chapter21 - Alternating Current Circuits and...

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Alternating Current Circuits and Electromagnetic Waves A. Resistors in an AC Circuit 1. AC circuit – combinations of circuit elements and an AC generator or an AC source, which provides the alternating current; represented by 2. Current and voltage vary identically with time a) Current and voltage reach their maximum values. They are in phase. b) Average value of current over one cycle is zero. Current is maintained in one direction for the same amount of time and at the same magnitude as in the opposite direction. c) Direction of current has no effect on the behavior of the resistor in the circuit. Collisions result in an increase in resistor’s temperature regardless of direction.
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3. Average value of current is the rms current – the direct current that dissipates the same amount of energy in a resistor that is dissipated by the actual AC. 4. Alternating voltages are discussed in terms of rms voltages 5. A resist impedes the current in an AC circuit just like DC circuit, therefore Ohm’s law is valid. B. Capacitors in an AC Circuit 1. DC source (battery) –As charge accumulates on the plates, the voltage across increases, opposing the current. After some time, the current approaches zero therefore the capacitor impedes the current. 2. AC source – No charge on the capacitor and large current starts out. Resistance between the two wires causes the current to decrease as the voltage increases. The voltage reaches a point and the current begins to increase in the opposite direction. Voltage decreases. Repeats the cycle. 3. In a purely resistive circuit, current and voltage are always in step. a) Except when capacitors are involved. The voltage across a capacitor always lags the current by 90 ° . The impeding effect is expressed in terms of a factor called the capacitive reactance X c .
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Units - ohms C. Inductors in an AC Circuit 1. Back emf impedes the current in the circuit 2. The effective resistance of the coil in an AC circuit is measured by a quantity called the inductive reactance, X L . a) Related to Ohm’s Law: 3. The voltage across an inductor always leads the current by 90 ° . D. The RLC Series Circuit 1. When inductor, capacitor, and resistor are combined: current is the same at al points and voltage across each element may or may not be in phase with current. 2. Relationship between instantaneous voltage and instantaneous current: a) v R across the resistor is in phase with the instantaneous current. b) v L across the inductor leads the current by 90. c)
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chapter21 - Alternating Current Circuits and...

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