Transcript 12. Energy-Thermo_09apr13a

Energy
• Multiple forms of Energy
– Mechanical Energy
• Kinetic Energy, E=1/2 mv2 , velocity dependence
• Potential Energy, ability to do work, stored energy
– Radiation Energy
• Electromagnetic Energy, E=h*frequency
• Nuclear Energy, conversion of mass to energy
– Chemical Energy (also food energy)
• Energy released when bonds break
• Largely based on photosynthesis of light by plants
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Conservation Rules
• Mass of Products = Mass of Ingredients
• Cannot create or destroy, just change
– Additional rules in Physics
• Conservation of Mass
– Chemical reactions, no free or lost atoms
• Conservation of Energy
– Light into heat, potential to kinetic, etc.
• Conservation of Momentum
– Recoil of a shotgun, action and reaction
• Conservation of Angular Momentum
– Ice skater’s spin (pull in arms to increase rotation)
Conservation of Mass
• Chemical change cannot create or destroy mass
• Reactions cause change in form but not total amount
• Total mass the same before and after reaction
• Mass is invariant to chemical reactions
• Same number + kind of elements before & after
• Reaction simply rearranges the relationships
• Conversion may happen between solid, liquids, gases
– Quantity of element atoms will be the same
Conservation of other things
• Conservation of energy easy to visualize
– Potential and Kinetic energy are inter-convertible
• Consider a boulder on a mountain top, rolling down
• Maximum Potential Energy at height of mountain
• Maximum speed (Kinetic Energy) at the bottom
– Roller Coaster Example
• Kinetic and Potential Energy change back and forth
Potential + Kinetic (+ Heat) = Constant
Total energy is “conserved”, but changes in form
Kinetic-Potential Energy Exchange Device
Hypersonic XLC at Paramount's Kings Dominion
Water in Hoover Dam
• High Potential Energy
– Water at top of dam
• High Kinetic Energy
– Water exiting dam
• PE + KE = constant
– Energy is “conserved”
– Energy changes form,
but not in total amount
Generators in Hoover Dam
• Water Kinetic Energy
– Spins rotor
– Loses some K.E.
• Electrical Energy
– Turning rotor for power
– Water loses energy
– Electrical energy sent
to users
• KE + EE =constant
– Only form is different
Conservation of Energy
• Sum of energy involved is a Constant
– “First law of Thermodynamics”
– Variation on earlier themes
• matter (or energy) can neither be created nor destroyed
– Can convert energy from one form to another
• Burning gasoline turns chemical into kinetic + heat
• Climbing stairs turns kinetic into potential
– Falling down the stairs turns potential into kinetic
• Convert food (chemical) to body heat & motion (kinetic)
– Heat is a measure of Kinetic Energy
• Temperature is a direct consequence of molecules in motion
• Heat transfer is a movement of energy between hot and cold
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1824
The modern-day definition of work, i.e. "weight lifted through a height",
was originally defined in 1824 by thermodynamicist Sadi Carnot in his
famous paper Reflections on the Motive Power of Fire.
Specifically,according to Carnot:
“We use here motive power (work) to express the useful
effect that a motor (fire) is capable of producing. This effect
can always be likened to the elevation of a weight to a
certain height. It has, as we know, as a measure, the
product of the weight multiplied by the height to which it is
raised.”
This paper became the inspiration for James Joule’s famous
experiment validating Carnot’s hypothesis.
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In 1845 the English physicist James Joule read his paper “On the
mechanical equivalent of heat” to the British Association meeting in
Cambridge. In this work, he reported his best-known experiment, that in
which the work released through the action of a "weight falling through a
height" was used to turn a paddle-wheel in an insulated barrel of water.
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Joule’s Experiment
In his experiment, friction and agitation of the
paddle-wheel on the body of water caused heat to
be generated increasing temperature of the water.
