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W11D3 Magnetic Forces EXAMINATION #3 Wednesday November 9th Calendar Thing Today (Watch for last WA before exam. Sorry about the intensity of these!) Quiz Who wants to present their experimental results? Collected now! Some Problems (No Evan Show! He is cutting class today)) Continue with Forces Next Week Monday - As much of remainder of chapter as possible. Nothing DIFFICULT from this session will be on exam. Wednesday Exam Second Hour & Friday … moving along! FINAL EXAMINATION Monday morning – December 3rd 7:30AM Physical Science Building First Floor Conference Room Location is SECRET!! TWO index cards allowed Calculator Writing Instrument YOU. In the circuit shown below, the emf of the battery is 7.6 volts. Resistor R1 has a resistance of 33 ohms, resistor R2 has a resistance of 47 ohms, and resistor R3 has a resistance of 57 ohms. A steady current flows through the circuit. a) What is the equivalent resistance of R1 and R2? (b) What is the equivalent resistance of all the resistors: R1, R2, and R3 (c) What is the conventional current through R3? Switch S in the figure below is closed at time t = 0, to begin charging an initially uncharged capacitor of capacitance C = 10.0 µF through a resistor of resistance R = 16.0 W At what time is the electric potential across the capacitor equal to that across the resistor? t = 0.111 ms . In the circuit of the figure below, = 2.0 kV, C = 5.5 µF, R1 = R2 = R3 = 0.63 MΩ. With C completely uncharged, switch S is suddenly closed (at t = 0). a) At t = 0, what is current i1 in resistor 1? 0.00212 A (b) At t = 0, what is current i2 in resistor 2? 0.00106 A (c) At t = 0, what is current i3 in resistor 3? 0.00106 A Repeat for t = infinity (that is, after many time constants.) (d) What is current i1? 0.00159 A (e) What is current i2? 0.00159 A (f) What is current i3? 0 A (g) What is the potential difference V2 across resistor 2 at t = 0? 667 V (h) What is V2 at t = ? 1000 V (i) Sketch V2 versus t between these two extreme times. (Do this on paper. Your instructor may ask you to turn in this sketch.) In the figure below, the battery has a potential difference of 10.0 V and the five capacitors each have a capacitance of 16.0 µF. (a) What is the charge on capacitor 1? 0.00016 C (b) What is the charge on capacitor 2? 3.2e-05 C MORE ON FORCES Remember Bil? Bil FORCES BETWEEN WIRES Opposites don’t always attract! The Wire in More Detail – Conventional Assume all electrons are moving with the same velocity vd. q I t I I L L vd F qv d B I L vd v d B IL B vector : F IL B Think “BIL” V ector L in the direction of the m otion of P O S IT IV E charge (I). B out of plane of the paper TWO WIRES B 0 2I 4 r TWO WIRES Now we can calculate the magnitude of the magnetic force F21 exerted on the lower wire by the field produced by the upper wire: F2 1 I 2 L using the right-hand rule with IΔL × B lower wire is attracted to the upper wire. 0 2 I1 4 d B , the direction of the force is up, so that the Let’s Get Dizzy! Trajectory of Charged Particles in a Magnetic Field (B field points into plane of paper.) + +B + v+ + + + + + + + + F + + + + F + + + + + + + + + + + + + + + + + + + B + + + + + v Magnetic Force is a centripetal force Review of Rotational Motion = s / r s = r ds/dt = d/dt r v = r s r = angle, = angular speed, = angular acceleration at ar at = r tangential acceleration ar = v2 / r radial acceleration The radial acceleration changes the direction of motion, while the tangential acceleration changes the speed. Uniform Circular Motion ar 16 = constant v and ar constant but direction changes v ar = v2/r = 2 r KE = ½ mv2 = ½ mw2r2 F = mar = mv2/r = m2r Radius of a Charged Particle Orbit in a Magnetic Field +B + v+ + + + + + F + + + + + r + + + + + + Centripetal Force mv = Magnetic Force 2 qvB r + r mv qB Note: as F v , the magnetic force does no work! Cyclotron Frequency +B + + v+ + + + + F + + + + + + + r + + + + + The time taken to complete one orbit is: T 2 r v 2 mv v qB f 1 T qB 2 m c 2 f qB m More Circular Type Motion in a Magnetic Field 19 Magnetic Sector - Mass Spectrometer 20 Velocity Selector r mv qB How Old is That?? Activity SIMS THAT’S ALL THERE IS! Magnetic Forces on Charges