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by: Mrs. Peter Toy


Mrs. Peter Toy

GPA 3.96


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This 88 page Study Guide was uploaded by Mrs. Peter Toy on Friday October 30, 2015. The Study Guide belongs to PHYS 2020 at University of Colorado at Boulder taught by Staff in Fall. Since its upload, it has received 23 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 111 review RIll 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 RlI2Does 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 ofa capacitor is xed 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 Rll3 A capacitor is attached to a batter which maintains a constant voltage V across the capacitor plates While the batter is attached the plates are moved further apart The energy stored in the capacitor A increased B decreased C remained constant Hint U 12QV 12Q2C 12 CV2 Answer V is xed by the battery Q is not xed since charge can go on or offthe plates through the attached wires Use U 12 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 Rll4 A charged capacitor is isolated so no charge can get on or off The plates of the capacitor are slowly pulled apart I I 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 Efield stays constant because E due to large plane of charge is constant independent of distance from plane The voltage difference increases since AV 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 so A d d increased RllS 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 By what factor is the capacitance of the new capacitor increased A l B 2 C 4 D 16 E None of these Answer 2 C so A d A increased by factor of 4 so ratio A d increased by factor of 2 Rll6A copper wire and an aluminum wire both have the same dimensions same length same crosssectional area Which one statement is true A Both wires have the same resistance R and the same resistivity p B The wires have different resistances and different resistivities C The wires have the same resistance but different resistivities D The wires have different resistances but the same resistivity Answer The wires have different resistances and different resistivities R p L A Different materials have different resistivity p Same L same A different p s so different R s RIl7 Which has higher resistance A 100 w bulb B 60 w bulb Hint P 1v 12R VZR Answer 60 W bulb has higher R All light bulbs are meant to be used at constant voltage V 120 VAC So use P v2 R Rll8 Consider the following circuit containing three ordinary household light bulbs a 100W light bulb a 60W bulb and a 40W bulb wwm Which one statement is true A The 100W bulb will be brightest B The 60W bulb will be brightest C The 40W bulb will be brightest D 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 higher resistance bulb has more power and is brighter But if the bulbs are in parallel then the voltage is the same and then according to P V2 R the larger resistance bulb has less power and is dimmer RII9 A 600W hairdryer is attached to 120VAC circuit What is the peak current through the hairdryer to within 5 A 10A B 7A C 5A HINT P 1v 12R VZR Answer 7A Use PIV works OK with rms values and Pavg ImsPV 600 120 5A Ipeak 2 21ms 7A RII10The 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 V2 B V3 C V4 D zero E V Answer Atough 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 K39s voltage law says the voltage V is split evenly between the two equal resistances R RIIl 1 Consider the circuit below What is the total resistance which the battery sees A 1 Q I IQ B 2 Q 1 R1 5E C 05 Q I 53 2 2 D 025 Q In the circuit above the resistor R2 is increased to SQ What happens to the current through from the battery A increases B decreases C remains constant Answers The total resistance is 1 ohm When any of the Rs is increased the battery current decreases since the total resistance seen by the battery increases RIl12 In the circuit below which resistors must have the same current owing through them Do not assume that the resistors are identical R2 R2 R5 only i 20 A B R3R4 only C R2 R5 and R3R4 only D R1 R2 R5 only Which resistors have the same voltage drop A R2 R5 only B R3R4 only C R2 R5 and R3R4 only D R1R2 R5 only Answers Only R2 and R5 areguaranteed to have the same current Only R3 and R4 are guaranteed to have the same voltage RIll3 A charged particle is orbiting in a uniform Bfield in the sense shown Is the particle positively or negatively charged Bin A positive B negative C impossible to telldon39t know Answer 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 RIll4 A coil of wire carrying current I can rotate freely about an aXis in a magnetic field If released from rest in the position shown which way does it rotate A right side will move out of page 39 B B left side will move out of page C loop will not rotate at all I I aX IS Answer Right side will move out of page Use right hand rule Force on left side of loop is into page force on right side ofloop is out of page POTENTIAL EXAM QUESTIONS 1 Which