Both the temperature change ∆T of the water and
the the falling height ∆h of the weight were
recorded. Using these values, Joule determined the
mechanical equivalent of heat as 819 ft•lbf/Btu.
In today’s terms this is equivalent to 4.41 J/cal,
while the modern value is 4.184 J/cal, a nice result
considering the instrumentation used.
The modern day definitions of heat, work,
temperature, and energy all have connection to this
famous experiment.
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Energy Dimensions
• Original definition is “calorie” (small c)
– Energy to raise temp.1 gram (1 ml) water by 1.0oC
– Turned out to be inconveniently small
• Usual quotation in kcal = “Calorie” (big C)
– Energy to raise temp 1.00 liter water by 1.0oC
– Calories are NOT in S.I. (MKS, ISO) dimensions
– Commonly used for food products
• Big Mac has 540kcal
• SI or ISO metric system unit of energy is “Joule”
– 1 watt for one second = 1 Joule
– Conversion is 4.184 Joule/calorie
– Same thing is 4.184 kJ/kcal = 4.184 kJ/Calorie
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ISO Energy Definition
• Units of Energy, definition of Joule
– Conforms to ISO system
– Equivalent to watt-seconds
– Derivation-Definition from basic dimensions
• Bottom line is 1 Joule = 1 Watt-Second
– A 100 watt light running 1 minute = 6kJ
• 60 sec/min * 1 min * 100 watts = 6000 W-sec = 6000 J
– Watt-seconds becoming a commonplace U/M
• Direct links between electricity & chemistry U/M
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Energy Unit Conversions
– ISO Definition: 1 Joule ≡ 1 Watt-Second
– Units conversion yields 4.184 Joule/calorie
– 100 watt device running 1 hour = 36,000 J = 360 kJ
• 100 watts*1 hour*3600 sec/hour = 3.6*10^5 W-s (or Joules)
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360 kJ / 4.18 kJ/kCal = 86 kcal = 86 Cal
One 12 oz can (355ml) Coke Classic = 146 kcal = 146 Cal
1.7 hr 100W light bulb use ~ energy 1 can “Coke Classic”
2.3 hr for 75W laptop with “Coke Classic” energy amount
– Watt-seconds becoming a commonplace U/M
• Direct links between electricity & chemistry U/M
• Usual specification units for camera flash
– 50 w-s flash lasts 1/1000 sec, intensity = 50,000 watts !
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Why is this important?
• We now have quantitative relationships
between heat and other forms of energy.
• Using ISO units ties it all together
– Equivalence between all forms of energy
• Electrical, Joule = watt-second
• Heat, calorie = 1.184 Joule
– Calories still in wide use due to simplicity, historical value
• Kinetic, Potential, etc are ALL related
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How to measure heat?
we will use a soda-can calorimeter
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Food sample is burned
Heat flows into water
Water temperature rises
Calorie = 1oC/ml water
“Heat Content” calculated
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Calories in the food
• Calories delivered into water, Q = m*c*∆T
• Q = heat in calories
• M = actual mass of water heated ( ≈ 100gram)
• C = specific heat of water = 1 cal/(gm-∆T)
– a “fudge factor” to make units come out right
• Q = 100gm*1cal/(gm*∆T)*∆T = calories
• Calories into water came from food
– Calories transferred / mass of food = cal/gram
• If 0.5 gram food (preburn - postburn) yields 2 kcal
• 2 kcal / 0.5 gram = 4 kcal/gram for the food
• 1.0 pound (454 gm) of this food yields ≈ 1800 kcal
Food energy differs from Burning
• Calorimeter = complete combustion
– All material consumed by fire
– Complete extraction of available heat
• Animals = partial utilization
– Animals do not digest cellulose (wood fiber)
• Termites an exception, a bio-fuel source?
– “Buffalo Chips” used by pioneers in campfires
• Remaining “fuel” energy available for burning
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“Buffalo Chips” (Meadow Muffins) are large pieces of dung left on the
prairie by Bison. They were collected and burned by Plains Indians,
settlers, and pioneers as a source of cooking heat and warmth.
