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# GENERAL PHYSICS 2 PHYS 2020

GPA 3.96

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This 40 page Class Notes was uploaded by Mrs. Peter Toy on Friday October 30, 2015. The Class Notes belongs to PHYS 2020 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 13 views. For similar materials see /class/232111/phys-2020-university-of-colorado-at-boulder in Physics 2 at University of Colorado at Boulder.

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Date Created: 10/30/15

Exam I Review RIl How many uV microvolts in one kV kilovolt A 1000 B 105 C 106 D 109 E None ofthese Answer 109 RI2 How many ohms in a coulomb A one gazillion B one coullion C 17 D None of these Answer None of these The question makes no sense It s like asking quotHow many pounds in February RI3 A calculator question 105 A 1E5 B lOES C something else 1E5 means 1x105 RI4 A box of mass m and net charge Q is on a frictionless plane inclined at angle 9 What is the magnitude of the electric force F QE necessary to keep the box from sliding down the incline A Not enough info given to answer the question B mg C mg cos9 D mg sine E mg sine Answer mg sine RIS Does the magnitude of the normal force N depend on the magnitude of the Efield A Yes B No Answer No the normal force is N mg cosG RI6 Two pith balls each of mass m are suspended by light strings from the same pivot point The two balls are each positively charged so they repel The left ball has a larger charge of 3Q while the right ball has a smaller charge of Q Which gure most accurately shows the equilibrium positions of the charged pith balls uncharged A B C Q 3Q 390 Q 3Q Answer A By Newton39s 3rd Law each ball feels the same magnitude force Since each has the same mass the angle will be the same for both RI7 Three equal mass charges are released from rest at the positions shown on the X aXis Which mass has the largest initial acceleration Q Q Q A A B B C C D 2 of the masses have the same acceleration E Some other answer Answer B feels the largest size force so it has the largest acceleration RI8 Four charges are alranged as shown all a distance r from the origin What is the direction of the E eld AT THE ORIGIN y Q 139 gt A m V V X Q 4Q A C 6 Q D B A Up B down C upper right D lower left E None of these Answer None of these The E eld at the origin points right 9 What is the magnitude of the E eld at the origin in units if kQr2 A 2 B 3 C 4 D None ofthese Answer None of these the magnitude of the E eld is SkQrz RII lO A point charge q is brought from in nity 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 200 0 Q Path 1 Path 2 Path 3 O Q A l B 2 C 3 D Same work on all three paths Answer Same work on all paths W APE q AVqVb 7 V00 qu 11 Only the three charges Q Q and 2Q are present q is absent and the point b is midway between Q and Q The voltage at point b is A positive B negative C zero Answer Positive RI12 TRUE A or FALSE B If the voltage throughout a region of space is constant the E eld throughout that region must be zero Answer True lAVl EH d If AV0 between any two points throughout a volume of space the E must be zero RIl3 TRUE A or FALSE B If the E eld in a region of space is constant the voltage throughout that region must be zero Answer False RIIl4 Two equal and opposite charges q and q are a distance r apart or a distance 2 r apart Which con guration r or 2r is the higher energy con guration A separation r is higher energy I H B separation 2r is higher energy q q C the energy is independent of separationdon t O 0 know 2r q q Answer separation 2r is higher more positive energy For separation r the PE iqur for separation 2r PE kq22r Although separation r has the higher magnitude PE it is more negative 15A 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 C remained constant Hint U 12QV 12QzC 12 cv2 Answer U decreases V is fixed by the battery C decreases So U 12 CV2 decreases This is a different from the case where Q is fixed 16 A parallelplate capacitor has square plates of edge length L separated by a distance d A second capacitor is made with L doubled and d decreased by a factor of 2 iL39 By what factor is the capacitance of the new capacitor increased A l B 2 C 4 D 16 E None of thesedon39t know Answer C sNd std IfL is doubled and d is halved then C increases by 8 l7Two capacitors labeled 1 and 2 are identical is size and shape except capacitor 2 has a dielectric between the plates while 1 has air The same charge Q is placed on both capacitors Which has the higher voltage V A the aircore capacitor B the dielectric capacitor CQV The one with the higher V will have the smaller C since both have same Q The air core capacitor has the smaller C and the higher V Chapter 19 Concept Tests CTl 9 l Vote TRUED if both statements below are always true Otherwise vote FALSEE TRUE D or FALSEE o For resistors in series the current through each resistor is the same 0 For resistors in parallel the voltage across each resistor is the same NOTE Vote quotDquot or quotEquot NOT A or B Ch1 92 A 1Q resistor is placed in parallel with a 10000 Q resistor as shown 1Q 10000 Q The total equivalent