of the following statements isare true about the following circuits Assume all batteries and all bulbs are identical B c m f39 A O x e X 9 X I I I a Bulb A does not turn on because the voltage difference across it is zero b Bulb B does not turn on because the voltage difference across it is zero c Bulb C does not turn on because the current flowing through it is zero Answers a is true b is true c is true 2 What is the correct order for the total power dissipated in the following circuits from least to greatest Assume a bulbs and all batteries are identical lgnore any internal resistance of the batteries a AltBCltDltE b DltCltBEltA c DltBltEltAltC d ABltDltCltE e BltAltCDltE Answer Assume that all bulbs have same resistance R and all batteries have same voltage V The total resistances are A 3R B R since 2 of the R are shorted by the Wire C R D R 3 E R The total voltages are A V B V C V since batteries in parallel D V E 2V since batteries in series The total power is Pm VLoLZ Rm A PVZ3R B VZR C VZR D VZR3 E 2VZR SOA ltBCltD ltE Concept Tests Final Review CTFl A square with edge length L is made larger and larger mm a length of perimeter As the square grows the ratio area A increases B decreases C remains constant CTFZ VX d X d Which of the following is a solution for X AX1 d BXd21 Cx12dd2 DX0 ENone CTF3 A box with mass M and charge Q is pushed along a rough surface at constant velocity by a uniform horizontal electric field of magnitude E The coefficient of kinetic friction between the box and the oor is u What is the net force on the box AQE BuMg C zero DQE MgFN uMg FN is the magnitude of the normal force on the box M Q E v constant CTF4 Charges Q and Q and a third unknown charge are all a distance R from point x What value of the unknown charge will produce a net electric field Em 0 at point x A Q Q R X R Q B 0 C 2Q D Q R E There is no value which will make Em 0 at point x CTF5 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 kq C None r 2kg kg 8 A B 5 D 6 CTF6 Consider a point in empty space near several point charges which might be positive negative or both Consider the following statements I The Efield can be zero while the voltage is nonzero H The voltage can be zero while the Efield is nonzero Which of these statements can be true A both can be true B neither can be true C only I can be true D only 11 can by true CTF7 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 m There is no gravity in this problem and you may assume that the motion of the electron is mrelativistic What is the final kinetic energy of the electron just before it collides with the positive plate A m e V B 2eV C 12mv2 D eV V CTF8 A resistor of resistance R is plugged into a 120VAC wall socket The graph below is either resistance R voltage V across current I in or power P dissipated in the resistor vs time What could the graph be A V only B I only C V or I only D R or P only E V l or P CTF9 Consider the circuit shown with the switch initially open When the switch is closed the current 11 through resistor R1 I bat A increases B decreases C stays the same CTFlO When the switch is closed the current from the battery Ibat A increases B decreases C stays the same CTFl 1 When a particular 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 A larger B smaller C Same R but larger power rating CTF12 A charge q is released from rest at point in empty space were there may be E andor Bfields There are no forces on the charge except for the forces due to the E andor Bfields no gravity etc The charge is observed for a short while and is seen to move along a curved path quotcurvedquot means quotnot straightquot but not necessarily circular Consider the following possibilities I There is only an Efield present and no Bfield II There is only a Bfield present and no Efield 111 There is both an Efield and a Bfield present Which possibilities could account for the observed motion A all three B l and 111 only C II and 111 D 111 only CTFl3 At time to a particle with charge q has instantaneous velocity v as shown and is moving in a uniform magnetic field B that is parallel to the XaXis A True or B False 7L Vt B As time increases the Xcomponent of the q x velocity vX remains constant CTFl4 A horizontal loop of wire is in a vertical magnetic field The magnetic field varies linearly with time changing smoothly from pointing upward to pointing downward as shown The Bfield gradually gets smaller goes to zero then grows in the other direction n nw At the moment when the magnetic field in the loop is zero is there an induced current in the loop B Yes C No CTF15 What is the direction of the induced current B Clockwise as seen from above C Counterclockwise CTF l 6 Two different radio towers labeled 1 and 11 both emit the same power PO Towerl emits isotropically uniformly in all directions Tower II emits only to the East uniformly over a hemisphere Consider a receiver at a distance R from both towers The towers broadcast at different frequencies so there is no constructive or destructive interference