Fundamentals of Physics 8/e 31 - Alternating Fields and Current D. Halliday, R. Resnick, and J. Walker 國立彰化師大教學卓越計畫 理學院普通物理整合型教材 主持人：郭艷光院長

彰化師大教學卓越－普物教材－理學院長郭艷光主持 Outline 31-1 What Is Physics? 31-2 LC Oscillations, Qualitatively 31-3 The Electrical–Mechanical Analogy 31-4 LC Oscillations, Quantitatively 31-5 Damped Oscillations in an RLC Circuit 31-6 Alternating Current 31-7 Forced Oscillations 31-8 Three Simple Circuits 31-9 The Series RLC Circuit 31-10 Power in Alternating–Current Circuits 31-11 Transformers 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 31-1 What Is Physics? We next turn to the associated applied physics, in which the energy stored in one location can be transferred to another location so that it can be put to use. In most parts of the world, electrical energy is transferred not as a direct current but as a sinusoidally oscillating current (alternating current, or ac). 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-2 LC Oscillations, Qualitatively 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-3 The Electrical–Mechanical Analogy This is a homogeneous, second order, linear differential equation which we have encountered previously. We used it to describe the simple harmonic oscillator (SHO) with solution: x(t)=Xcos(ωt+φ) 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-4 LC Oscillations, Quantitatively The energy of the electric field has a maximum value of Q2/2C at t=0, T/2, T, 3T/2… The energy of the magnetic field has a maximum value of Q2/2C at t=T/4, 3T/4, 5T/4… Note: When UE is maximum UB is zero, and vice versa 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-4 LC Oscillations, Quantitatively 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-5 Damped Oscillations in an RLC Circuit 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 31-6 Alternating Current “direct current” or “dc” “alternating current ” or “ac” 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 31-7 Forced Oscillations Whatever the natural angular frequency ω of a circuit may be, forced oscillations of charge, current, and potential difference in the circuit always occur at the driving angular frequency ωd. 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits A convention 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits A resistive load 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits A capacitive load O 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits O An inductive load The current lags behind the voltage by a quarter of a period. The voltage and current areout of phase by 90 ˚. 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-8 Three Simple Circuits Circuit element Average Power Reactance Phase of current Voltage amplitude Resistor R Current is in phase with the voltage Capacitor C Current leads voltage by a quarter of a period Inductor L Current lags behind voltage by a quarter of a period 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-9 The Series RLC Circuit 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-9 The Series RLC Circuit XL > XC → ψ > 0 The current phasor lags behind the generator phasor. The circuit is more inductive than capacitive. XC > XL → ψ < 0 The current phasor leads ahead of the generator phasor. The circuit is more capacitive than inductive. XC = XL → ψ = 0 The current phasor and the generator phasor are in phase. 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-9 The Series RLC Circuit Resonance 彰化師大教學卓越－普物教材－理學院長郭艷光主持

31-10 Power in Alternating-Current Circuits 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 31-11 Transformers Power Station Transmission lines Erms =735 kV , I rms = 500 A home 110 V T1 T2 Step-up transformer Step-down transformer R = 220Ω 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 31-11 Transformers If Ns > Np → Vs > Vp We have what is known a “step up” transformer. If Ns < Np → Vs < Vp We have what is known a “step down” transformer. 彰化師大教學卓越－普物教材－理學院長郭艷光主持

彰化師大教學卓越－普物教材－理學院長郭艷光主持 End of chapter 31! 彰化師大教學卓越－普物教材－理學院長郭艷光主持