There was plenty of energy left after digestion … so food calories
measured by burning are not always equivalent to nutritional calories.
I would gain no weight eating any amount sawdust … I cannot digest it.
Calories for Women
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Calories for Men
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Body Energy
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Human Energy
• At 2000 kcal / day = the USDA benchmark value
– 2.00E6 cal/day * 4.184 j/cal = 8.369E6 J/day
• same as 8.369E6 watt-seconds/day
• 60sec/min*60min/hr*24hr/day=8.64E4 sec/day
• (8.369E6 w-s/day) /(8.64E4 sec/day) = 96.8 watts
– Human energy output ≈ 100 watt light bulb!
• 20 watts to keep brain going
• 80 watts to keep warm, locomotion, organ function
• Issues for A/C and critical environments
• Classroom of 50 people generates 5,000 Watts of heat!
• Clean rooms adjust A/C to match number of people
• Sleeping together keeps us warm (Penguin movie)
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March of the Penguins
2005 National Geographic Movie
Penguins keep warm in sub-zero climate by huddling
together, rotating positions from inside to outside the flock.
Net effect is to distribute and share their body heat
HP Pavilion seats 17,562-19,190
19,000 people*100watts=1,900,000 watts of heat
(same energy 1900 @ 1000 watt space heaters)
which heat must be removed by air conditioning …
especially for an ice hockey event.
Brain Energy
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540 Calories = 27% daily amount
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One burger = 71% of daily 2000 kcal
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Calories for a week?
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Order a diet coke with that !
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Somewhat lighter fare …Karl’s Jr. latest offering
Morgan Spurlock’s film
“Supersize Me”
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1 month only fast food
100% at McDonald’s
Ate around the USA
Used “Super Size” option
Tried everything on menu
5000 kcal per day
– 21 megajoules equivalent
– Equivalent to 9 BigMac
• He gained 25 pounds
– Took 14 months to lose it
• Academy award nominee
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Fast Food Thermodynamics from Chem-1A
early AM class discussion excel sheet has more details
Breakfast
"Lite" Cream Cheese
Bagel
Coffee
"Lite" Cream Cheese, 2 oz = 60 gram
large = 123 gram
Caffe Latte, whole milk 356ml
kcalories
120
363
200
683
% of 2kcal
34%
Lunch
Sandwich
French Fries
Regular Soft Drink
Carl's Junior
McDonalds
Coca-Cola Classic
Dbl West Bacon CheesB
Large
12 fluid oz. = 355ml
970
540
146
1656
83%
460
210
710
1380
69%
3719
186%
Dinner
Side Order
Sandwich
Milk Shake
Daily Total
Carl's Junior
Subway
Carl's Junior
Onion Rings
Roast Beef
Chocolate Shake
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New diet plan
• Heat of fusion for ice = 80 calories/gram
– Same as 80 kcal per kilogram
• Can we offset food calories by eating ice?
– Calorie content of BigMac = 540 kcal
• Exothermic, “burning” food yields heat
– Melting Ice absorbs energy, endothermic
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Equal exothermic Big Mac with endothermic ice
540/80 = 6.75 kg = 14.9 pounds of ice
Eating 15 lb of ice along with BigMac = 0 calories!
We’ll start franchising tomorrow.
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Today’s Experiment
• We will use a “soda can” calorimeter
– Inexpensive, we’re careful with taxpayer money.