at the receiver both signals are received independently Which signal will be stronger A Signal 1 B Signal 11 C Both are equal strength 0 receiver CTF l 7 Is the image formed by the lens in your eye real or virtual A real B virtual CTF18 An object is placed nearer to a converging lens than the focal length f of the lens The image formed is A Real and the image is further from the lens than 2f B Real and image is between f and 2f distance from the lens C Virtual and closer to the lens than the object D Virtual and further from the lens than the object object A I 3 w quot D C U f f CTFl9 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 A gt I slit2 A I d l gt gt slitl gt fringes x screen A 5 B 3 C 2 D 15 E None of these Don39t forget to review Modern Physics and Special Relativity Chapter 16 Concept tests Q161 Two uniformly charge spheres are attached to frictionless pucks on an air table The charge on sphere 2 is three times the charge on sphere 1 Which force diagram correctly shows the magnitude and direction of the electrostatic forces on the two spheres A C E Answer D The forces are equal in magnitude and opposite in direction by Newton39s Third Law and by the form of the Coulomb force equation Q162 A charge Q is xed in space A second charge q is brought a distance r away Then q charge is removed and another charge 2q is brought a distance 2r away Then the 2q charge is removed and a charge 5q is brought a distance 2r away A B C QO O q 4 D r QO O 2q 21 QO O 5q 21 Which charge feels the largest force A q B 2q C 5q D The two ofthe charges feel the same size force Answer C 5q The force on the 5q is of magnitude 54 qurz Q163 Two vectors A and B are shown Consider the vector sum C A B What is Cy the y component of C y A3 B2 C 2 D 4 E None of thesedon39t know Answer Cy Ay By 13 2 Q164 An electric dipole consists of two equal and opposite charges q and q separated by some xed distance 6 A charge Q is brought near the dipole and is positioned so that the distances to the q and the q charges are identical as shown below OQ B What is the direction of the net electrostatic force on the Q charge Answer Q165 An electric dipole Q and Q separated by a distance 6 is placed along the XaXis as shown A positive test charge q is placed at position A to the right of the dipole 4 q Q 0 a gt0 The test charge feels a force that is A zero B to the right C to the left If the test charge q is removed electric eld at position A is A zero B to the right C to the left If a negative test charge is placed at A it feels a force A zero B to the right C to the left Answers Part 1 C The force on the q is to the left I Q Q q O O OT x 1 2 gt Fuel Part 2 C The Efield at A is to the left Part 3 B A negative test charge at A feels a force to the right Q166 Two positive charges each of size Q are equal distances from the origin as shown What is the direction of the electric eld at the point in empty space which forms a square with the two charges and the origin y A B Field Q here X C D Q E No Field there Answer Etot k y E Q I gt 1 E1 2 Q Q167 Two charges Q and Q are located on the XaXis as shown what is the magnitude of the electric eld at point A R R R kQ 1 1 kQ 1 1 Agt F39 6 13gt 2 kQ 1 C Zero D 6 1 E None ofthese Answer A Magnitude is the size of the E eld Magnitude is positive by de nition Q Q E A E 1 2 Chapter 17 Concept tests Q171 A person carefully lowers a book at constant speed a distance h The work done by the person done by gravity and done by the net force on the book are person gravity net force A B C O D O E None of these Ql72 A positive test charge q is carefully moved by some external agent at constant speed a distance X between two capacitor plates in the direction along the electric field gt a The work done by the agent done by the electric field and done by the net force on the book are a ent net force A B C 0 D 0 E None of these Q173 A negative charge q is moved from position i to position f between the plates of a charged capacitor as shown As the test charge q was moved from i to f the potential energy PE increased or decreased and the voltage V at the position of the test charge increased or decreased 5 gt I A PE increased and V decreased B PE decreased and V increased C PE increased and V increased D PE decreased and V decreased E None of these What would the answer be if the test charge were positive q Q174 Two test charges are brought separately into the Vicinity of a charge Q First test charge q is brought to a point a distance r from Q The this charge is removed and test charge q is brought to the same point The potential energy of which test charge is greater Assume that PE0 and V0 at rinfinity Situatio O gt Situatio O o A q B q C Same for both CTl75 Consider 4 charges Q Q Q and Q arranged in a square with points X and Y located midway between a pair of charges as shown 0 O O 0 At point X the voltage is A positive B negative C zero At point Y the voltage is A positive B negative C zero At point Y the electric field A is zero B points right C points