Transcript Lecture 8
PHY 2049: Physics II Tutoring Center is open in room NPB 1100, M-F 10:00AM -3:00PM. It is free. Hopefully all webassign problems have been solved. Please see me immediately after the class if there is still an issue. PHY 2049: Physics II About bicycle riding: If some one shows you a trick, it means that, It is possible to do that trick The person who showed you, can do the trick. It does not mean that you can do the trick. Try it a lot of times and you can too. PHY 2049: Physics II Recall: Faraday’s law Lenz’ law: the kickback B B.dA V E.dl B V t PHY 2049: Physics II Recall more Inductors and inductance RL circuits Emf induced in a coil when the current changes Solenoids L = μ0n2lA I = (1-e-t/τ)V/R τ = L/R Energy in a magnetic field U = ½ Li2, u = B2/2μo In a coaxial cable the surface is most stressed. PHY 2049: Physics II Mutual induction E2 = -M21 di1 /dt M21 = M12 Coil in a coil M o N1 N 2 R2 2 2R1 PHY 2049: Physics II Three circuit elements: Capacitor Resistor Inductor V = Q/C V = IR I = dQ/dt V =- LdI/dt A general circuit is a combination of these elements. The analysis is a bit subtle. PHY 2049: Physics II PHY 2049: Physics II a. Long after the switch is closed In each case the current through R is shorted by the inductor. The inductor carries all of the current. b. Switch is opened. The equivalent resistances are (1) R (2) 2R and (3) R/2. Hence by current, the ranking is 3 1 2. c. Long after the switch is opened, there is no current anywhere. PHY 2049: Physics II PHY 2049: Physics II Problem 32: The area is v2t2, you can convince yourself that the two sides of the triangle are each 2vt/√2. Area A = ½ (2vt/√2)2 At t= 3 s. Flux = BA = 0.35x 5.22 x32 = 85.2 Wb Emf around: 56.8 V Emf = 18.93t., n = 1 PHY 2049: Physics II L1 L2 L3 L4 = = = = 30 50 20 15 mH mH mH mH PHY 2049: Physics II L1 L2 L3 L4 = = = = 30 50 20 15 mH mH mH mH L23 = 14.3 Leq = 30 +15 + 14.3 = 59.3 mH PHY 2049: Physics II PHY 2049: Physics II E =Ldi/dt i = Et/L t = iL/E = 1.5s PHY 2049: Physics II LC oscillations For a spring Ld2Q/dt2 +Q/C = = Md2x/dt2 +kx =0 Solution is x = A cosωt ω = √(k/m) Here ω = 1/ √(LC) When there is a natural frequency, there is violent response => resonance Quantity mech Mass m Spring k Position x electronic L 1/C Q PHY 2049: Physics II Inductors and inductance Currents are slowed down by Faraday’s law Eddy currents More later