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Physical Science 1 Mid-Term Exam Name: _______________________________________ MULTIPLE CHOICE. Circle the one alternative that best completes the statement or answers the question. Each question is worth five points. 1) The fact that desert sand is very hot in the day and very cold at night is evidence that sand has a A) low specific heat capacity. B) high specific heat capacity. When a material changes temperature very easily, it doesn’t take much energy to change the temperature. That means it has low heat capacity. The answer is A. 2) If you run a refrigerator in a closed room with the refrigerator door open, the room temperature will A) increase. B) decrease. C) remain unchanged. Recall that if you want to move energy from an object at low temperature to another object at high temperature, you have to do work! So, a refrigerator moves heat from its cold interior to an outside hot exterior. The work that has to be done to do this process also heats the outside. Opening the door, means the =refrigerator keeps running and puts out more energy into the room! The answer is A. 3) It is commonly thought that a can of beverage will cool faster in the coldest part of a refrigerator. Knowledge of Newton's law of cooling A) supports this common knowledge. B) shows this common knowledge is false. Common thinking is correct. So, Newton’s law says the bigger the temperature difference the more energy gets transferred. So we want to cool something rapidly, put it in the coldest place you can find so that the temperature difference is greatest. The answer is A. 4) Hydrogen and oxygen molecules in a sample gas have the same temperature. This means the hydrogen molecules, on average, have the same A) kinetic energy, but more speed. B) speed and the same kinetic energy. C) kinetic energy, but less speed. D) speed, but less kinetic energy. E) speed, but more kinetic energy. \ Same temperature means same average kinetic energy. Remember KE = 1/2 mv2. Since hydrogen molecules have less mass than oxygen, if they have the same energy they must move faster. The answer is A. March 10, 2009 Page 1 Physical Science 1 Mid-Term Exam Name: _______________________________________ 5) When heat is added to boiling water, the water temperature A) increases. B) decreases. C) stays the same. As long as there is water to boil, the temperature of the boiling water will remain at 100oC. The answer is C. 6) When the distance between two charges is halved, the electrical force between the charges A) doubles. B) halves. C) is reduced by one-quarter. D) quadruples. E) none of these Coulomb’s Law says the force varies inversely with the square of the distance. That is the force law has distance squared in the denominator. If that distance is halved the denominator is made smaller by a factor of 4, so the force goes up by a factor of 4. The answer is D. 7) The current through a 10 ohm resistor connected to a 120-V power supply is A) 1 A. B) 12 A. C) 10 A. D) 120 A. E) none of these Use Ohm’s Law, I = V/R. So I = 120/10 = 12 ams. The answer is B. 8) What is the power rating of a light bulb when 0.8 A flow through it when connected to a 120-V outlet? A) 60 W B) 96 W C) 12 W D) 120 W E) 15 W Power is Volts x Amps, P=VI. So, P = 120 x 0.8 = 96 Watts. The answer is B. March 10, 2009 Page 2 Physical Science 1 Mid-Term Exam Name: _______________________________________ 9) Metal detectors, like the ones used at airports, are activated by A) magnetic fields. B) electromagnetic induction. C) alternating current. D) electric fields. A metal detector uses a changing magnetic field (that of the earth) when metal objects are near a loop of wire. A voltage is induced in the loop due to the changing magnetic field. This is electromagnetic induction. The answer is B. 10) A device that transforms electrical energy to mechanical energy is a A) transformer. B) motor. C) generator. D) magnet. E) none of these Motors are device that change electrical energy into mechanical energy. The answer is B. Problems. Show all work to receive as much partial credit as possible. 11. In the ice ages, cave men and women didn’t have pots and pans they could set on the fire to heat water for tea. Instead, they had tightly woven waterproof bags and dropped hot rocks from the fire into the water-filled bag to heat the water. If the specific heat of water is 1 cal/gm-deg and the specific heat of rock is 0.1 cal/gm-deg, how many grams of rocks at 200oC must be added to 100 grams of water to raise the water temperature from 20oC to 80oC? (Hint, from the first Law of Thermodynamics, the heat energy gained by the water is equal to the heat energy lost by the rocks and both water and rocks have the same final temperature.) (15 points) energy _ lost _ by _ rocks energy _ gained _ by _ water mrock crock Trock mwatercwaterTwater mrock 0.1 (200 80) 100 1 (80 20) 12mrock 6000 mrock 500 grams March 10, 2009 Page 3 Physical Science 1 Mid-Term Exam 12. Name: _______________________________________ A 100-gram ice cube at 0oC is dropped into 100 grams of water at 20oC. When the ice melts, what will be temperature of the water? (Hint: As in the last problem, conserve energy. The heat of fusion of water is 80 cal/gram.) (10 points) If you melt all the ice the energy needed is Q heat _ of _ fusion mice Q 80 100 8000calories Water temp will go from 20 to zero Q mcT Q 100 1 (20 0) 2000 Not enough heat in water to melt all the ice So, final situation is water and some ice at 0 C 13. Three light bulbs are connected in series to a 6-volt battery. The resistances of the light bulbs are 10, 20 and 30 ohms. Calculate the current that flows in the circuit. What is the power dissipated in each light bulb? (15 points) Resistors in series add, so effective resistance is 10+20+30=60 ohms. Current is V/R=6/60=0.1A. Power in a bulb is I2R. P10=0.01(10)=0.1 Watt. P20=0.2. P30=0.3 14. A neon-sign operates at 12,000 volts. A transformer is used to convert 120 volts coming from the wall plug to the required high voltage. If the neon light has a current of 0.01 amperes flowing through it, what is the current coming from the wall plug? (10 points) VinIin = VoutIout 120Iin = 12000(0.01)=120. So, Iin=1, March 10, 2009 Page 4 Physical Science 1 Mid-Term Exam Name: _______________________________________ MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question. 1) A 2) A 3) A 4) A 5) C 6) D 7) B 8) B 9) B 10) B March 10, 2009 Page 5