1 of 11 More B field Concept Tests B2 1 A current carrying wire is in a B field The wire is parallel to the B field as shown What is the direction of the magnetic force on the wire A right B left C up D down E None of these B I Answer the force is zero so no direction B2 2 A circular loop of wire with radius R is carrying current I as shown The B field at the center of a circular loop is A zero B out of the page C into the page D None of these K K 0 I d l r Use Biot Savart dB 4 r2 I R Answer out of the page Phys1120 Concept Tests M Dubson University of Colorado at Boulder 2 of 11 B2 3 Which point A or B has the larger magnetic field A B C The B field is the same at A and B I I B A Answer Case B has the larger magnetic field Use the Biot Savart Law to get the directions of the B field due to the two semi circular portions of the loop In A the two fields oppose each other in B they add B2 4 A long straight wire is carrying current I The magnetic field at point x has magnitude Btotal Btotal x r I B1 2 r G True A or False B The magnetic field B1 2 at point x due only to the current to the G left of x points in the same direction and has 1 2 the magnitude of B total Phys1120 Concept Tests M Dubson University of Colorado at Boulder 3 of 11 G G 0 dl r I 2 The field element dB always points in r 4 the same direction so the vectors just add like scalars numbers Integrating over the left half of the wire gives half the result z G Answer True Btot G dB z B2 5 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 B field at the center of the square y out out x in out B Hint C D E None of these Answer Each of the four currents labeled 1 2 3 4 below creates a B field at the cent of the square The net B field is the vector sum of these 4 fields and it points to the upper right y 1 out 3 2 out 1 4 x 2 3 out 4 in Phys1120 Concept Tests M Dubson University of Colorado at Boulder 4 of 11 B2 6 A magnetic compass is placed at the points A B and C near an electric circuit which has the following twisty shape A C B The deflection of the compass needle is a measure of the strength of the magnetic field The relative deflection of the needle in order from biggest to smallest deflection is A ABC D ACB B CBA E None of these C BCA Answer The correct order is CAB Big currents make big magnetic fields 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 B2 7 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 B field from the long straight wire I A i B D C E Net force is zero Answer The net force is up The B field due to the long straight wire decreases in Phys1120 Concept Tests M Dubson University of Colorado at Boulder 5 of 11 oI So the upward force on 2 r the top of the wire loop F ILB is greater than the force on the bottom of the loop magnitude as you move further from it according to B Another way to see the answer Parallel currents attract and anti parallel 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 field a bigger force B2 8 The perhaps incorrect magnetic field lines around an object are partly obscured by a screen Is this partial field line diagram possible B A Yes B No Answer Yes The field lines can be coming in toward the center from above and below behind the screen This would be the case when two bar magnets have their North poles pointing toward each other Phys1120 Concept Tests M Dubson University of Colorado at Boulder 6 of 11 B2 9 The imaginary loop L near a wire with current I has 4 segments labeled 1 2 3 and 4 as shown What is K K B d A for each of the segments For 1 2 3 4 the line integral is A 0 B 0 0 C 0 0 D 0 0 E None of these 3 1 4 I in 2 Answer 0 0 B2 10 We need a sign convention for I thru Place the fingers of your right hand around the imaginary loop and your thumb points in the direction of positive I thru What is I thru here where all three wires have 5 A 5A 5A A 15 A B 10 A 5A C 10 A D 5 A E None of these Answer 5 A Phys1120 Concept Tests M Dubson University of Colorado at Boulder 7 of 11 B2 11 A long straight copper wire has radius b and carries a constant current of magnitude I The current density of magnitude J I b2 is uniform throughout the wire What is the current contained in the circular loop with radius r b centered on the wire s center as shown 2 r r B I A I b b b 3 r C I b r D None of these J constant How does the magnitude of the B field a distance r b from the center of the wire depend on r A B r B B constant C B 1 r 2 E None of these don t know D B 1 r Referring to the same wire for the new loop shown below what is the value of v G G B dA L A 0 I 2 B 0I 2 b C 0I 2 J constant Phys1120 Concept Tests M Dubson University of Colorado at Boulder 8 of 11 Answers FG IJ H bK Part 1 Current thru loop of radius r is r 2 I Current thru smaller circle total current I area of smaller circle area of larger circle I r 2 b 2 Part …
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