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