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W11D3. Magnetic Forces. Reminder. EXAMINATION #3 Wednesday November 9 th. 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!

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## W11D3

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**W11D3**Magnetic Forces**Reminder**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. More Info Later**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 ARepeat 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. L I Think “BIL” B out of plane of the paper We usually calculate force per unit length.**TWO WIRES**Now we can calculate the magnitude of the magnetic force F21exerted on the lower wire by the field produced by the upper wire: using the right-hand rule with , the direction of the force is up, so that the lower wire is attracted to the upper wire. B**(B field pointsinto plane of paper.)**v Trajectory of Charged Particlesin a Magnetic Field B B + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + v F F Magnetic Force is a centripetal force**Review of Rotational Motion** = s / r s = r ds/dt = d/dt r v = r s = angle, = angular speed, = angular acceleration r at = r tangential acceleration ar = v2 / rradial acceleration The radial acceleration changes the direction of motion, while the tangential acceleration changes the speed. at ar Uniform Circular Motion = constant v and ar constant but direction changes ar KE = ½ mv2 = ½ mw2r2 ar = v2/r = 2 r v F = mar = mv2/r = m2r**Radius of a Charged ParticleOrbit in a Magnetic Field**Note: as , the magnetic force does no work! Centripetal Magnetic Force Force = v B + + + + + + + + + + + + + + + + + + + + F r**Cyclotron Frequency**v B + + + + + + + + + + + + + + + + + + + + F r The time taken to complete one orbit is: V cancels !**Magnetic Sector - Mass Spectrometer**Velocity Selector**that’s all there is!**Magnetic Forces on Charges

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