2Energy Transformation Electric energy can be transformed into other more useful formsPower is the rate of energy transformationP = ΔPE/tΔPE = QVP = QV/ tI = Q/ tP = IV units – Watt (W)
3ExampleWhat is the resistance of a 40 W car headlight designed for a 12V car battery?
4KiloWatt - 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
5ExampleA 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?
6ExampleA 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.
7OverloadAll wires have resistance and some of the electric energy is changed to heatOverload is when wire carries more than the safe amount of currentFuses and circuit breakers protect from overloadCauses of overload: too many devices drawing too much current, “short” which is crossed wiring
8ExampleA 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?
9Alternating Current (AC) Direct current (DC) is produced from a battery – flows steadily in one directionElectric generating plants produce AC current – this reverses direction many times per secondACDC
10Sinusoidal Nature Current and voltage produced in AC are sinusoidal V = Vosin 2πftI = Io sin 2πftVo and Io are called peak voltage and currentF is the frequency of oscillation (60 Hz)
11Power Even though average current is zero, power is always delivered P = Io2RSince Io is squared, power is always +
12Average Current, Voltage, Power Root mean square averagesThese are the average current, voltage, and power delivered by an ac lineIavg = .5 Io2 = .707 IoVavg = .707 VoPavg = (Vavg)2/ RPavg = (Iavg)2R
13Examples 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