical Energy
  • Electrostatic energy (associated with charged
    particles, like electrons) is stored in the
    chemical bonds of substances.
  • Rearranging these bonds can release energy (some
    reactions require energy to be put in)
  • Typical numbers are 100200 kJ per mole
  • a mole is 6.022?1023 molecules/particles
    (Avogadros number)
  • works out to typical numbers like several
    thousand Joules per gram, or a few Calories per
    gram (remember, 1 Cal 1 kcal 4184 J)

7
Chemical Energy Examples
  • Burning a wooden match releases about one Btu, or
    1055 Joules (a match is about 0.3 grams), so this
    is 3,000 J/g, nearly 1 Cal/g
  • Burning coal releases about 20 kJ per gram of
    chemical energy, or roughly 5 Cal/g
  • Burning gasoline yields about 39 kJ per gram, or
    just over 9 Cal/g

8
Energy from Food
  • We get the energy to do the things we do out of
    food (stored solar energy in the form of chemical
    energy).
  • Energy sources recognized by our digestive
    systems
  • Carbohydrates 4 Calories per gram
  • Proteins 4 Calories per gram
  • Fats 9 Calories per gram (like gasoline)

9
Our Human Energy Budget
  • A 2000 Calorie per day diet means 2000?4184 J
    8,368,000 J per day
  • 8.37 MJ in (24 hr/day) ?(60 min/hr)?(60 sec/min)
    86,400 sec corresponds to 97 Watts of power
  • Even a couch-potato at 1500 Cal/day burns 75 W
  • More active lifestyles require greater Caloric
    intake (more energy)

10
Nutrition Labels
  • Nutrition labels tell you about the energy
    content of food
  • Note they use Calories with capitol C
  • Conversions Fat 9 Cal/g
  • Carbs 4 Cal/g
  • Protein 4 Cal/g
  • One serving of whole milk has 72 Cal from fat, 48
    Cal from carbohydrates, and 32 Cal from protein
  • sum is 152 Calories compare to label
  • 152 Cal 636 kJ enough to climb about 1000
    meters (64 kg person)
  • Young children need milk fat a good source of
    lecithin to help develop their nervous system
    (among other things). Until a few years ago,
    adults often chose to reduce their fat intake.

11
Mass-energy
  • Einsteins famous relation
  • E mc2
  • relates mass to energy (units OK?)
  • In effect, they are the same thing
  • one can be transformed into the other
  • physicists speak generally of mass-energy
  • Seldom experienced in daily life directly
  • Happens in the center of the sun, in nuclear
    bombs and reactors
  • Actually does happen at barely detectable level
    in all energy transformations, but effect is tiny!

before the bad hair days. . .
12
E mc2 Examples
  • The energy equivalent of one gram of material
    (any composition) is (0.001 kg)?(3.0?108 m/s)2
    9.0?1013 J 90,000,000,000,000 J
  • Man, thats big! (read Angels and Demons)
  • If one gram of material undergoes a chemical
    reaction, losing about 9,000 J of energy (i.e., 3
    matches burn), how much mass does it lose?
  • 9,000 J ?mc2, so ?m 9,000/c2 9?103/9?1013
  • 10-10 kg (would we ever notice?)

13
Solar Energy is Nuclear, Using E mc2
  • Thermonuclear fusion reactions in the suns
    center
  • Sun is 16 million degrees Celsius in its center
  • Enough energy to ram protons together (despite
    mutual repulsion) and make deuterium, then helium
  • Reaction per atom 20 million times more energetic
    than chemical reactions, in general

2 neutrinos, 6 photons (light)
4He nucleus mass 4.0015
4 protons mass 4.029
2 electrons mass 0.00055
14
E mc2 in Sun
  • Helium nucleus is lighter than the four protons
    2 electrons!
  • Mass difference is 4.029 - 4.0015 0.0276 a.m.u.
  • 1 a.m.u. (atomic mass unit) is 1.6605?10-27 kg
  • difference of 4.58?10-29 kg
  • multiply by c2 to get 4.12?10-12 J
  • 1 mole (6.022?1023 particles) of protons ?
    2.5?1012 J
  • typical chemical reactions are 100-200 kJ/mole
  • nuclear fusion is 20 million times more potent
    stuff!

