Transcript Document
19 Overview • magnetic force fields & sources • forces on charges & currents • Homework: • 3, 7, 10, 33, 45, 55, 75. 1 Magnetic Field, B • SI Unit: tesla, T = N/A·m • Direction shown below 2 Magnetic Poles • Do ______ have _____ electric charge • _______________ plates easily make uniform electric fields. • Uniform B field difficult to achieve 3 Magnetic Force on Charge • F = qvB sin θ • ___________ points in direction of force on a ________________ charge (Flat RHR) • Negative charge feels ________________ directed force 4 Properties of the Magnetic Force • F = qvB sin θ • Proportional to: charge, speed, field, & sine of angle between v and B. • perpendicular to both v and B. • / 5 Ex. An object with a charge of 2.0 mC is moving perpendicular to a magnetic field of strength 1.0 T at a speed of 1.0 x 103 m/s. a) Sketch the situation. b) Find the magnetic force on the particle. c) Indicate the direction of the force on your sketch. 6 Applications of Magnetic Force • • • • electric motor Mass spectrometer magnetohydrodynamic propulsion colliders 7 Cyclotron Motion • Magnetic force is __________________ to velocity • causes a free charge to move in a circle • Does no ___________ 8 Cyclotron Motion • qvB = _____________ • radius of motion r = mv/qB • period of motion = T ( vT = 2pr ) • frequency = 1/T 9 Ex. A particle, m = 1gram, v = 10m/s, q = 1mC, moves perpendicular to a uniform field B = 1.0 T directed into the screen. Sketch the motion and calculate its radius. 10 Magnetic Force on a Current Carrying Wire • net magnetic force = net magnetic force on all moving charges • qv = IL • (C)(m/s) = (C/s)(m) • F = ILB sin θ • // 11 Force on a Current Carrying Wire • F = ILB sin θ • Direction is given by a right hand rule 12 Ex. A wire of length 1m is perpendicular to a uniform magnetic field of strength 0.50 T. a) Sketch the situation. b) Choose a direction for the current and indicate the direction of the force on the wire for your choice. c) If the magnitude of the force is 10 N, find the current in the wire. 13 Electric Motors The ILB force causes a motor coil to turn. Current __________________ is changed by a split ring. 14 Torque on Coil • • • • • area, A number of turns, N current in wires, I in field, B angle between coil normal & B: q. torque = NIABsinq. 15 torque example • Tesla Motors Car produces __________________ = 271 m·N NIAB = 271 m·N. • estimated reasonable? specs: • with B = 0.3T: NIA = 904. • with A = 0.05sq.m.: NI = 18,000. • with I = 20A: N = 900turns 16 Straight Wire & Loop o I B 2pr Bcenter N o I 2R 17 Solenoid Bcenter N o I L 18 Ferromagnetism • Unmagnetized • Induced • Permanent 19 Summary • • • • • • F = qvB sinq = ILB sinq (Flat RHR) B fields around wires (Curled RHR) B field inside a solenoid is nearly uniform electric motors utilize B force charge motion (v, I) causes B fields // 20