– CRV value about $.05, insulation ≈ $0.10
• Calibrate the calorimeter
– Simple but not a high efficiency tool
– We’ll calibrate with a well defined source of heat
• You will measure food items by burning
– Your choice of two food items for the report
• Calibration factor used to update food data
– e.g. 100kcal observed / 70% = 143 kcal corrected
– We assume that calorimeter efficiency is constant
Calibrating the Calorimeter
• Calibrating a “soda can” calorimeter
– weigh can, then add ≈ 100-150 grams water
– Put thermometer in can, record initial temperature
• Weigh candle before lighting
– Light and place burning votive candle under can
– Burn for about 5 minutes or a 15oC temp. rise
• Re-measure temperature & re-weigh candle
– Water temperature will be higher, candle mass less
• Calculate the calories
– Mass of water * temp. rise = calories
– Compare to literature value for candle = 42kJ/gram
How to measure heat?
we will use a home-made calorimeter
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Food sample is burned
Heat flows into water
Water temperature rises
Calorie = 1oC/ml water
“Heat Content” calculated
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Calculations
• Calories = grams water * c * temp. rise
– Energy input is measured by temp. rise
• ‘c’ is a factor so other dimensions cancel
– c = cal/(gram * oC) = 1.00 for water
• calories = gm * c [cal/gm-deg] * deg
• We have calories into water, grams of food
– Need kcal/gram for literature comparison
• Example:
– 300 cal / 0.1 gram * (1/1000) = 3.0 kcal/gram
– A fairly typical result for several food items
Calculations for Candle
• Calories = water mass * temp. rise
– Example: ΔT=14oC*100gm=1400cal= 1.4kcal
• Mass of wax consumed by burning
– Example:12.2-12.0= 0.2 gm wax consumed
• Energy per gram can be calculated
– Example: 1.4kcal / 0.2 gram= 7.0 kcal/gram
• Compare to published value = 42kJ/gram
– Our data 7.0kcal/gm*4.182kJ/kcal = 29.3kJ/g
– We got 29.3/42 = 70% of theoretical
– We’ll use this “efficiency factor” when
calculating values for food burning
Today’s Experiment
Calories in Food
• We use the same “soda can” calorimeter
– REPLACE the WATER, always start with cool water
– Put thermometer in can, record temperature
• Weigh food item and water before burning
– Use stick & pin or wire to hold food items
• Ignite food with Bunsen Burner, place under can
– Do this quickly to minimize heat loss
– Burn until food is consumed or fire goes out
• Re-measure temperature & residual food mass
– Water temperature will be higher, food mass less
• Repeat process for second food item
Calculations for Food
• Calories = water mass * temp. rise
– Example: ΔT=8.4oC*100gm=840cal= 0.84kcal
• Mass of food consumed by burning
– Example:12.2-12.0= 0.2 gm consumed
• Energy per gram
– Example: 0.84kcal / 0.2 gram= 4.2 kcal/gram
– We had 70% efficiency, 4.2 / 70% = 6.0 kcal/gram
• Pine nut published value = 6.7kcal/gram
– Our result was not far off
• 6.0/6.7 = data at 90% of literature value
• (6.7-6.0)/6.7= 10% error
– Sources of error?
• Heat loss (flame missed the can)
• Measurement errors, perhaps temperature
Lots of food choices
• Nuts
– pine nut, walnut, peanut (oil + dry roast)
– Cashew, pecan, almond, pine nuts
• Chips & crackers
– Potato chip, corn chip, cereals
– Cheez-its, triscuit, wheat thins
– Pretzels, bread sticks
• Cereals (new this semester)
– Cheerios (oats), Shredded wheat, Corn Chex
• Burn your lunch?
Typo Corrections
• Page 4
– Omit 2nd line, there to show dimensions of c
• Page 5, line 10a
– Calculation should be 9a/7c = kcal/food gram
• Page 5, line 10d
– Calculation should be 9b/7c = kcal/food gram
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Lets go for it
• Where to burn
– Bench top (may smell a bit)
– under hood is optional, not as handy
• How to burn
– Candle is easy, light it away from calorimeter
– Food is ignited remotely with Bunsen burner
• Some loss of heat getting it started & moving it
• Check your data & calcs before leaving
– Post data on white board, compare results
– Easy to redo a bad result when you’re in lab