resistance of these two resistor in parallel is closest to A a little less than 1Q B a little more than 1Q C 5000 Q D a little less than 10000Q E a little more than 10000Q Answer a little less than lQ You could use the formula Rtot RT 172 R 09990Q Or just think IQ is a very low resistance compared to 10000 i lOOOOQ so almost all the current will ow through the IQ resistor the circuit will behave almost as if the lOOOOQ resistor is not present and the equivalent resistance is close to lQ The question is is the equivalent resistance a little less or a little greater than lQ Adding the lOOOOQ resistor in parallel provides another current path of the ow of charge More ow means lower resistance Q1 93 The circuit below consists of a battery attached to two resistors in series Resistor R1 is variable When R1 is decreased the voltage V2 across R2 A increases B decreases C stays the same fill a a R2 12 V2 FJ l lt Answer A increases This is easiest to see ifyou let R1 go all the way to zero Kirchhoff39s 2nd Law says that the battery voltage VV1V2 The voltage drop is split between resistors l and 2 So V2 is smaller than the battery voltage V But if R10 then the full battery voltage V is across R2 V2 increased as R1 goes down Q1 94 Two resistors R1 and R2 are hooked to a battery in parallel R1 is twice as large as R2 How does the current IE from the battery compare to the current 11 though R1 Hint IE 1112 1 V R1 11 R2 112 l 2 R2 A 113 11 1132211 1132311 D IB411 E None of these Answer C The voltage across R1 is the same as the voltage across R2 but R1 is twice as large as R2 so 11 is halfthe size ofIz since IVR same V R twice as big means I halfas large So ifll 1A 12 would be 2A and 110113 11 12 lA 2A 3A which is three times as large as 13 13311 Q195 The four light bulbs shown are identical Which circuit puts out more light Hint more power more light AAA Vquot m J U VVV l I I I I IV 12V V 12quot A B C They both put out the same amount of light Answer The total equivalent resistance which the battery in the A circuit sees is W2 two resistors each of resistance R in parallel The total equivalent resistance which the battery in the B circuit sees is 2R two resistors in series The total power coming from the battery is V2 P tot more light Smaller Run with xed V results in a larger power P So A puts out Q196 In the circuit below what happens to the brightness of bulb 1 when bulb 2 burns out When a bulb burns out its resistance becomes infinite A Bulb 1 gets brighter 2 m w B Bulb 1 gets dnnmer C Its brightness remains the same AAA m V39 I V I 1 3 Hint What happens to the current I I from the battery when bulb 2 burns I I out V 12V Answer When bulb 2 burns outs the lament inside breaks and R2 becomes in nitely large The total equivalent resistance which the battery sees increases since bulb 2 is gone there are fewer paths for the current ow so less ow more total resistance Since the battery sees a larger Run the current from the battery 11m VRm is reduced Less current from the battery means less current through bulb 1 less light Bulb 1 gets dimmer thus B Q197 The three light bulbs A B and C are identical How does the brightness of bulbs B and C together compare with the brightness of bulb A A All A Total power in BC power V in A m A B Total power in BC gt power quotI w 111 A B C C Total power in BC lt power in A Answer If each light bulb has the same resistance R the series resistance ofB and C is 2R Power PV2R Larger total resistance for the B C pair means less power Total power in BC lt power in A therefore the correct answer is C CTl98 If you wanted to measure the m current through the battery where viiz I R in the circuit would you place an A k 1 B ammeter I I K VA39A39A 11 l 1 A B C D or R1 D R3 C E None of these will work I 1quot L i IV If you wanted to measure the current through resistor R2 where would you place and ammeter A B C D or E None of these will work Answer Use location A since all of the current that is going through the battery is also running through this point You wouldn t want to use D because it is the set up for measuring the voltage across the battery B and C have only part of the total current running through them Answer For the current running through R2 use location B Only the current going through R2 runs though this point A is the current through R1 and is the total current C is the current through R3 only Q1 99 A circuit with two batteries is shown below The directions of the currents have been chosen guessed as shown Which is the correct current equation for this circuit A12211I3 B11IQI3 CI321112 E None of these 11 quotI V R1 V1 R2 gllz V2 Loop 1 R3 AAA 13 Answer C 13 11 12 Consider the upper junction marked with a dot The current into the junction is 13 current arrow pointing towards the junction means current into the junction The current out of the junction is 11Iz Kirchhoff39s 2nd law says total current in equals total current out You get exactly the same answer if you consider the lower junction CTl 910 Which equation below is the correct equation for Loop 1 A V211R1 12R20 BV211R1 12R20 C V2 11R112R20 V211R1 12R220 E None of these 11 39I39A39A