Last time…Exam 2 is Tuesday Oct. 28Magnets and magnetic fieldsDefinition of Magnetic FieldMagnetic force on moving charged particleMagnetic force on electric chargesMagnetic force on charged particleFB on a Charge Moving in a Magnetic Field, FormulaQuick QuizSlide 10Force on moving charged particleMore About Magnetic ForceMore force examplesDirection of Magnet Force on Moving ChargesMagnetic Force on a Current‘Homopolar’ motorSurprise! Electric current produces magnetic fieldMagnetic field from a currentMagnetic field from long straight wire: DirectionMagnetic field from loop of currentSlide 21Torque on a current loopTorque on current loopCurrent loops & magnetic dipolesTue. Oct. 21, 2008 Physics 208 Lecture 15 1Last time…RC circuitsMagnetic dipole torqueQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.Magnetic fields € r τ =r μ ×r BTue. Oct. 21, 2008 Physics 208 Lecture 15 2Exam 2 is Tuesday Oct. 28Students w / scheduled academic conflict please stay after class TODAY to arrange alternate time.5:30-7 pm, 2103 Ch (here)Covers: all material since exam 2.Bring: CalculatorOne (double-sided) 8 1/2 x 11 note sheetExam review: Thursday, Oct. 23, in classTue. Oct. 21, 2008 Physics 208 Lecture 15 3Magnets and magnetic fieldsMagnetic dipole aligns with magnetic fieldDue to torque QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture. € r τ =r μ ×r BTue. Oct. 21, 2008 Physics 208 Lecture 15 4Definition of Magnetic FieldDefined electric field based on electric force on a charged particle.Observation: Magnetic field as no effect on stationary charged particleIs there a magnetic force on charged particle?Tue. Oct. 21, 2008 Physics 208 Lecture 15 5Magnetic force on moving charged particleqFvmagEmpirical facts: a) magnitude: to velocity of q b) direction: to direction of qQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.STue. Oct. 21, 2008 Physics 208 Lecture 15 6Magnetic force on electric chargesEffect of uniform B-field on charged particleCharged particle not moving no effectCharged particle is moving:force exerted perpendicularto both field and velocityTue. Oct. 21, 2008 Physics 208 Lecture 15 7Magnetic force on charged particle Magnitude of force is proportional toCharge of particle, qSpeed of particle, vStrength of magnet field, Bsin(), =angle between and € FB= qvBsinθDirection of force is perpendicular to both and € r F = qr v ×r B vector ‘cross product’ € r B € r v € r B € r vTue. Oct. 21, 2008 Physics 208 Lecture 15 8FB on a Charge Moving in a Magnetic Field, FormulaFB = q v x BFB is the magnetic forceq is the chargev is the velocity of the moving chargeB is the magnetic fieldSI unit of magnetic field: tesla (T)CGS unit: gauss (G): 1 T = 104 G (Earth surface 0.5 G)€ T =NC ⋅m /s=NA ⋅mTue. Oct. 21, 2008 Physics 208 Lecture 15 9Quick QuizThe three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. Which particle experiences the greatest magnetic force?F = q v B sin()A. 1B. 2C. 3D. All sameB123Tue. Oct. 21, 2008 Physics 208 Lecture 15 10Quick QuizThe three charges below have equal charge and speed, but are traveling in different directions in a uniform magnetic field. The force on all the particles is in the same direction.All forces are into pageMagnitude F = q v B sin()A. TrueB. FalseB123Tue. Oct. 21, 2008 Physics 208 Lecture 15 11Force on moving charged particle € r F = qr v ×r B € r F = 0 € r F is max € r F intermediateTue. Oct. 21, 2008 Physics 208 Lecture 15 12More About Magnetic ForceThe magnetic force acts only a moving charged particle The magnitude of the magnetic force on a charged particle is FB = |q| vB sin FB is zero when v and B are parallel or antiparallelFB is a maximum when v and B are perpendicularThe magnetic force NEVER does work on the particle.Magnetic force perpendicular to the particle displacementB field can alter direction of velocity, but not the speed or the kinetic energyTue. Oct. 21, 2008 Physics 208 Lecture 15 13More force examplesForce from B-field on moving charged particleFx x x x x x x x x x x x x x x x x xvBqvBqF = 0vBqFUnits: 1 T (tesla) = 1 N / Am1G (gauss) = 10-4 T € r F = qr v ×r BTue. Oct. 21, 2008 Physics 208 Lecture 15 141) Up2) Down3) Right4) Left5) ZeroDirection of Magnet Force on Moving ChargesVelocity B Forceout of page right upout of page left downRight Hand RuleThumb v, Fingers B, palm Fout of page up ?vBFTue. Oct. 21, 2008 Physics 208 Lecture 15 15Magnetic Force on a CurrentSNICurrentMagnetic fieldMagnetic force € r F = IBL € qr v ×r B Force on each chargeForce on length of wire € dr s € Idr s ×r B Force on straight section of wire, length LTue. Oct. 21, 2008 Physics 208 Lecture 15 16‘Homopolar’ motorWhy does the motor turn?A. Electric Coulomb force pushes on magnetB. Magnetic field pushes on current from batteryC. Current from battery heats the magnetD. Current through wire produces magnetic fieldTue. Oct. 21, 2008 Physics 208 Lecture 15 17Surprise! Electric current produces magnetic fieldCurrent (flow of electric charges )in wire produces magnetic field.That magnetic field aligns compass needleCurrentMagnetic fieldTue. Oct. 21, 2008 Physics 208 Lecture 15 18Magnetic field from a currentIron filings align with magnetic field linesMagnetic field loops around the current.Tue. Oct. 21, 2008 Physics 208 Lecture 15 19Magnetic field from long straight wire:DirectionWhat direction is the magnetic field from an infinitely-long straight wire?Ixy € r B =μoI2πr€ μo= 4π ×10−7N / A2= permeability of free spacer = distance from wireTue. Oct. 21, 2008 Physics 208 Lecture 15 20Magnetic field from loop of currentTue. Oct. 21, 2008 Physics 208 Lecture 15 21Quick QuizA current I flows in a square loop of wire with side length L.A constant B field points in the -z direction,
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