1 of 7 RIII 1 R C R A capacitor C is charged up to a voltage V The switch is closed at time t 0 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 A IR Ic B IR 1 2 Ic C IR 2 Ic E IR 0 Answer IR Ic The circuit elements are in series Immediately after the switch is closed how does the voltage across the second resistor on far right compare with the voltage across the capacitor A VR Vc B VR 1 2 Vc C VR 2 Vc D VR 0 Answer VR 1 2 Vc By Kirchhoff s voltage law we have Vc VR1 VR2 2VR Phys1120 M Dubson University of Colorado at Boulder 2 of 7 RevIII 2 The circuit shown has two identical capacitors both with capacitance C and two identical resistors both with resistance R Initially the switch is open and both capacitors are completely uncharged What is the initial current I through the battery immediately after the switch is closed V 2V V A zero B C D 2R R R R V R C E None of these Answer Iinit C V When a capacitor is uncharged it acts like a short a wire 2R RevIII 3 A metal bar not attached to any outside circuitry is moving through a uniform magnetic field as shown The electric field E within the bar is A non zero and downward B in B non zero and upward V C zero don t know Answer Non zero and downward Although there is no current there is a charge separation charges piles up at the top of the bar charges at the bottom The Efield in the bar is such that the force from the E field F qE and the force from the Bfield F qvB cancel everywhere in the bar and charges move neither up nor down RevIII 4 A charged particle is orbiting in a uniform B field in the sense shown Is the particle positive or negative B in A positive B negative C impossible to tell Phys1120 M Dubson University of Colorado at Boulder 3 of 7 Answer The direction of acceleration and net force must be toward the center of the circle For CW rotation and B in and a negative particle the force on particle is toward the center of the circle RevIII 5Two 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 A Neither the orbits have the same size B The larger mass particle will have a larger orbit C The smaller mass particle will have a larger orbit Answer 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 mv qB The physical argument is this both particles feel the same size sideways force F qvB The more massive particle has more inertia and is therefore harder to turn so it turns through a larger orbit RevIII 6 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 B B left side will move out of page C loop will not rotate at all I axis Answers Right side will move out of page Use Fwire I L B Phys1120 M Dubson University of Colorado at Boulder 4 of 7 RevIII 7 Two bar magnets are brought near each other as shown The magnets A attract B repel C exert no net force on each other N S S N Hint N B field comes out of North end S atomic current on rim like solenoid B field enters South end Answer Parallel currents attract Think about the current directions on the sides of the magnets The two magnets will attract RevIII 8 A long U shaped wire carries a current I in the sense shown Consider the magnetic field B at the point A which is equi distant from the two corners of the U and in the same plane as the U Which one of the following statements is true A B has a z component only which is due entirely to the bottom segment of the U I B B has a z component only which has contributions from the bottom and sides of the U C B has a non zero x component as well as a z component y x A Phys1120 M Dubson z out D B has a non zero y component as well as a z component University of Colorado at Boulder 5 of 7 Answer B has a z component only which has contributions from the bottom and sides of the U Use the Biot Savart Law to see this RIII 9 A long U shaped wire carries a current I in the sense shown Consider the imaginary loop of radius r centered on the right side of the U as shown True or False For the loop shown z B dl 0I r A True I A B False Answer True Ampere s Law is always true for steady currents and no changing Efields RIII 10 True or False At point A the magnitude of the magnetic field is B A True B False oI 2 r False That formula is for an infinitely long straight wire with no other wires nearby It doesn t apply here because the other side of the U breaks the symmetry of the situation In this messy situation with a U shaped wire Ampere s Law is true but not useful since the integral is very messy To get the field at A one would use Biot Savart RIII 11 Consider the following configuration of B field lines solid lines and the imaginary rectangular loop dotted line Which one of the following statements is true Phys1120 M Dubson University of Colorado at Boulder 6 of 7 A There must be a non zero current thru the imaginary loop going into the page B There must be a non zero current thru the loop coming out of the page C There is no current going thru the loop Answer There must be a non zero current thru the loop going into the page Apply Ampere s Law to the loop Note carefully The B field lines show the total field due to all currents present For instance the field lines on the right are not due just to a current in the center of those lines RevIII 12 A coil of wire attached to a resistor is in an oscillating magnetic field as shown The B field vs time is shown At what point in time is the magnitude of the current through the resistor a maximum B oscilating B field T B C 0 …
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