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lecture 6 - Chapter 20 Electric Circuits Outline Section...

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Electric Circuits Chapter 20
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Outline Section 20.05: Alternating Current Section 20.09: Internal Resistance Section 20.10: Kirchhoff’s Rules Section 20.11: Measuring Voltages and Currents Section 20.12: Capacitors in Series and Parallel Section 20.13: RC Circuits
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Alternating Currents (AC) The voltage and current change polarity and magnitude periodically V = V0Sin2 π ft; Ι = Ι0 Sin2 π ft Peak voltage frequency Peak current Power (P) = VI = I0V0Sin22 π ft Average power = I0V0/2 Root- mean-square (rms) values Irms = I0/ ; 2 = /√ Vrms V0 2
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20.9 Internal Resistance So far we’ve just considered batteries and generators as contributing their emf to a circuit. In reality, they too have some resistance. This is called internal resistance, r . In batteries it’s due to the chemicals, and in generators it’s due to wire resistance. So, if a battery is connected to a load resistor, R , then the internal resistance, r , is in series with the load: Notice: R and r are in series! Thus, the voltage across the battery (known as the terminal voltage, VT ) is less than the full voltage V , since some is lost across r . VT Thus, r T V V V - = Terminal voltage Battery voltage Voltage drop across the internal resistance, r
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Kirchhoff’s Rules In many circuits, applying the series or parallel methods is not sufficient to analyze them. There are two other rules we can use called Kirchhoff’s Rules : 1. Junction Rule – Current into a junction has to equal current out. It is based on conservation of charge. I flows into junction, and I 1 and I 2 flow out, thus: I I2 I1 2 1 I I I + = 2. Loop Rule – Around any closed circuit loop, the sum of the potential (voltage) drops has to equal the sum of the potential rises.
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