left D points up E points down Q176 The equipotential contours around two charges negative charge 1 and positive charge 2 are shown Which charge has larger magnitude Q A Charge 1 B Charge 2 Q177 An electron is moved at constant speed from position i to position t near charges 1 and 2 What is the work required to move the electron from i to t A 115 eV B More than 115 eV C 85 eV D115 eV E None of these Q178 Consider two in nite uniform planes of equal and opposite charge as you might find on a parallel plate capacitor The planes are seen edgeon in the diagram below In each of the three regions I H and III is the electric field up down or zero A 1 Up 11 Zero 111 Up B 1 Up 11 Down 111 Up C 1 Zero 11 Down 111 Zero D 1 Up 11 Down 111 Zero Hint The electric field near a single in nite layer of charge is constant and uniform as shown below The field does not get smaller as you move further away because no matter how far you are from the plane you still see the plane covering half your quotskyquot Q179 The plates of a parallel plate capacitor are charged with equal and opposite charges Q and Q The plates are electrically insulated so their charges cannot change The plates are pulled apart a little bit so that the separation d of the plates is greater As the plates are pulled apart the capacitance of the plates A increased B decreased C stayed the same Qe The energy stored in the capacitor A increased B decreased C stayed the same Hint Did the person pulling the plates apart do positive work negative work or no work As the plates were pulled apart the energy density energy per volume u 1280E2 A increased B decreased C stayed the same Chl 710 A parallelplate capacitor with a dielectric between the plates is charged so that Q resides on one plate Q on the other With the plates isolated and the charge Q constant the dielectric is pulled out from between the plates The energy stored in the capacitor A increased B decreased C stayed the same Hints Did the electric field between the plates change Was work done when the dielectric was removed CTR l The instructor goes crazy and grabs the terminals of a fullycharged 12V battery He will A immediately be killed B not be killed but he will start hollering because it really hurts C not feel anything This action is harmless CTR2 Which graph of voltage vs current shows the behavior of an quotohmicquot resistor a resistor obeying Ohm s Law 4 E None of these CTR3 Two cylindrical resistors are made of the same material same resistivity p Resistor 2 is twice as long and has twice the diameter of 7 R The arrow shows the direction of 1 resistor 1 What is the ratio current ow l l A 2 B 4 C 12 D 14 E1 CTR4 A battery with emf or voltage V is attached to a resistor of resistance R The circuit diagram is shown below The point A is at zero volts The correct voltages at the points B C and D are A VB 0V Vc 5V VD 0V B VB 5V Vc lt 5V VD gt 0V C VB 5V Vc lt 5V VD 0V D VB 5V Vc 5V VD 0V E None of these CTR5 A light bulb is attached to a battery with constant voltage V The light lament has resistance R The circuit diagram is shown below When the light bulb is first turned on by attaching to the battery the filament heats rapidly and as it heats its resistance R increases due to increased scattering of electrons by thermal Vibrations As the light bulb filament heats up the currentl in the filament A increases B decreases C stays the same CTR6 What is the correct way to connect the battery to the lightbulb to make it light up W K hol insul m W W E None of these will work CTR7 A 60W light bulb and a 100W light bulb each has a lament with a certain resistance when the bulb is on and hot How do the resistances of the laments compare A R60W R100W B R60W gt RIOOW C R60W lt RIOOW D Impossible to tell Without further information Hint P VZR CTR8 Please answer now The voltage provided by your household wall sockets is A AC voltage B DC voltage C Depends on the appliance which you plug into the socket CTR9 Two light bulbs A and B are in series so they carry the same current Light bulb A is brighter than B Which bulb has higher resistance HINT brighter means more power A A B B C Same resistance VAd pvg Answer B has lower resistance A has higher resistance Both bulbs have the same current 1 According to P 12R if the currents are the same the bulb with the larger R will have the larger power P Chapter 19 Concept Tests CTl9l 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 Ch192 A 1Q resistor is placed in parallel with a 10000 Q resistor as shown 1Ffff 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 lOOOOQ 10000 i so almost all the current will ow through the IQ resistor the circuit will behave almost as if the 100009 resistor is not present and the equivalent resistance is close to IQ The question is is the equivalent resistance a little less or a little greater than IQ Adding the 100009 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 batter 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 EM a LV T 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 