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Lecture%2028%202220%20Spring%202009 - Reminders and...

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1 Reminders and Announcements HW 16 is due today; HW 17 is due next Tuesday; HW 18 is due next Friday • No quiz this week but some of recitation No quiz this week, but some of recitation time will be used for course evaluation. We will be grading the exams early next week. Help Room Hours (Physics 117): – M 4-8 pm, T 3-6:30, W 3-6:30, R 4:30-8, F 2-6 Resistors in an AC Circuit Consider a circuit consisting of an AC source and a resistor The AC source is symbolized by Δ v = Δ v R = Δ v max sin ω t Δ v R is the instantaneous voltage across the resistor Resistors in an AC Circuit, 2 The instantaneous current in the resistor is sin sin max max I R R v V i ω t ω t R R Δ Δ = = = The instantaneous voltage across the resistor is also given as Δ v R = I max R sin ω t Phasor Representation rms Current and Voltage The average current in one cycle is zero • The rms current is the average of importance in an AC circuit rms stands for root mean square Alternating voltages can also be discussed in terms of rms values 0707 2 max max I I . I rms = = 0707 2 max max . rms V V V Δ Δ = = Δ Power The rate at which electrical energy is dissipated in the circuit is given by P = i 2 R, where i is the instantaneous current The average power delivered to a resistor that carries an alternating current is 2 I av rms P R =
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2 Notes About rms Values rms values are used when discussing alternating currents and voltages because – AC ammeters and voltmeters are designed to AC ammeters and voltmeters are designed to read rms values – Many of the equations that will be used have the same form as their DC counterparts Which of the following statements might be true for a resistor connected to a sinusoidal AC source? 25% 25% 25% 25% 1. 2. 1 2 3 4 3. 4. 0 of 126 Inductors in an AC Circuit Kirchhoff’s loop rule can be applied and gives: 0 or 0 max , sin L v v di v L dt di v L V ω t dt Δ + Δ = Δ = Δ = = Δ Current in an Inductor The equation obtained from Kirchhoff's loop rule can be solved for the current max sin max cos V V i ω t dt ω t Δ Δ = = −
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