General Physics ph 213 Midterm Exam II (Ch 24 27) November 14, False, they don t have to be flat but they must be perpendicular to E-field.

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1 General Phsics ph 13 Miterm am II Ch 7 November 1, 005 Name: Tpe am is close boo an close notes. Use onl our note car. Write all wor an answers in the papers provie. Show all our wor an eplain our reasoning No creit will be given for an answer that oes not inclue the necessar solution or eplanation, ecept for truefalse or multiple choice questions Partial creit ma be aware for a correct metho of solution, even if the answer is wrong. Part I True or False.5 points each: For questions 1 11, state whether each statement is true or false. 1. The change in electric potential energ, Ub Ua, is the wor one on a charge b the electric force as it moves from point a to point b. False, W a b = Ua Ub, = Ub Ua. The potential ue to a sphericall smmetric charge istribution is the same as that of a point charge. False onl outsie of the sphere 3. quipotential surfaces are alwas flat an perpenicular to the electric fiel. False, the on t have to be flat but the must be perpenicular to -fiel.. The units of, the permittivit of free space, ma be epresse as Fm. True 5. The fatter a given length of wire is the higher its resistance will be. False 6. In an alternating current, the charge carriers switch between electrons an protons in a repeate fashion. False W toaster raws more current than a 1000-W toaster. False, it epens on operating voltage Part II Multiple choice questions: For questions 8 16 select the onl one choice that best answers or completes each statement. 8. circular ring of uniforml istribute charge of total charge an raius a penetrates a spherical gaussian surface of raius b at points so that onl part of the ring is enclose insie the surface of the sphere. The net flu leaving the gaussian surface is 3. What is the angle mae b the lines from the center of the ring to the two points where the ring penetrates the spherical surface? 30 egrees C 10 egrees B 90 egrees D not enough information given 9. The units of electric potential are J B Jg C JC D Cm 10. The potential of a uniforml charge sphere is lowest at the center of the sphere B the surface of the sphere C a istance from the sphere equal to its raius D infinit Ph 13 Miterm am II, f05 1

2 11. The potential halfwa between two equal charges separate b a istance is 0 C B 8 D 1. If an electric fiel of magnitue 5 Vm maes an angle of 30 o with a path of length 10 m, then the integral of over this path has a value 50 V B -50 V C 15 V D 17 V 13. circular is of raius a has its central tion of raius b remove forming an annulus. charge is uniforml istribute over the surface of the annulus. The potential at the center of the annulus is [ a + b] C a + b 1 B a - b D a b 1 1. The minimum capacitance that can be mae using five 10 nf capacitors is 50 nf B 10 nf C 0. nf D nf mf, a mf, an a 3 mf capacitor are connecte in parallel, the combination being connecte across a 9 volt batter. The capacitor with the greatest charge on it is the 1 mf C the 3 mf B the mf D neither since the all have the same charge 16. If the voltage on a capacitor is ouble, then the energ store b the capacitor stas the same C quaruples B oubles D halves 17. parallel plate capacitor has a capacitance of 10 mf an is charge with a 0 V power suppl. The power suppl is then remove an a ielectric of ielectric constant is use to fill the space between the plates. The voltage now across the capacitor is Zero C 0 V B 5 V V = V 0 since stas the same D 80 V 18. If a 10 current flows for two minutes, how man electrons have passe an point in the circuit? C B D electrons on't move in a circuit 19. resistor is attache to a voltage Vo. s the current flows through the resistor the resistor heats up, oubling it resistance. B what factor oes the power issipate b the resistor change as it heats up? 0.5 C 1 no change B D 0. metal bar of imensions b b 10 units is to be use as a resistor. To get the smallest resistance from this bar, ou shoul attach leas to opposite sies that have the imensions b units. D ll connections will give the same resistance. B b 10 units. None of the above is correct. C b 10 units. Ph 13 Miterm am II, f05

Capacitor C 1 an C are in series an combination is in parallel with C 3. 1 1 1 C 1 = 6 mf C1 C1 C n C 13 = C 1 + C 3 C 13 = 18 mf Finall: C 13 is in series with C an C 5 : 1 1 1 1 C eq =.

