4 A sinusoidal current or voltage is usually referred to as an alternating current (or AC) or voltage and circuits excited by sinusoids are called ac circuits. Sinusoids are important for several reasons: 1.Nature itself is characteristically sinusoidal (the pendulum, waves, etc.). 2.A sinusoidal current or voltage is easy to generate. 3.Through Fourier Analysis, any practical periodic signal (one that repeats itself with a period T) can be represented by a sum of sinusoids. 4.A sinusoid is easy to handle mathematically; its derivative and integral are also sinusoids. 5.When a sinusoidal source is applied to a linear circuit, the steady-state response is also sinusoidal, and we call the response the sinusoidal steady-state response.
5 Complex Numbers What is the solution of X 2 = -1 real imaginary 1 j -j Complex Plane Note:
6 Complex Numbers real imag A Euler's equation: Complex Plane measured positive counter-clockwise Note:
7 Sinusoidal Functions and Phasors http://www.kineticbooks.com/physics/trialp se/33_Alternating%20Current%20Circuits/1 3/sp.html
10 Comparing Sinusoids Note: positive angles are counter-clockwise
11 KVL This is a differential equation we must solve for i(t). How? Guess a solution and try it! Note that It turns out to be easier to use as the forcing function rather than and then take the real part of the solution. This is because which will allow us to convert the differential equation to an algebraic equation. Let's see how. VRVR VLVL + - - +
12 Solve the differential equation for i(t). Instead, solve and take the real part of the solution Guess that Divide by and substitute in (1) (1) VRVR VLVL + - - +
13 Solve the differential equation for i(t). (1) (2) Rearrange (3) Recall that (3) Therefore (4) can be written as (4) (5) VRVR VLVL + - - +
14 Solve the differential equation for i(t). (1) Therefore, from (5) (5) (2) (6) Substituting (6) in (2) and taking the real part VRVR VLVL + - - +
15 Phasors XMXM Recall that when we substituted cancelled out. We are therefore left with just the phasors A phasor is a complex number that represents the amplitude and phase of a sinusoid. in the differential equations, the
16 Solve the differential equation for i(t) using phasors. (1) (2) Substitute (2) and (3) in (1) Divide by and solve for I (3) (4) (5) VRVR VLVL + - - +
17 By using phasors we have transformed the problem from solving a set of differential equations in the time domain to solving a set of algebraic equations in the frequency domain. The phasor solutions are then transformed back to the time domain.
18 Impedance Note that impedance is a complex number containing a real, or resistive component, and an imaginary, or reactive, component. Units = ohms VRVR VLVL + - - +
22 Re Im Re Im Re Im V V V V V V I I I I I I I in phase with V I lags V I leads V
23 Expressing Kirchoff’s Laws in the Frequency Domain KVL: Let v 1, v 2,…v n be the voltages around a closed loop. KVL tells us that: Assuming the circuit is operating in sinusoidal steady-state at frequency we have: or Phasor Since Which demonstrates that KVL holds for phasor voltages.
24 KCL: Following the same approach as for KVL, we can show that Where I k is the phasor associated with the k th current entering a closed surface in the circuit. Thus, both KVL and KCL hold when working with phasors in circuits operating in sinusoidal steady-state. This implies that all of the circuit analysis methods (mesh and nodal analysis, source transformations, voltage & current division, Thevenin equivalent, combining elements, etc,) work in the same way we found for resistive circuits. The only difference is that we must work with phasor currents & voltages and the impedances &/or admittances of the elements.