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Physics 6B Inductors and AC circuits Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Inductance Mutual Inductance of two coils: Some of the magnetic flux through one coil also passes through the other coil, inducing a voltage. Inductance is magnetic flux/current. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Self-Inductance Changing current through the wires in a coil will induce a voltage that opposes the CHANGE in the current. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB R-L Circuit When connected in a circuit with a resistor, an inductor will have the effect of slowing down changes in the current through the resistor. When the current is steady (the switch has been closed for a long time), the inductor has no effect, but there is potential energy stored in the inductor. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB R-L Circuit If switch S1 is closed in the circuit, current will begin to flow through the resistor and inductor as shown. This increasing current will induce current to flow the opposite direction, slowing the growth of the current. We can write down a formula for the current as a function of time: π π‘ = π 1 − π− π π πΏ π‘ πΏ The quantity π = π is called the “time constant” for this exponential decay. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB R-L Circuit Once the current reaches a steady value we can flip the switches, opening S1 and closing S2. Then current will keep flowing for while as the inductor opposes this decreasing current. A similar formula describes this decaying current as a function of time: π π‘ = πΌ0 π − π πΏ π‘ Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: A 35.0V battery with negligible internal resistance, a 50.0Ω resistor and a 1.25mH inductor are connected in series with an open switch. The switch is suddenly closed. (a) How long after closing the switch will the current through the inductor reach half of its maximum value? (b) How long after closing the switch will the energy stored in the inductor reach half its maximum value? Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: A 35.0V battery with negligible internal resistance, a 50.0Ω resistor and a 1.25mH inductor are connected in series with an open switch. The switch is suddenly closed. (a) How long after closing the switch will the current through the inductor reach half of its maximum value? (b) How long after closing the switch will the energy stored in the inductor reach half its maximum value? As soon as the switch is closed, current begins to flow around the circuit, increasing toward a maximum value given by Ohm’s Law. Here is the formula: π π‘ = π 1 − π− π π πΏ π‘ We want to find the time when the current is half of the maximum. π π‘ = 1 2 π π = 1 − π− π π − π πΏ π‘ = ln 1 2 π πΏ →π‘=− β ln π πΏ π‘ 1 2 1 2 → = 1− π π− πΏ π‘ 1.25 β 10−3 π» =− β ln 50Ω 1 2 → π π− πΏ π‘ 1 = 2 = 1.73 β 10−5 π = 17.3ππ For part b) we want the energy to be half of its maximum, so use the energy formula: π = 12πΏπ 2 = 12 12πΏπΌ 2 → π(π‘) = πΌ 2 Using the formula for current again: 1 2 = 1 − π− π πΏ π‘ → π‘ = 30.7ππ Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB L-C Circuit A circuit containing a capacitor and an inductor will exhibit an oscillating current, with potential energy transferring back and forth. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB L-C Circuit The oscillations in an L-C circuit should look familiar. This situation is directly analogous to an undamped mass-spring system that we saw previously. All of the formulas we developed for that case are repeated here, with charge, q, taking the place of displacement, x. The capacitor is related to the spring constant, and the inductance is like mass. To add in the damping, we just include a resistor in the circuit… Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: In an L-C circuit, L=85.0mH and C=3.20μF. During the oscillations the maximum current in the inductor is 0.850mA. (a) What is the maximum charge on the capacitor? (b) What is the magnitude of the charge on the capacitor at an instant when the current in the inductor has magnitude 0.500mA? (c) How long does it take for the capacitor to go from maximum charge to zero charge? Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: In an L-C circuit, L=85.0mH and C=3.20μF. During the oscillations the maximum current in the inductor is 0.850mA. (a) What is the maximum charge on the capacitor? (b) What is the magnitude of the charge on the capacitor at an instant when the current in the inductor has magnitude 0.500mA? (c) How long does it take for the capacitor to go from maximum charge to zero charge? (a) What is the maximum charge on the capacitor? We can use energy for this if we want to. When all the energy is in the inductor it will have maximum current. When all the energy is in the capacitor it will have maximum charge. ππππ = 12πΏπΌ 2 = 12(85 β 10−3 π»)(0.85 β 10−3 )2 = 0.307ππ½ πππππ = 2 1π 2 πΆ = 0.307ππ½ → π = 44.3ππΆ Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: In an L-C circuit, L=85.0mH and C=3.20μF. During the oscillations the maximum current in the inductor is 0.850mA. (a) What is the maximum charge on the capacitor? (b) What is the magnitude of the charge on the capacitor at an instant when the current in the inductor has magnitude 0.500mA? (c) How long does it take for the capacitor to go from maximum charge to zero charge? b) We can use energy again, or we can use the formula for the charge as a function of time. πΈπ‘ππ‘ππ = 1 2 πΏπ 2 + 2 1π 2 πΆ Total energy can be found from max current or max charge. Should be the same either way. We can solve for the charge when the current is as given: π2 1 1 −8 −3 −3 2 πΈπ‘ππ‘ππ = 3.07 β 10 π½ = 2 85 β 10 π» 0.5 β 10 π΄ + 2 → π = 3.58 β 10−7 πΆ = 358ππΆ 3.2 β 10−6 πΆ Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB Example: In an L-C circuit, L=85.0mH and C=3.20μF. During the oscillations the maximum current in the inductor is 0.850mA. (a) What is the maximum charge on the capacitor? (b) What is the magnitude of the charge on the capacitor at an instant when the current in the inductor has magnitude 0.500mA? (c) How long does it take for the capacitor to go from maximum charge to zero charge? To go from no charge to fully charged is a quarter of a cycle, so we need to find the period of the oscillation. We have a formula for angular frequency: π= 1 = 1917πππ π πΏπΆ Rearrange this to get the period, then divide by 4: π= 2π = 0.00328π → 14π = 0.82ππ π Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB L-R-C Series Circuit The AC voltage source drives the oscillating current in the circuit. Frequency of the source will affect the overall impedance (Z) of the circuit. π= 1 − ππΏ ππΆ 2 + π 2 At resonance the impedance will be smallest, and the current will be largest. This happens when the capacitor and inductor terms balance out. We can solve for the resonant frequency: ππΏ = 1 → ππππ = ππΆ 1 πΏπΆ Notice that at this frequency the impedance just equals the resistance R. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB L-R-C Series Circuit If there is a driving voltage V(t) our equation for the circuit looks like this: βπ(π‘) π π‘ πΏ + π(π‘)π + = π(π‘) βπ‘ πΆ Solving this equation gives a general solution for i(t): π(π‘) = π= π0 π ππ ππ‘ + π π 1 − ππΏ ππΆ π = arctan 2 + π 2 1 1 − ππΏ π ππΆ Z is called IMPEDANCE. It can be used in place of resistance for an AC circuit. This is the phase angle. You can use it to determine whether the circuit voltage leads or lags the current. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB