Gabrielse AC Electricity Gabrielse DC voltage from a

Gabrielse AC Electricity

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

Gabrielse Why three connectors? need high voltage insulation current no current flows except in emergency radio 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 Measure “Wall” Voltage Over Time Graph this voltage (Hot) The cycle repeats 60 times per second = 60 Hertz 0 Volts (Neutral) 0 Volts (Ground) 0. 017 seconds = 1/60 seconds 156 Volts -156 Volts

Gabrielse Why is this called 110 Volts AC? Alternating Current 0. 017 seconds = 1/60 seconds Vmax = +156 Volts average voltage = 0 Volts Vmin = -156 Volts peak-to-peak voltage = 312 Volts

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

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

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

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

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

Where does the electricity come from?

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

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

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

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

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

Gabrielse High voltage transmission line Photo by G. Gabrielse.

Gabrielse Notes