3 Part III uestions an Problems Please eplain or show our wor clearl an completel for each of the following questions or problem. 1. Five 1 mf capacitors are connecte across a 1 V batter as inicate in the figure below. What is the capacitance of the combination? Capacitor C 1 an C are in series an combination is in parallel with C C 1 = 6 mf C1 C1 C n C 13 = C 1 + C 3 C 13 = 18 mf Finall: C 13 is in series with C an C 5 : C eq =.5 mf C eq C13 C C5 B What is the charge on the bottom capacitor in the iagram? The capacitors can be partiall combine to form the circuit shown at right. Now potential across each branch is 1V. Since capacitors C an C 5 are in series therefore the charge on each is the same an it is the same as the combination of the two C 5. 5 = = 5 = C 5 V 5 = V 5 = 0.07 C. The resistivit of the filament of a light bulb tungsten material increases approimatel linearl from 56 n m at 93 K an 1.1.m at 3500 K. ssuming the filament temperature is about 500 K, an that 100-V DC suppl is use to power a 0-W light bulb, an 0.5-cm length of the filament remains constant, estimate the resistance of the tungsten. P = V R 0 R 0 = 100V 0W R 0 = 50 at 93K room temperature R = R T R = K K 93K R = 730 B the iameter of the tungsten filament. temperature coefficient of tungsten is K -1. Since the resistivit increases linearl, we can calculate the resistivit at 500K: 1100 n 56 n 500 K T T 3500 K 500 K n m R = 1.3 m R m 0.005m 9 10 n Ph 13 Miterm am II, f K R 75.7 n 730

4 Ph 13 Miterm am II, f05 3. thin ro of length is centere on the ais as shown. The ro carries a uniforml istribute charge. Determine the potential V for points along the ais. et be the uniform linear charge ensit. = q r q V Taing V to be zero at infinit, the total potential ue to the entire ro is V ln To evaluate the integral use trig substitution: et = tan = recall 1 + tan tan ln tan where tan = an = 1 + V ln ln V B Determine the electric fiel ue to the charge ro along the -ais. lectric fiel is the graient of the potential. V V ln 1 fter simplifing the epression ou will have:

5 . Paper has ielectric constant of K = 3.7 an a ielectric strength of Vm. Suppose that a tpical sheet of paper has a thicness of mm. You mae a capacitor b placing a sheet of inch paper between two aluminum foil sheets. The thicness of the aluminum foil is 0.00 mm. What is the capacitance C 0 of our evice? ssume the entire paper will be between aluminum foil. 0 C in 11in0.05 cm in 0.05 cm in C m C 0 = F = 66 nf B bout how much charge coul ou store on our capacitor before it breas own? Use the ielectric strength of paper to calculate the maimum voltage can be applie to the capacitor before its breaown: V ma = Vm m. V ma = 50 V = CV ma = F 50 ma = C = 9.6 C C B overlaing sheets of paper an aluminum ou can mae a parallel combination. If ou mae 100 such capacitors, an connect the ens of the sheets in parallel, so that ou have a single large capacitor of capacitance 100C 0, how thic our new large capacitor be? I ran out of time, hope ou can o this part ourself D What is the maimum voltage ou can appl to this 100C 0 capacitor without breaown? I ran out of time, hope ou can o this part ourself Note: Your test on ugust 15, 011 will have questionsproblems from magnetism topics as well. Ph 13 Miterm am II, f05 5

1. The electron volt is a measure of (A) charge (B) energy (C) impulse (D) momentum (E) velocity

AP Physics Multiple Choice Practice Electrostatics 1. The electron volt is a measure of (A) charge (B) energy (C) impulse (D) momentum (E) velocity. A soli conucting sphere is given a positive charge Q.