Subjects

Sections | |||
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Alternating Voltages and Currents | 18 mins | 0 completed | Learn |

Inductors in AC Circuits | 13 mins | 0 completed | Learn |

RMS Current and Voltage | 10 mins | 0 completed | Learn |

Phasors for Inductors | 7 mins | 0 completed | Learn |

Phasors | 21 mins | 0 completed | Learn |

Impedance in AC Circuits | 19 mins | 0 completed | Learn |

Resistors in AC Circuits | 10 mins | 0 completed | Learn |

Series LRC Circuits | 11 mins | 0 completed | Learn |

Phasors for Resistors | 8 mins | 0 completed | Learn |

Resonance in Series LRC Circuits | 10 mins | 0 completed | Learn |

Capacitors in AC Circuits | 17 mins | 0 completed | Learn |

Power in AC Circuits | 6 mins | 0 completed | Learn |

Phasors for Capacitors | 8 mins | 0 completed | Learn |

Concept #1: Power in AC Circuits

**Transcript**

Hey guys, in this video we're going to talk about power in AC circuits. What elements are emitting power, what elements are not emitting power, what the average power is and things like that. Alright, let's get to it. In AC circuits the only element to have an average power not equal to zero is, what do you guys think it is? It's the resistor. This is because whatever energy enters a capacitor or an inductor equals the energy that leaves it. Capacitors and inductors are elements that store energy. A capacitor stores charge to store electric potential energy and an inductor stores current to store a magnetic potential energy but either way they only store energy, the resistor is what's actually bleeding from the circuit. So if you were to plot the other element's power as a function of time you would see that on average all the peaks would cancel all the valleys and you would have no power on average.

The maximum power of a resistor is going to be that maximum voltage across the resistor times the maximum current. Now as a function of time you can say that the power equals the current as a function of time squared times the resistance. This gives us the following graph of power and current versus time. You can see that current, like we expect, is going to have no average value because anything that's positive any peak above the horizontal cancels with the negative peaks below the horizontal but power stays above the horizontal it just bounces above the horizontal so it's always positive and therefore it has nothing to cancel it out so on average it is absolutely not zero. The average power emitted by an AC circuit is going to be one half of the maximum power, this is because the power's peaks are completely symmetric so the average is going to be one half of the maxima so it's going to be one half the V max times I max and if you substitute the maximum values for their RMS values you find that this actually equals VRMS times IRMS. So the average power depends upon the RMS voltage and the RMS current which is an interesting result. We're not talking about the RMS power here we're talking about the true average of the power and it doesn't depend upon the average of the voltage or the average of the current. It can't because those are zero but it does interestingly enough depend upon the RMS values of the voltage and the RMS value of the current.

Let's do a quick example. An AC source operating at a maximum voltage of 120 volts is connected to a 10 ohm resistor. What is the average power emitted by the circuit? Is it equivalent to the RMS power which would be IRMS squared times R? Don't forget the average power we're just going to say is one half times the maximum voltage times the maximum current. So first what's the maximum current? Well that's just the maximum voltage which we know divided by R which is 120 volts that's the maximum voltage divided by 10 ohms which is 12 amps. So we can say that the average power is one half times 120 volts right which is the maximum power, sorry, the maximum voltage times 12 amps which is the maximum current and that whole thing is going to equal 720 watts.

Now if I take this maximum current I can then say that the RMS current is the maximum current over the square root of 2 which is 12 over the square 2 which is going to be 8.49 amps. That I can take and I can find IRMS squared times R is 8.49 squared times 10 which is indeed 720 watts so yes that does match up and this is something we touched upon earlier that the average power depends upon RMS values so this is a form of power emitted by a resistor we should absolutely be able to just plug in RMS values for it and get the average power out. Alright guys, that wraps up our discussion on power and AC circuits. Thanks for watching.

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Concept #1: Power in AC Circuits

Consider the AC circuit in which a capacitor and a lightbulb are connected in series with the AC source. The frequency of the AC source is adjusted while its voltage amplitude is held constant. The lightbulb will glow the brightest at which of the following?
A) same at all frequencies
B) high frequencies
C) low frequencies
D) zero frequency (DC)

A series RLC circuit with L = 25 mH, C = 0.8 μF and R = 7Ω is driven by a generator with a maximum emf of 12V and a variable angular frequency ω. If the inductance L is doubled (keeping all other quantities given, including the generator frequency, the same), compared to the power dissipated when the circuit was at resonance, the power now dissipated in the resistor willa) increase.b) decrease.c) stay the same.

In the circuit shown below, the AC generator supplies an EMF of the form ε = 15sin (100t - π/3) volts. A student measures the current to be I = 6 sin(100t) amps. Thus, the generator voltage lags the current by π/3 radian (i.e. 60°). The average power delivered by the AC generator is:a. 22.5 Wb. 45.0 Wc. 39.0 W

A series circuit consists of a 15-Ω resistor, a 25-mH inductor, and a 35-μF capacitor. If the frequency is 100 Hz the power factor is:A. 0.89B. 0C. 1.0D. 0.20E. 0.45

What is the rms value Vrms of the voltage plotted in the graph?Part BWhen a lamp is connected to a wall plug, the resulting circuit can be represented by a simplified AC circuit, as shown in the figure. Here the lamp has been replaced by a resistor with an equivalent resistance R = 120 Ω. What is the rmsvalue Irms of the current flowing through the circuit?What is the average power Pavg dissipated in the resistor?

You buy a 75-W lightbulb in Europe, where electricity is delivered to homes at 240 V. If you use the lightbulb in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V Bulbs? [ Hint: assume roughly that brightness is proportional to power consumed]

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