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AC Electricity Gabrielse

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DC voltage from a battery Gabrielse Measure the voltage of a battery over and over again. A “DC” voltage does not change in time. Direct Current

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Why three connectors? radio current no current flows except in emergency need high voltage insulation metal shield For some devices the metal shield is left off the insulation had better be good the different sizes of pins can be used to make sure high and low voltages go to the right places Gabrielse

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Measure “Wall” Voltage Over Time The cycle repeats 60 times per second = 60 Hertz Graph this voltage (Hot) 0 Volts (Ground) 0 Volts (Neutral) Gabrielse seconds = 1/60 seconds 156 Volts -156 Volts

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Why is this called 110 Volts AC? Alternating Current Gabrielse seconds = 1/60 seconds V max = +156 Volts V min = -156 Volts peak-to-peak voltage = 312 Volts average voltage = 0 Volts

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Solution: Use (RMS) Root Mean Squared Voltage (do it backwards) Average = 0 Volts Not a good measure Gabrielse Problem: How to measure a voltage that changes with time? Ssquare first Mtake the average (mean) Rtake square root last

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Square first (S in RMS) Gabrielse Squaring: makes all the negative parts positive. stretches the wave out. The average isn’t zero anymore

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Take the Average (Mean) of V 2 (M in RMS) Gabrielse Mean is how mathematicians say average. Now we have a number but it is way bigger than what we measured.

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Take the square root (R in RMS) Gabrielse USA: V RMS = 110 volts Most of the rest of the world: V RMS = 220 volts zoom Now we have a measure for AC Voltage, V RMS

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Circuit breakers for a house Gabrielse main circuit breaker 200 amp service 30 amp electric stove 15 and 20 amp circuit breakers

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Where does the electricity come from?

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“Telephone” poles carry many services Gabrielse cable TV power telephone “Telephone” pole in front of my parent’s house

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Electrical power transmission is at high voltage earth power line V r r: unavoidable resistance in the transmission line Gabrielse I R in town R >> r

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Electrical power transmission is at high voltage earth power line V r r: unavoidable resistance in the transmission line Gabrielse I R in town Definition of electrical power: P = IV Use Ohm’s law: V = IR Result: P = IV = I (IR) P = I 2 R R >> r

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Electrical power transmission is at high voltage earth power line V r r: unavoidable resistance in the transmission line Gabrielse I R in town Power lost in the transmission line: I 2 r (becomes heat) minimize I Power used in town IV make V big to get same IV Definition of electrical power: P = IV Use Ohm’s law: V = IR Result: P = IV = I (IR) P = I 2 R R >> r

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Transformer on telephone pole Gabrielse Transformer on “telephone” pole in Burlington, MA.

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High voltage transmission line Gabrielse Photo by G. Gabrielse.

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Notes Gabrielse

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