15
Cold Fusion a Checkered History
  • Fusion releases stored energy and ought to be a
    spontaneous process.
  • At low temperature, the rate is immeasurably low
    because the electrical repulsion between protons
    keeps them separated (Coulombs Law)
  • The high temperature at the center of the sun
    gives the protons enough kinetic energy to climb
    the electrical barrier - just like we climb a
    gravitational barrier when we climb a hill
  • Many attempts have been made to lower the
    required temperature. The latest attempt still
    festers. . .
  • Bob Park from the APS Information Office
    regularly lampoons these efforts. I encourage
    you to go to his site and subscribe to his weekly
    newsletter. . .
  • And then theres much ado about a star in a jar
    sonoluminescence, Chain Reaction, Keanu Reeves, .
    . .

16
Radiant Energy
  • Really just a special case of electromagnetic
    energy in the form of waves traveling at the
    speed of light c 3x108 m/sec (a classical
    notion of light)
  • Encompasses everything from radio waves to gamma
    rays the entire EM spectrum of wavelengths and
    frequencies
  • We will have more to say about this later in the
    context of photovoltaic and solar energy, e.g.,
    the solar constant on average, 164 J of radiant
    energy strikes every square meter of the earth
    surface every second.

17
Energy from Light
  • The tremendous energy from nuclear reactions in
    the sun is released as light. So light carries
    energy.
  • How much??
  • Best way to get at this is through the process of
    blackbody radiation, or thermal radiation
  • All objects emit light
  • Though almost all the light we see is reflected
    light
  • The color and intensity of the emitted radiation
    depend on the objects temperature

18
Emitted Radiations Color and Intensity depend on
Temperature
Temperature 30 C 500 C 1700 C 2700 C
5500 C
Color Infrared (invisible) Dull red Dim
orange Yellow Brilliant white
Object You Heat Lamp Candle Flame Bulb
Filament Suns Surface
The hotter it gets, the bluer the emitted
light The hotter it gets, the more intense the
radiation (more energy)
Well talk more about temperature scales next
week. . .
19
Blackbody, or Planck Spectrum
20
Same thing, on logarithmic scale
Sun peaks in visible band (0.5 microns), light
bulbs at 1 ?m, we at 10 ?m. (note 0C 273K
300K 27C 81F)
21
Okay, but how much energy?
  • The power given off of a surface in the form of
    light is proportional to the fourth power of
    temperature!
  • F ?T4 in Watts per square meter
  • the constant, ?, is numerically 5.67?10-8
    W/ºK4/m2
  • easy to remember constant 5678
  • temperature must be in Kelvin
  • ºK ºC 273
  • ºC (5/9)?(ºF 32)
  • Example radiation from your body
  • (5.67 ?10-8) ?(310)4 523 Watts per square meter
  • (if naked in the cold of space dont let this
    happen to you!)

22
Radiant Energy, continued
  • Example The sun is 5800ºK on its surface, so
  • F ?T4 (5.67?10-8)?(5800)4 6.4?107 W/m2
  • Summing over entire surface area of sun gives
  • 3.9?1026 W
  • Compare to total capacity of energy production on
    earth 3.3?1012 W
  • Single power plant typically 0.51.0 GW (109 W)
  • In earthly situations, radiated power out
    partially balanced by radiated power in from
    other sources
  • Not 523 W/m2 in 70ºF room, more like 100 W/m2
  • goes like ?Th4 ?Tc4

23
The Energy of Heat
  • 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 its kinetic energy, its 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 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 1800s. 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. . .
24
Energy of Heat, continued
  • Food Calories are with the big C 1 Cal 1
    kilocalorie (kcal)
  • 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
  • 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. . .

25
Heat Capacity
  • Different materials have different capacities for
    heat
  • Add the same energy to different materials, and
    youll 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)
  • 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)

26
And those are the major players
  • Weve now seen all the major energy players well
    be discussing in this class
  • work as force times distance
  • kinetic energy (wind, ocean currents)
  • heat energy (power plants, space heating, OTEC,
    really random KE)
  • electromagnetic energy (generators, transformers,
    etc.)
  • radiant energy (solar energy, really the same
    things as EM)
  • chemical energy (fossil fuels, batteries, food,
    biomass, also EM)
  • gravitational potential energy (hydroelectric,
    tidal)
  • mass-energy (nuclear sources, suns energy)

27
The Physics 161 Formula List
  • Lots of forms of energy coming fast and furious,
    but to put it in perspective, heres a list of
    formulas that youll need to use
Write a Comment
User Comments (0)
About PowerShow.com