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HW #4 due 0900 Monday March 3

HW #4 due 0900 Monday March 3. 1. (5 points) A step-up transformer with 60 Hz AC power has an input voltage of 1000 V at a current of 2 Amperes. The output voltage is 5000 Volts. What is the current flowing out of the transformer?

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HW #4 due 0900 Monday March 3

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  1. HW #4 due 0900 Monday March 3 1. (5 points) A step-up transformer with 60 Hz AC power has an input voltage of 1000 V at a current of 2 Amperes. The output voltage is 5000 Volts. What is the current flowing out of the transformer? 2. (5 points) What is the power into that transformer? The power out? (See the posted ‘not in the text’)

  2. 3. (10 Points) Table 8.4 of your text lists the ‘energy density’ for several ways an automobile may carry the energy needed for travel. Explain to a smart, but ill-educated, national policy maker what she should do about the future of US automobile traffic, from these numbers. 4. (10 points) Plug and chug: What are the rolling and the aerodynamic drag forces on my Subaru Legacy, with a weight of 3423 pounds, a frontal area of 25 square feet, the CD of a Ford Escort, a Cr of 0.01, at 60 mph? 5. (10 points) How much total work is done, in foot-pounds and in kWh, to drive twenty miles at that speed? Don’t do it, but make a mental outline of how you would compute the fuel consumed from these numbers.

  3. 6. (10 points) Western coal holds 0.12 ppm (parts per million by weight) of mercury. From your CAPA answer to #4 from the Feb. 24 assignment, how many pounds of mercury would the Valmont plant emit each day if nothing were done to clean it up? Since your CAPA problem differs from all others, it is absolutely necessary to show the grader your work in a very clear way!

  4. Today The science of traffic safety(R and K 8.4) The costs of transportation (R and K 8.5) Ethanol for cars(R and K 8.3.5) All power point images are only for the exclusive use of Phys3070/Envs3070 Spring term 2014

  5. Minireview of automotive friction • Rolling friction force, needed to be countered by force from the engine to maintain a speed v (in mph) for a car of mass (pounds/32) = Cr *(weight/32) *v in pounds • Air friction force, again needed to be countered by engine force, for a car of shape Cad (Table 8.3) and frontal area Af (in square feet) = Cad *Af* v2 / 370 in pounds. Like a recipe with teaspoons, cups, and sticks of butter.

  6. Friday analysis • Mileage(mpg) = distance / energy in fuel • Friction work(energy) = friction force*distance • Mileage ~ 1 / friction forces • Mileage (mpg) should DEcrease as v for rolling, as v2 for aerodynamic drag • True?

  7. mpg v

  8. A collision Kinetic energy changes into work energy KE = ½ m v2 (chapter 1) Work = force * distance, With the distance being the amount of car that becomes shorter—a crumple zone. The force is that needed to change the state of motion of the car and all it contains, a deceleration = m a = m Dv/Dt (page 247). To a stop, Dv= car speed. (climbers use a stretchy rope—longer distance to stop—to spread a smaller force over a longer distance)

  9. Damage = distance • If the force on the car is determined by the strength of the construction, the damage ( a distance) is proportional to the kinetic energy, or to the square of the speed.

  10. But-- • The greater the crumple distance, the less the force on the car and all within. • Choice– a collision can do little damage to a strong car, but the larger forces also act on all those inside.

  11. Acceleration of people By experiment ! People can stand accelerations no more than 100 m/sec2. Heads, probably less (concussions). So decelerate people over the greatest distance, not running into hard things. Seat belts, let the car crumple and stay with it. Air bags—sorta soft.

  12. Determining speed from skid marks? The force of friction of the tires is nearly a constant fraction of the weight of the car. That force acts over the length of the skid marks to remove the kinetic energy. Work done to stop = (Force = m Mg) times( skid length L) Kinetic energy = ½ M v2 So mMg L= ½ Mv2, v =square root of (2mgL).

  13. Example m = 0.8 (depends on road condition, tires) g = 9.8 m/sec2 L=20 m v = square root (mgL) = square root (0.8 * 9.8 m/sec2*20m) = square root (156.8 m2/sec2) = 12.5 m/sec. = (12.5 m/sec * 3600 sec/hour) / (1000 m/ 1 km) = 45 km/hour = about 28 mph

  14. Costs of transportation • Table 8.2, ‘passenger-mile/106 Btu’ and ‘ton-miles/106 Btu’. • AND you know (from the text) the Btu/bbl of fuel. • And, you can find the cost of a gallon of fuel • Thus—dollars per passenger-mile. • Mass transportation—more passenger miles per vehicle. • Car pool lanes— ”

  15. car vs bus • How much of the weight of a car is passengers? Car= 3600 pounds, one passenger=180 pounds. 180 pounds/3600 pounds = 5 % ! • A bus weighs 25,000 to 40,000 pounds, and has seats for 80 people. 80 * 180 pounds / 40,000 pounds = 36% !

  16. Relative bus/auto friction forces —rolling ~ m Bus weighs 10 times the car, rolling friction force, ten times the car rolling friction force --aerodynamic ~ Cd * Af 4 times the frontal area, twice the Cd (Table 8.3) = eight times the car drag force. Near 40 mph these are equal, average nine times the friction force and friction work for a bus compared to an auto.

  17. analysis Car with one passenger, 5% of the weight Full bus 36% of the weight A ratio in favor of the bus by 36/4 = 9 But the bus has 9 times the friction force, 9 times the work done against friction, nine times the fuel cost. A draw! (see the analysis in R and K, with more features)

  18. Ethanol fuel (text uses methanol) • Ethanol = C2H6O • Complete combustion C2H6O + 3 O2 2 CO2 + 3 H2O (2*12+6*1+16) + 3(2*16) 2(12+2*16) + 3(2*1+16) 46 + 96  88 + 54 • So 46 tons of ethanol burns to make 88 tons of CO2 One ton of ethanol burns to make 88/46 tons of CO2 , etc.

  19. mpg? Heat content (page 269) = 84,600 Btu/gallon for ethanol vs 125,000 Btu/gallon for gasoline. 30 mpg for gasoline30 (mi/gal. of gas)*84,600 (Btu/gal. of eth.)/125,000 (Btu/gal. of gas) =20.3 mi per gal. of ethanol

  20. Questions to decide on an ethanol policy? • Can ethanol be produced at 2/3 the price per gallon of gasoline? • Per gallon, which fuel produces more CO2? • Per Btu, which fuel produces more CO2? • Is Thot better or worse for one fuel? Carnot efficiency? • Are there more subtle differences? (yes—the extra atom of oxygen in ethanol makes combustion more complete, less CO. A 10% mixture)

  21. Is ethanol ‘green’? The CO2 combustion product is metabolized by plants to make sugars, etc, which can be turned into ethanol, for no net CO2 production. A sustainable fuel. Is this really true? Wait for Chapter 5.

  22. Science 324, p. 587 1 May 2009

  23. Wednesday Cars without gasoline—hydrogen, electric, hybrid, flywheel. R and K 8.3

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