How Do You UseMagnetic Fieldsin YourEveryday Life!?Lecture 18: FRI 26 FEBLecture 18: FRI 26 FEBMagnetic fieldsMagnetic fieldsCh.28.1–5Physics 2102Jonathan Dowling“I’ll be back….Aurora BorealisSecond Exam Review:6-7PM WED 04 MARNicholson 130Second Exam (Chapters 24–28):6–7PM THU 05 MARLockett 6Electric vs. Magnetic FieldsElectric vs. Magnetic FieldsElectric fields are created:• microscopically, by electric charges (fields) of elementary particles (electrons,protons)• macroscopically,by adding the field of many elementary charges of the same signMagnetic fields are created :• microscopically, by magnetic “moments” of elementary particles (electrons, protons, neutrons)• macroscopically, by• adding many microscopic magnetic moments(magnetic materials); or by• electric charges that move (electric currents)Magnetic Field DirectionFROM North PolesTO South PolesB+–Compare to ElectricField DirectionsERitchie’s Rule for MagnetsOpposite Poles AttractLike Poles RepelWe know that an electric fields exists because it accelerates electric charges, with a force independent of the velocityof the charge, proportional to the electric charge: FE = qEWe know that a magnetic field exists because it accelerateselectric charges in a direction perpendicular to the velocityof the charge, with a magnitude proportional to the velocityof the charge and to the magnitude of the charge: FB= q v x BMagnetic forces are perpendicular to both the velocity of chargesand to the magnetic field (electric forces are parallel to the field).Since magnetic forces are perpendicular to the velocity,they do no work! (W=F · r)Speed of particles moving in a magnetic field remains constantin magnitude, ONLY the direction changes. Kinetic energy is constant! (no work).Magnetic vs. ElectricMagnetic vs. ElectricForcesForcesMagnetic vs. Electric ForcesMagnetic vs. Electric Forces ! ! F E= q! E Electric Force onCharge Parallel to E: ! ! F B= q! v "! B Magnetic Force onCharge Perpendicularto B and v.+q ! ! F E ! ! E ! ! F B ! ! B +q ! ! vDefinition of Magnetic FieldDefinition of Magnetic Field ! ! E =! F EqDefinition of Electric Field: ! B=! F Bq! v Definition of Magnetic Field:! Units : B =NewtonCoulomb " meter/sec( )# $ % & ' ( =NewtonCoulomb/sec( )" meter# $ % & ' ( =NewtonAmpere " meter# $ % & ' ( =NA" m# $ % & ' (ThompsonExperimentForcesBalance:v=E/BL! E " 0, B = 0; qE = FE= maa = FE/m = qE /mL = vt; y =12at2; Solve : y =qEL22mv2yII: E≠0, B=0I: E=0, B=0III: vB=E! v = E / B; y =qEL22mv2y =qEL2B22mE2=qL2B22mE!
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