# Magnetic Force and Field HW-solutions - Magnetic Force and...

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MagneticForceandFieldHWyeazell(58010)1Thisprint-outshouldhave26questions.Multiple-choice questions may continue onthe next column or page – find all choicesbefore answering.00110.0pointsAtoneinstantanelectron(charge=1.6×1019C) 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:1.7.5×1014Ncorrect2.6.4×1014N3.3.84×1014N4.0 N5.5.1×1014NExplanation: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(1.6×1019C)(0.8 T)×radicalBig(3×105m/s)2+ (5×105m/s)2vextendsinglevextendsinglevextendsinglevextendsingle= 7.5×1014N00210.0pointsThe 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.right2.into the page3.left4.down5.upcorrectExplanation: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.into the pagecorrect2.out of the page3.to the right4.to the left5.the field is zero at the centerExplanation: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
MagneticForceandFieldHWyeazell(58010)2by a positive charge carrier moving in thatdirection.Doing this, you find that the field at thecenter should be out of the page, but sinceelectrons are negative charge carriers, thenthe actual field created is negative.00410.0points

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