Q194 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 13 211 7 g 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 Ihalf as large So ifll 1A 12 would be 2A and 110113 11 12 1A 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 mm 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 V P R7 Smaller Run with xed V results in a larger power P So A puts out more 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 m B Bulb 1 gets dimmer m m w quot 39 C Its brightness remains the same Hint What happens to the current from the battery when bulb 2 burns out 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 1m 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 V A Total power in BC power in A B Total power in BC gt power in A I C Total power in BC lt I I 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 BC 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 1quot current through the battery where in 1 Iquot the circuit would you place an I x ammeter If AAA m I 39 I V39 39 V V 39 l I l 4 l 1 A B C D or F1 L E None of these will work i h l i I I E I I V 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 R runs though this point A is the current through R1 and is the total current C is the current through R3 only Q199 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 Vvv ILA quot39 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 quotquot39A l iii 1 Tn AA 1 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 Vz 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 12 so we have a voltage drop and a change of 4sz 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 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 le2 A 0135A None of these 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 100 V C T T R 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 ofthese 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 E TVR1 01 0 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 TV7R 1 070 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 Chapter 20 Concept Tests Reading Quiz Pick the answer which is most nearly correct Magnetic fields are caused by A stationary electric charges B magnetic monopoles C moving charges currents D particles with both mass and charge E None of these is correct Answer C Q201 A negative particle and a positive particle are moving with certain velocities in a constant uniform magnetic field B as shown The direction of the Bfield is to the right The particle is moving directly left the particle is moving directly up V V V V V The forces on the two particles are in into page out out of page A B C D E in zero out in zero out zero in None of these Answer C The particle is moving antiparallel to the Bfield The angle 9 is 180 and the force is FBqu sine 0 The particle is moving at right angles to the eld By the righthand rule the direction of the force is out of the page Q202 A positive particle is released from rest in a region of space where there is constant uniform electric field E and a constant uniform magnetic field B The electric field points up and the magnetic field points out of the page in the diagram below Which path will the positive particle follow A B and C are in the plane of the o o o G G G G Bout3 D it will remain E it will move out of the plane of the The work done by the magnetic field on the particle is A positive B negative C Zero Answer The particle will feel a force FE qE due to the E eld along the direction of the E eld As it starts moving along the E eld direction it will acquire a velocity and it will start to feel a force FBqVB due to the B eld The direction of the force is to the right by the righthandrule Answer C The magnetic eld does no work since the force is perpendicular to the motion Answer C Q203 A magnetic compass is placed at the points A B and C near an electric circuit which has the following twisty shape The de ection of the compass needle is a measure of the strength of the magnetic field The relative de ection of the needle n order from biggest to smallest de ection is I A ABC B CAB C BCA D ACB E None of these Answer The correct order is CAB Big currents make big magnetic elds Near C there are two currents in the same direction which are effectively one big current Near B there are two equal currents going in opposite directions which if they were right on top of each other would give a net current of zero Point A is the middle case Q204 A rectangular loop of wire is carrying a current i in the clockwise direction and is near a long straight wire carrying a current I as shown What is the direction of the net force on the rectangular loop due to the Bfield from the long straight wire FA Net force is Answer The direction of the net force is up The B fleld created by the long straight wire is into the page at the location of the loop by Right Hand Rule 11 The upper portion of the loop feels an upward force and the bottom portion of the loop feels a downward force by the Right hand rule But the eld is larger at the top potion