R1 V1 R2ltllz V2 Loop 1 R3 Ill 13 Answer As we move around loop 1 we go through the battery from the to the side a voltage drop so the voltage change is V2 Then we go through R1 from the low V side to the high V side since we are moving against the current ow 11 a voltage rise so the change is 11R1 Finally we go through R2 in the same direction as the current I 2 so we have a voltage drop and a change of Isz The total change as we moved around the loop must be zero since we nished at the point where we started V2 11R1 12R 0 Therefore the answer is C Q1 91 1 A capacitor in an RC circuit is initially charged up to a voltage of 10V and is then discharged through an RlOQ resistor as shown The switch is closed at time t0 001 F 109 What is the current through the resistor immediately after the switch is closed at time t 00 s A 1A B 05A C le A 037A D None ofthese What is the time constant for this circuit A 001 s B 01 s C l s D 10 s D None ofthese What is the current through the resistor at time t 02 s A 1A B 05A C le A 037A D None ofthese Answer The time constant for this circuit is RCIOQ 0010F 010 sec So at time t02 sec two time constants have passed After one time constant the voltage charge and current have all decreased by a factor of e After two time constants everything has fallen by e2 The initial current is lA So after two time constants the current is le A 0135A None ofthese Therefore A B D Q1912 An RC circuit is shown below Initially the switch is open and the capacitor has no charge At time t0 the switch is closed What is the voltage across the capacitor immediately after the switch is closed time 0 R 109 V10V C00010F 39 R 109 A Zero B 10 V C 5V D None of these What is the voltage across the resistor on the far right in parallel with the capacitor at time 0 A Zero B 10 V C 5V D None of these What is the initial current quotthroughquot the capacitor immediately after the switch is closed A 1A B zero C 05A D None of these A long time after the switch has been closed what is the voltage across the capacitor A 5V B 10 V C zero D None of these Answer A zero Before the switch is closed the charge Q on the capacitor is zero and the voltage across the capacitor V Q C 0 Right after the switch is closed the charge has not had time to build up on the capacitor and the charge and voltage are still zero Answer A zero The capacitor and the resistor on the right are in parallel and therefore have the same voltage drop across them Since the capacitor is acting like a wire there is no voltage drop across it which is the same for the resistor on the right Answer A 1A Initially when the capacitor has zero charge it behaves like a short circuit zero resistance because it is easy to put charge on an uncharged capacitor The circuit is then effectively V10V IVR10101A The capacitor acting like a zero resistance wire which all the current ows through initially The other resistor is not involved in the initial current ow Answer A 5V After a long time the capacitor becomes fully charged and current stops owing through it When this happens it behaves like an infinite resistor and the circuit is effectively V10V C00010 F IV2R102005A As far as current ow is concerned the capacitor is gone The voltage across the capacitor is the same as the voltage on the right since they are in parallel The voltage across each resistor is 5V Same R s same I in each so the voltage across each must be the same and they must add up to 10V Exam 111 review RIIl A copper wire and an aluminum wire both have the same dimensions same length same crosssectional area Which one statement is true B Both wires have the same resistance and the same resistivity C The wires have different resistances and different resistivities D The wires have the same resistance but different resistivities E The wires have different resistances but the same resistivity Answer C RII Z Which has higher resistance B 100 w bulb C 60 w bulb Hint P IV 12R VZR Answer the 60 W bulb has higher resistance Bulbs are labeled assuming that the voltage will be 120 VAC RII 3 Consider the following circuit containing three ordinary household light bulbs a 100W light bulb a 60W bulb and a 40W bulb i i i Which one statement is true B The 100W bulb will be brightest C The 60W bulb will be brightest D The 40W bulb will be brightest E All the light bulbs glow with the same brightness Answer the 40W bulb is brightest Since it has the largest R In comparing the power or brightness of two light bulbs you must first be clear whether the current or the voltage is the same for the two It makes all the difference If the two bulbs are in series then the current is the same and according to P I2 R the larger resistance bulb has more power and is brighter But if the bulbs are in parallel the voltage is the same and then according to P V2 R the larger resistance bulb has less power and is dimmer RII 4 A 600W hairdryer is attached to 120VAC circuit What is the peak current through the hairdryer to within 5 B 10A C 7A D 5A HINT P IV 12R VZR Answer 7A Use PIV works OK with rms values and Pavg ImsPV 600 120 5A Ipeak 0 Irms 7A RII S 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 