1 of 7 Exam II review RII 1 How many ohms in a coulomb A one gazillion B one coullion C 17 D None of these Answer None of these Ohm is unit of resistance coulomb is unit of charge RII 2 Consider a positive point charge Q just outside an imaginary sphere as shown How does the magnitude of the electric flux thru the left half of the sphere K K E da compare to Left the magnitude of the electric flux thru the right half K K E da Right A Flux Left Flux Right B Flux Left Flux Right C Flux Left Flux Right Q Answer This is a tough one Flux Left Flux Right Some of the flux lines that enter the right half also exit the right half so those lines do not contribute to the flux through the right half All of the lines which exit the left half also enter the right half and those are the only lines that contribute to the flux on the right half does not contribute to flux thru right half Q 10 12 2009 Phys1120 M Dubson University of Colorado at Boulder 2 of 7 RII 3 Consider a point charge Q off center within a spherical metal shell Does Gauss s Law allow you to compute the total charge on the inside surface of the shell A Yes B No Q Answer Yes you can compute the total charge on the inside G G E surface Consider the flux v da over a spherical surface S within the metal Since the E field must be zero everywhere within the conductor then G G E v da Qenc o 0 Since Qenc 0 there must be a charge Q spread over the inside surface S of the conductor Since the point charge Q is off center the surface Q will not be spread uniformly over the inside surface RII 4 A point charge q is brought from infinity to a point b near 3 other charges Q Q and 2Q The charge q is brought along 3 different paths in turn path 1 path 2 and path 3 as shown Along which path is the most work done by the external agent carrying the charge q A 1 B 2 C 3 D Same work on all three paths 2Q Q path 1 b path 2 Q path 3 Answer Same work on all paths W PE q V q Vb V qVb RII 5 TRUE A or FALSE B If the E field throughout a region of space is zero the voltage throughout that region must be zero 10 12 2009 Phys1120 M Dubson University of Colorado at Boulder 3 of 7 Answer False If E 0 that means that the voltage is not changing with position The voltage can have any non zero constant value RII 6 TRUE A or FALSE B If the voltage throughout a region of space is zero the E field throughout that region must be zero Answer True E field is rate of change of voltage with position E V x If V constant zero or any constant then E V x 0 RII 7 Does the capacitance of a capacitor depend on the charge Q on the capacitor A Yes B No C Depends on the type of capacitor Answer No The capacitance of a capacitor is fixed by its size and shape C Q V is a constant ratio for a given capacitor if Q increases V increases so that the ration Q V remains constant RII 8 A capacitor is attached to a battery which maintains a constant voltage V across the capacitor plates While the battery is attached the plates are moved further apart The energy stored in the capacitor A increased B decreased V C remained constant Hint U 1 2 QV 1 2 Q2 C 1 2 CV2 Answer V is fixed by the battery Q is not fixed since charge can go on or off the plates through the attached wires Use U 1 2 CV2 C decreases V constant so U decreases Both the battery and the external agent do work so work arguments are not easy to apply in this case 10 12 2009 Phys1120 M Dubson University of Colorado at Boulder 4 of 7 RII 9 A charged capacitor is isolated so no charge can get on or off The plates of the capacitor are slowly pulled apart After the plates are pulled apart a bit the electric field between the plates A increased B decreased C remained constant The voltage difference between the plates A increased B decreased C remained constant The capacitance A increased B decreased C remained constant Answers The E field stays constant because E due to a large plane of charge is constant independent of distance from plane The voltage difference increases since V E d E is constant d increases The capacitance decreased We can see this either from C Q V Q stayed constant V increases or from C A d d increased RII 10 A copper wire and an aluminum wire both have the same dimensions same length same cross sectional area Which one statement is true A Both wires have the same resistance R and the same resistivity B The wires have different R and different C The wires have the same R but different D The wires have different but the same R Answer The wires have different resistances and different resistivities R L A Different materials have different resistivity Same L same A different s so different R s RII 11 Which has higher resistance A 100 W bulb Hint P IV I2R V2 R Answer 60 W bulb has higher R 120 VAC So use P V2 R 10 12 2009 Phys1120 M Dubson B 60 W bulb C Same R in both All light bulbs are meant to be used at constant voltage V University of Colorado at Boulder 5 of 7 RII 12 The voltage between points a and b in the circuit shown is measured with an ideal voltmeter What does the voltmeter read Hint Recall that an ideal voltmeter has infinite internal resistance A V 2 C V 4 B V 3 D zero R R V a R b E V Answer A tough one The answer is V 2 The current thru the voltmeter is nearly zero since it has nearly infinite resistance So no current flows through the R in the upper right K s voltage law says the voltage V is split evenly between the two equal resistances R RII 13 Consider the circuit below What is the total resistance which the battery sees I 1V R 1 2 2 A increases 1 R2 2 B decreases A 1 B 2 C 0 5 D 0 25 In the circuit above the resistor R2 is increased to 5 What happens to the current through from the battery C remains constant Answers The total resistance is 1 ohm When any of the R s is increased the battery current decreases since the total resistance seen by the battery increases 10 12 …
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