of the loop because the B fleld is larger closer to the straight wire So the upward force on the upper wire in the loop is larger than the downward force on the lower wire Another way to see the answer Parallel currents attract and antiparallel currents repel So the upper portion of the loop feels an upward force and the bottom portion of the loop feels a downward force But the upper portion is closer to the straight wire so there is a bigger eld a bigger force Q205 A square loop of wire carrying a current I is in a constant uniform magnetic field B The magnetic field is perpendicular to the plane of the loop The loop can rotate freely about the axis gt 314K The loop tends to rotate so that the near edge moves A to the right B to the left C The net torque on the loop is zero so it does not tend to move i 1in gt lt Iout Answer Since from above the situation is as shown below The direction of the forces are from the Right hand rule The net torque is zero there is no tendency to rotate The net torque is zero Q206 4 parallel Wires each carry a current I 3 of the Wires carry current out the page 1 carries current into the page as shown What is the direction of the Bfield at the center of the square F3 None of these Answer B Q207 A permanent bar magnet is broken in half The two pieces are interchanged keeping their orientations fixed as shown below Do the pieces attract or repel t A Attract C No force at all Answer The bar magnet can be thought of as a solenoid of current Parallel currents attract When the magnet is broken and rearranged the currents on the ends are parallel so there is an attraction They attract Answer A In the diagram below we choose the right end to be North If we had chosen the right end to be South all the currents would be reversed but the end currents would still be parallel South t t South North South North h South North South North Q208 A proton charge q e moving to the right with an initial speed v passes into a region where there is a vertical electric field pointing down and a horizontal magnetic field pointing into the page Is it possible that the forces from the E and Bfields cancel and the proton moves straight through undeflected Ignore gravity in this problem B Yes the forces can cancel and the particle moves straight through C No the forces cannot perfectly cancel the particle path will be bent I 7 A on L V E 38 V The E and Bfields of the velocity selector are adjusted so that protons with a certain speed V pass through unde ected Now electrons with the same speed are shot into the velocity selector with same E and Bfields as before Do the electrons also pass through unde ected B Yes the electrons go straight through C No the electrons are bent Answer B The forces from the E and the B field still cancel This time they are in the opposite direction because the charge of the electron is negative Q209 A row of positive charges sites stationary on the ground A person with a gaussmeterwhich measures the magnetic field is running to the right along the row of charges at the same height as the charges and in front of them in the diagram below What is the direction of the Bfield which the observer measures backward directi Answer From the person s point of view the person39s reference frame the person is at rest and the charges are moving to the left so there is a current to the left The B field produced at the person39s position is up gt lt p g Q208 A loop of wire is brought near a long straight wire canying a current I The direction of the current induced in the loop is T HT A Clockwise B Counterclockwise C Not enough information to tell Answer B F qV X B V is up and B is into the page With the right hand rule the force on the charges in the loop of wire is left making a counter clockwise current Chapter 21 Concept Tests Q211 In which situation is the magnetic ux through the loop the smallest A Don t vote A Abuttons are unreliable gt gt 5 are a 2A area area A constant uniform Q212 A loop of wire is sitting in a uniform constant magnet field Suddenly the loop is bent into a smaller area loop During the bending of the loop the induced current in the loop is B zero C clockwise D counterclockwise Hey Don39t vote A Answer The ux is decreasing as the loop area decreases To ght the decrease we want the induced B to add to the original B By the right hand rule version 11 a clockwise induced current will make an induced B into the page adding to the original B Clockwise Q213 A bar magnet is positioned below a horizontal loop of wire with its North pole pointing toward the loop Then the magnet is pulled down away from the loop As viewed from above is the induced current in the loop clockwise or counterclockwise EVA l B D Answer The B eld from a bar magnet points out of the North pole As seen from above the eld through the loop is out toward the observer As the magnet is pulled away the ux is decreasing To ght the decrease the induced B eld should add to the original B eld B ind


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