mittal (im5936) – Magnetic Force and Field HW – yeazell – (58010)1Thisprint-outshouldhave26questions.Multiple-choice questions may continue onthe next column or page – find all choicesbefore answering.00110.0pointsAtoneinstantanelectron(charge=−1.6×10−19C) is moving in thexyplane,the components of its velocity beingvx=3×105m/s andvy= 5×105m/s.A mag-netic field of 0.8 T is in the positive z direc-tion.At that instant the magnitude of themagnetic force on the electron is:1.3.84×10−14N2.0 N3.5.1×10−14N4.6.4×10−14N5.7.5×10−14NcorrectExplanation:The magnetic force is given byqvectorv×vectorB.The velocity and theBfield are perpendic-ular, so we can now ignore vector notation,and directly calculate the magnitude of theforce:vextendsinglevextendsinglevextendsinglevectorFvextendsinglevextendsinglevextendsingle=qvB=qBradicalBigv2x+v2y=vextendsinglevextendsingle(−1.6×10−19C)(0.8 T)×radicalBig(3×105m/s)2+ (5×105m/s)2vextendsinglevextendsinglevextendsinglevextendsingle= 7.5×10−14N00210.0pointsThe diagram shows a straight wire carryinga flow of electrons into the page. The wire isbetween the poles of a permanent magnet.The direction of the magnetic force exertedon the wire is:1.upcorrect2.right3.into the page4.left5.downExplanation:The magnetic field points toward the southpole and away from the north pole.We can then use the right-hand rule todetermine the direction ofvectorv×vectorB, with theresult that it is downward.But, the magnetic force is given byFB=qvectorv×vectorB, and the charge on the electron isnegative, so the answer isupward.00310.0pointsElectrons are going around a circle in acounterclockwise direction in the plane of thispaper. They produce a magnetic field whosedirection at the center of the circle is:1.out of the page2.into the pagecorrect3.to the left4.the field is zero at the center5.to the rightExplanation:Using the right hand rule, one can pointtheir thumb in the direction of a current car-rying wire, and then their hands should curlint he direction of the magnetic field created
