Exam III review CT3 1 An electron and a proton have the same momentum p mv and enter a region where there is a uniform magnetic field Which particle is bent more A electron B proton C both are bent the same amount Answer Both the electron and proton are bent into circles with the same radius of curvature R mv qB Same p mv same q same B so same R The electron has larger acceleration and speed however CT3 2 A charged particle is orbiting in a uniform B field in the sense shown Is the particle positive or negative A positive B in B negative C impossible to tell 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 CT3 3 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 I C loop will not rotate at all axis Answer Right side will move out of page Use Fwire I L B CT3 4 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 D B has a non zero y component as well as a z component y x A z out 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 CT33 5 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 r shown A True z A I B dl 0I B False Answer True Ampere s Law is always true for steady currents and no changing Efields 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 CT3 6 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 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 CT3 7 A bar magnet with the ends properly labeled N and S is placed near an unlabeled bar magnet as shown The magnets attract The left side of the unlabeled bar magnet must be a A South pole S B North pole N Answer South Opposite poles attract Recall that a bar magnet is like a currentcarrying solenoid with the B field coming out of the N end Parallel currents attract so the orientation must be such as to cause the currents in the near ends to be parallel CT3 8 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 A CW R B CCW Answer CW When R decreases the current I V R in the circuit increases causing an increasing B field out of the page at the position of the loop A CW current in the loop will fight the increase in B CT3 9 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 E field in the bar is such that the force from the E field F qE and the force from the B field F qvB cancel everywhere in the bar and charges move neither up nor down CT3 10 A solenoid has an increasing current which creates an increasing B field in its interior The electric field at point c is I increasing B increasing a b c A up B down C left D right E zero Answer down Apply Faraday s Law and Lenz s Law At which of the three points a b or c is the E field maximum A a B b C c D the E fields at b and c have the same magnitude and are larger than the E field at a Answer point b CT3 11 A uniform solid sphere of copper rotates about a stationary axis in a uniform magnetic field B Are there eddy currents A Yes there are eddy currents B B No no eddy currents ANSWER NO EDDY CURRENTS The flux through any loop rotating with the sphere does not change Therefore no emf no eddy currents CT3 12 The switch in the circuit below has been closed for a long time and then is opened at t 0 What is the current in the inductor at t 0 A zero R1 10 V 10V R2 10 B 1A C 0 5A L 10H C 1F D some other answer Answer 1A Just before the switch was opened the current thru the inductor was 1A V R1 The current thru an inductor cannot change instantly At t 0 what is the current through resistor R2 A zero B 1A C 0 5A D some other answer Answer 1A The inductor the capacitor and the resistor are in series so they must all have the same current The current thru R2 is the same as …
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