Physics 1070 First Hourly Exam October 13, 1999



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This is a closed book exam. Be careful not to share your answers with anyone else, since the hallmark of science is honesty and integrity. Write all your answers carefully, completely, consciously, and legibly in the spaces provided on these exam sheets. Anything that can't be easily read and understood by the grader will be marked wrong. The 100 points for this exam will be worth 15% of your final grade.

1. True or False: Mark the following statement either true (T) or false (F) on the basis of the reading and discussions of this class. (30 points, 3 for each correct item).

F a) The formation of polar "ozone holes" is the result of satellites punching holes through the atmosphere on their way into orbit.

T b) When a 10N weight falls through a distance of 10m, near the earth's surface, its kinetic energy is increased by 100J in the absence of air resistance.

T c) An ideal transformer does not change the power in an electric circuit.

T d) A 240W light bulb will carry a current of 2 amps when plugged into a 120V outlet.

F e) The amount of time it requires to double a population depends upon how many people there were originally as well as on the rate of population growth.

T f) Co-generation of electrical energy was common at the beginning of the electric distribution system a century ago.

F g) The inner electrons are the most important in determining how atoms react chemically.

F h) All atoms in a particular gas are moving at the same speed, one that is characterized by the absolute temperature of the gas.

T i) Field construction of energy-generating facilities would be less of a problem if there were a common utility industry-wide design standard for generating units.

T j) The entropy of one kg of steam is greater than that of one kg of ice.

2. Multiple choice: Pick the numbered item which best completes the statement (20 points, 5 for each correct item).

__a) An object weighing 500N is pulled up a 20m long ramp that rises from ground level to a height of 3.0m. By how much does the gravitational potential energy change in the process?

1. 25J             4. 1500J

2. 167J             5. 10,000J

3. 600J             6. Answers not determined by information given.

 _b) A device draws 10.0A from 120V circuit. How much energy does the device use in 12 seconds?

1. 10.0J         4. 1440J

2. 12.0J         5. 14,400J

3. 1200J         6. None of these is correct.

_ c) Water hyacinths can double their number in one week. If a lake is about a quarter covered by water hyacinths, how long will it be before the entire lake chokes on water hyacinths (barring outside intervention)?

1. 1 week         4. 4 weeks

2. 2 weeks         5. 5 weeks

3. 3 weeks         6. 6 or more weeks

    - d) Suppose an electric company's boilers run at a temperature of 600ºC. The facility uses river water to cool its spent steam. Assume that the mean river temperature is 20º. Suppose that an identical twin boiler is air-cooled on Baffin Island where the mean air temperature is -20ºC. The power station on Baffin Island in northern Canada is theoretically about

1. 100% more efficient         4. 5% less efficient

2. 100% less efficient             5. equally efficient

3. 5% more efficient

than the electric company's.

3. Matching: choose the item in the column on the right that, on the basis of course readings and discussions most closely associates with the description on the left. (30 points; 3 for each correct matching) [Bonus: extra 5 points; Define, in one or two sentences, any one of the items listed in the right hand column.]
a) the average of the mean kinetic energies of the random part of molecular motion 1) potential energy

2) kinetic energy

b) increases as the string in an archer's bow is drawn back 3) friction

4) photosynthesis

c) equal amounts of lead and aluminum, subject to the input of equal amounts of heat energy, undergo different temperature changes  5) mechanical advantage

6) semilogarithmetic graph paper

d) the ability to do work in virtue of motion 7) entropy
e) in reality, mechanical energy is never completely conserved 8) temperature

9) heat capacity

10  f) energy never spontaneously flows from a cool body to a body at higher temperature 10) Second Law

11) First Law

g) the conversion of carbon dioxide and water into sugar and oxygen 12) Coulomb's Law

13) efficiency

h) making exponential growth appear as a linear variation 14) chemical energy

15) electrical energy

12  i) the force between charged bodies varies inversely as the square of the distance between them 16) electric fields

17) magnetic fields

17  j) only created by moving electronic charges 18) transformer

19) screening curve

20) Pauli exclusion principle

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4. Short essays: Answer any two of the following questions (20 points, 10 for each correct essay).

a) Explain why coal burning produces more carbon dioxide than the burning of natural gas (methane). Coal is all carbon, which leads to carbon dioxide and carbon momoxide upon combustion, Methane contains both carbon and hydrogen so that it produces water as well as carbon dioxide and carbon momoxide when burnt.

b) Discuss (defend, or criticize) the assertion that the statement "entropy tends to a maximum" must be wrong because there are many occurrences here on Earth that lead to increases of order (petroleum formation, life, and so forth). The Second Law says that enropy always increases in a closed system (=isolated sytem or the complete universe). Entropy can certainly decrease in some part of a larger system. Thus life on earth means that entropy of earth is decreasing even though entropy of entire solar system, containing earth, is increasing.

c) Explain why air cooling is more complicated than water cooling for transporting waste heat away from a generating facility. Water has greater heat capacity than air; hence given amount of water can carry away much more waste heat than that amount of air. Thus to obtain the same amount of cooling, you must move much greater quantities of air through the cooling system than water. For a given amount of required cooling and cooling substance -air or water, the temperature of the air will increase much more than that of the water meaning that the environmental effects on the neighborhood are likely to be less with water than with air cooling. To eliminate such bad effects will require a more complicated system.

d) How can the chemical properties of two atoms be very similar if the nuclei of the atoms differ substantially in their makeup? The chemical properties of an atom only depends upon the configuration of its outer electrons. This in turn only depends upon the number of protons in the nuclei of the atoms, i.e., the positive chage on the nucleus must exactly balance the negative charge on the electrons, meaning that number of protons equals number of electrons. Two nuclei can have the same number of protons but different numbers of neutrons. Hence their atoms will have the same chemical properties even though their nuclear properties are quite different (isotopes).

e) Contrast and compare "soft energy" and "hard energy" paths. The "hard energy " path calls for increasing centraliztion of the electric power generation system: fewer and bigger centrally located generating plants, owned by monopoly utilities, distributing their generated electric power over very large distances. The "soft path" implies many small plants, cogenerating heat and process steam, competing with each other to distribute their power locally.

f) Discuss the advantages and disadvantages of superconductivity (resistanceless current transmission) for the large-scale transfer of electric power. If the electric power distribution lines were superconducting, there would be now transmission losses due to resistance in the wires: you would receive all of the power sent out by the generating plant; you would get what you pay for with n o waste to the environment. But, superconductivity requires very low temperatures, near absolute zero. We would have to refrigerate the transmission lines which would entail enormous expense and also produce adverse environmental effects.

g) A friend of yours devises a clever scheme to get an unending source of energy. She will carry a ball up a hill on a gently winding path, then roll the ball down hill along a steeply descending path. The crux of her argument is that this extra energy comes because it takes less energy to get the ball up the hill than down the hill. Explain whether your friend is right or wrong and why? The friend is confusing "power" and energy.. As long as the height gained equals the height lost, the energy change on the way up is exactly the same , in magnitude, as that on the way down. Going up, or down, faster implies requiring or releasing more power, not more energy.