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Power and AC

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**Energy Transformation**

Electric energy can be transformed into other more useful forms Power is the rate of energy transformation P = ΔPE/t ΔPE = QV P = QV/ t I = Q/ t P = IV units – Watt (W)

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Example What is the resistance of a 40 W car headlight designed for a 12V car battery?

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**KiloWatt - Hour Electric companies charge by the amount of energy used**

Larger unit than Joule is used – called the kilowatt-hour (kW•hr) 1 kW•hr = 3.6 x 106 J

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Example A heater draws 15.0A on a 120V line. How much power does it use and how much does it cost per month (30 days) to operate if it runs 3.0 hours per day and the electric company charges 10.5 cents per kW•hr?

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Example A lightning bolt transfers 109 J of energy across a potential difference of 5 x 107V in .2 s. Find the charge transferred, the current, and the power of the bolt.

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Overload All wires have resistance and some of the electric energy is changed to heat Overload is when wire carries more than the safe amount of current Fuses and circuit breakers protect from overload Causes of overload: too many devices drawing too much current, “short” which is crossed wiring

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Example A 100W lightbulb, 1800W heater, 350W stereo, and 1200W television are all connected to the same circuit which has a 20A fuse. Can all of these be operating at the same time?

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**Alternating Current (AC)**

Direct current (DC) is produced from a battery – flows steadily in one direction Electric generating plants produce AC current – this reverses direction many times per second AC DC

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**Sinusoidal Nature Current and voltage produced in AC are sinusoidal**

V = Vosin 2πft I = Io sin 2πft Vo and Io are called peak voltage and current F is the frequency of oscillation (60 Hz)

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**Power Even though average current is zero, power is always delivered**

P = Io2R Since Io is squared, power is always +

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**Average Current, Voltage, Power**

Root mean square averages These are the average current, voltage, and power delivered by an ac line Iavg = .5 Io2 = .707 Io Vavg = .707 Vo Pavg = (Vavg)2/ R Pavg = (Iavg)2R

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**Examples What is the peak voltage of a 120V ac line?**

A stereo receiver is capable of average power output of 100W into an 8Ω loudspeaker. What is the rms voltage and rms current fed to the speaker a) at max power of 100W and b) at 1.0 W

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