in nite internal resistance V B V2 C V3 D V 4 E zero Answer A tough one The answer is V2 The current thru the voltmeter is nearly zero since it has nearly in nite resistance So no current ows through the R in the upper right RII 6 Consider the circuit below What is the total resistance which the battery sees B 1 Q C 2 Q 19 D 05 Q E 025 Q 2 29 In the circuit above the resistor R2 is increased to SQ What happens to the current through from the battery B increases C decreases D remains constant Answers The total resistance is 1 ohm The battery current decreases since the total resistance seen by the battery increases RII 7 The capacitor C is charged up to a voltage V The switch is closed at time t0 connecting the capacitor to two identical resistors as shown Immediately after the switch is closed how does the current through the second resistor on far right compare with the current through the capacitor B IRIC CIRl2Ic DIR21c E IR 0 Immediately after the switch is closed how does the voltage across the second resistor on far right compare with the voltage across the capacitor BVRVC CVRl2Vc DVR2Vc E VR 0 don t know Answers IR IC The circuit elements are in series VR 12 Vc By Kirchhoff39s voltage law we have VC VR1 VRz 2VR RII S In the circuit below which resistors must have the same current owing through them Do not assume that the resistors are identical B R2 R5 only C R3R4 only A D R2 R5 and R3R4 only E RlRz R5 only Which resistors have the same voltage drop B R2 R5 only C R3R4 only D R2 R5 and R3R4 only E RlRz R5 only Answers Only R2 and R5 are guaranteed to have the same current since they are in series Only R3 and R4 are guaranteed to have the same voltage RII 9 A charged particle is orbiting in a uniform B field in the sense shown Is the particle positive or negative 69 Ban B positive C negat1ve D impossible to telldon39t know Two particles with the same charge but different masses are moving in circular orbits in a magnetic field They have the same speed Which one will have the larger radius orbit B Neither the orbits have the same size C The larger mass particle will have a larger orbit D The smaller mass particle will have a larger orbit Answers The direction of acceleration and net force must be toward the center of the circle For a negative particle the force on particle is toward the center of the circle The larger mass particle will have a larger orbit You can see this from a physical argument or from the formula for the radius of an orbit R mVqB The physical argument is this both particles feel the same sideways size force F qVB The more massive particle has more inertia and is therefore harder to turn so it turns through a larger orbit RII 10 A coil of wire carrying current I can rotate freely about an aXis in a magnetic eld If released from rest in the position shown which way does it rotate B right side will move out of page C left side will move out of page D loop will not rotate at all Answer Right side will move out of page Use righthandrule with Fwire I L B RII ll Two bar magnets are brought near each other as shown The magnets B attract C repel D exert no net force on each other m m Answer Repel Antiparallel currents repel Think about the current directions on the sides of the magnets RII 12 A coil of wire attached to a resistor is in an oscillating magnetic eld as shown The B eld vs time is shown At what point in time is the current through the resistor zero 4 B osc ilating g B C p D E E e 0 0 d E time Answers B or E At those points the rate of change of B is zero So the rate of change of ux is zero so no emf by Faraday s law RII 13 A circuit with a battery and a variable resistor is near a loop of wire as shown When the resistance R is decreased the induced current in the loop is B cw I R C ccw D ZERO Answer CW When R decreases the current I VR in the circuit increases causing an increasing B eld out of the page at the position of the loop A CW current in the loop will ght the increase in B RII 14 A uniform solid sphere of copper rotates about a stationary aXis in a uniform magnetic eld B Are there eddy currents B Yes there are eddy currents B C No no eddy currents ANSWER NO EDDY CURRENTS The ux through E loop rotating with the sphere does not change Therefore no emf no eddy currents Concept Tests Final Review CTFl A square with edge length L is made larger and larger mm a length of perimeter A th th t s e square grows e ra 10 area B increases C decreases D remains constant Answer The ratio is 4LL2 4L The ratio decreases CTF2 A box with mass M and charge Q is pushed along a rough surface at constant velocity by a uniform horizontal electric eld of magnitude E The coefficient of kinetic friction between the box and the oor is u What is the net force on the box BQE CuMg Dzero EQEMgFNuMg FN is the magnitude of the normal force on the box M Q E v constant Answer Acceleration is zero so the net force must be zero All the forces cancel CTF3 Points A and B are distances r and 3r respectively from a point charge q What is the voltage difference between points A and B r 2r 0 q A B E a kq B 3r C 3r D 9 r E None ofthese Answer Voltage due to