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
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) = ConstantTotal energy is “conserved”, but changes in form
Kinetic-Potential Energy Exchange DeviceHypersonic XLC at Paramount's Kings Dominion
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 Water 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 Generators in Hoover Dam
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
1824The 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.
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.
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.
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
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
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) • 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 !
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
Food sample is burned Heat flows into water Water temperature rises Calorie = 1oC/ml water “Heat Content” calculated How to measure heat?we will use a soda-can calorimeter
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
“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.
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)
March of the Penguins2005 National Geographic MoviePenguins 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 17,562-19,19019,000 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.
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 Morgan Spurlock’s film“Supersize Me”
Fast Food Thermodynamics from Chem-1A early AM class discussionexcel sheet has more details
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 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. 35
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 Food sample is burned Heat flows into water Water temperature rises Calorie = 1oC/ml water “Heat Content” calculated 38
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 ExperimentCalories 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
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