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Announcements

Announcements. HW#2 is due next Friday, 10/20. The first ‘further activity’ is due next Monday, 10/16 Read chapter 4 for Wednesday, 10/11

jenette-miller
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Announcements

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  1. Announcements • HW#2 is due next Friday, 10/20. • The first ‘further activity’ is due next Monday, 10/16 • Read chapter 4 for Wednesday, 10/11 • 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. • The Physics Department help room has been set up. The schedule can be found at http://hendrix2.uoregon.edu/~dlivelyb/TA_assign/index.html

  2. Lecture 5 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 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. . .

  3. 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. . .

  4. Heat Capacity • 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) • 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)

  5. And those are the major players… • We’ve now seen all the major energy players we’ll 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, sun’s energy)

  6. The Physics 161 Formula List • 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:

  7. Power, Energy Exchange and Conservation of Energy

  8. Power: Rate of Energy Flow • 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. . .

  9. Power Examples • 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.)

  10. Electrical Power • If energy = Volts x charge, then what does volts x current correspond to? • Recall current = charge/second, so Volts x current = Volts x charge/second = energy/second • 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).

  11. Electrical Power, continued • 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) • 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

  12. Series and Parallel Circuits R=2W R=2W R=2W R=2W I = ? R=2W R=2W V = 6 V V = 6 V Series circuit Parallel circuit

  13. 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?

  14. Energy Exchange and Conservation of Energy • When we lift a rock, we do work against the force of gravity. This work is stored as potential energy (PE). • If we drop the rock, then gravity does work on the rock, and the PE is converted into kinetic energy (KE). • 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 = D(PE+KE) Demos: roller coaster, bowling ball pendulum

  15. Energy Exchange and Energy Conservation, cont. • 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 = D(TE+PE+KE) • 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.

  16. Energy Exchange and Energy Conservation, cont. • 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 = D(TE+PE+KE) • 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).

  17. Conservation of Energy in a Hydroelectric Dam • Water stores PE; we want to convert this to do useful work W • Water with little KE falls to a turbine, reducing its PE (DPE), increasing its KE, and probably slightly increasing its TE due to turbulence • 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 (DKE) • The turbine drives a generator which produces electric power that can be transported to do useful work W. • The slight increase in the water’s thermal energy (DTE) is expelled to the environment as heat Q. • Overall, these must balance: Q+W = D(TE+PE+KE) • 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.

  18. Conservation of Energy in a Fossil Fuel Power Plant Some steps are the same, except • PE is stored in fuel, not mgh • Fuel is burned to produce heat which boils water • Steam drives a turbine • KE in and out are small • . . . and we need to expel some heat at ambient T, leading to lower overall efficiency. Why is that? Q+W = D(TE+PE+KE): Q is larger than for hydroelectric (why?) so W must be smaller and the efficiency must be lower

  19. Efficiency • Generally, we define efficiency in terms of the benefit derived by a particular process divided by effort expended. • 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. • These definitions will change in other circumstances, e.g, for a refrigerator. • In all these cases, the really final end result is to convert a usable energy source to heat – in your toaster, for example.

  20. 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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