a point charge is kqr The voltage difference between points A and B is kqr kq3r 2kq 3r CTF4 Consider a point in empty space near several charges which might be positive negative or both Consider the following statements I The Efield can be zero while the voltage is nonzero II The voltage can be zero while the Efield is nonzero Which of these statements can be true B both can be true C neither can be true D only I can be true E only 11 can by true Answer Both can be true CTFS A capacitor has a voltage V across its plates An electron initially at rest is released from a point very close to the negative plate of a capacitor and it accelerates toward the positive plate The electron has charge e and mass In There is no gravity in this problem and you may assume that the motion of the electron is M relativistic What is the nal kinetic energy of the electron just before it collides with the positive plate B m e V C 2eV D 12mv2 E eV Answer lAKEl lAPEl lqAVl eV CTF6 A resistor is plugged into a 120VAC wall socket The graph below is either voltage V across current I in or power P dissipated in the resistor vs time time What could the graph be B V only C I only D V or I only E V I or P Answer The graph could be V or I only Power is always positive CTF7 Consider the circuit shown with the switch initially open When the switch is closed the current 11 through resistor R1 B increases C decreases Ibat D stays the same Answer stays the same 11 VR1 regardless of whether or not the switch is closed What does change is the battery voltage CTF8 When the switch is closed the current from the battery Ibm B increases C decreases D stays the same Answer increases With the switch closed the battery must supply current to R as well as to R1 CTF9 When a resistor is plugged into a standard AC outlet the fuse blows and all the lights go out If we want to repeat this dangerous experiment and not have the fuse blow we need a resistor that is B larger C smaller D Same R but larger power rating Answer larger At constant voltage here V 120 VAC the power is P VZR Larger resistance draws less power CTF 10 A charge q is released from rest at point in empty space were there may be E and or B fields There are no forces on the charge except for the forces due to the E and or B fields no gravity etc The charge is observed for a short while and is seen to move along a curved path Consider the following possibilities I There is only an Efield present and no B field II There is only a B field present and no Efield 111 There is both an Efield and a Bfield present Which possibilities could account for the observed motion B all three C I and 111 only D II and III E 111 only Answer I and 111 only A curved Efield and no B field could produce the motion shown CTFl 1 At time to a particle with charge q has instantaneous velocity V0 as shown and is moving in a uniform magnetic field B B True or C False V 3 y B As time increases the Xcomponent of the velocity vx remains constant q X Answer True The force exerted by the magnetic field is into the page righthand rule This force has no Xcomponent Since there is no Xcomponent of the force the X component of the acceleration must be zero and the Xcomponent of the velocity must be zero CTF12 A horizontal loop of wire is in a vertical magnetic eld The magnetic eld varies smoothly from pointing upward to pointing downward as shown The B eld gradually gets smaller goes to zero then grows in the other direction gt gt gt WWQW At the moment when the magnetic eld in the loop is zero is there an induced current in the loop B Yes C No Answer Yes Even though the B eld is instantaneously zero it is still in the process of changing from up to down So the timerate of change of B is nonzero and by Faraday s Law there must be an emf and an induced current CTFl3 What is the direction of the induced current B Clockwise as seen from above C Counterclockwise Answer Counterclockwise Lenz s Law CTFl4 An radio tower is emitting a singlewavelength monochromatic EM wave in all 39 39 39 39 39 39 measures the electric eld amplitude E at 100 K r J meters from the tower and again at 400m from the tower What is the ratio ElooE400 Hint Intensity is proportional to the square of the amplitude 1 oc E2 ltltgtgtgtEmm s O B 2 C 4 D 16 E 162 256 Answer 4 The intensity is 16 times greater at 100 m than at 400 m but the Efield is only 4 time greater CTF15 Is the image formed by the lens in your eye real or virtual B real C virtual CTFl6 A double slit interference pattern is seen on a screen The path difference pd for a given point on the screen is the quantity distance to slit 1 distance to slit 2 What is the path difference at point A slit2 I d i gt slitl U fringes screen B 5x C 3x D 2x E None of these Answer None of these The first min occurs when the path difference is M 2 2 CTFl7 The rest mass energy of an electron is Brest 2 1 1 100 2 051 1M6V An electron is accelerated from rest across a voltage V At what voltage V is the final kinetic energy of the electron equal to its rest mass energy B V0511 MV C 1022 MV D 1533 MV E None ofthese Answer V0511 MV

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