AC Electricity

Gabrielse

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

Why three connectors?

radiocurrent

no current flowsexcept 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

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

-200

-150

-100

-50

0

50

100

150

200

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Vol

tage

(V

)0.017 seconds = 1/60 seconds

156 Volts

-156 Volts

Why is this called 110 Volts AC?

Alternating Current

Gabrielse

-200

-150

-100

-50

0

50

100

150

200

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Vol

tage

(V

)

0.017 seconds = 1/60 seconds

Vmax = +156 Volts

Vmin = -156 Volts

peak-to-peak voltage = 312 Volts

average voltage = 0 Volts

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?

S square firstM take the average (mean)R take square root last

-200

-150

-100

-50

0

50

100

150

200

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Vol

tage

(V

)

Square first (S in RMS)

Gabrielse

Squaring:

• makes all the negative parts positive.

• stretches the wave out.

-2500

2500

7500

12500

17500

22500

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Vol

tage

(V

)

The average isn’t zero anymore

Take the Average (Mean) of V2 (M in RMS)

Gabrielse

-2500

2500

7500

12500

17500

22500

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Vol

tage

(V

) Mean is how mathematicians say average.

Now we have a number but it is way bigger than what we measured.

Take the square root (R in RMS)

-2500

2500

7500

12500

17500

22500

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Volta

ge (

V)

VoltsVRMS 110

Gabrielse

USA: VRMS = 110 voltsMost of the rest of the world: VRMS = 220 volts

-200

-150

-100

-50

0

50

100

150

200

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

Time (s)

Voltage (

V)

zoom

Now we have a measure for AC Voltage, VRMS

Circuit breakers for a house

Gabrielse

main circuitbreaker200 ampservice

30 amp electricstove

15 and 20 amp circuit

breakers

Where does the electricity come from?

“Telephone” poles carry many servicesGabrielse

cable TV

power

telephone

“Telephone” pole in front of my parent’s house

Electrical power transmission is at high voltage

earth

powerline

Vrr: unavoidable resistance

in the transmission line

Gabrielse

I

R in townR >> r

Electrical power transmission is at high voltage

earth

powerline

Vrr: unavoidable resistance

in the transmission line

Gabrielse

I

R in town

Definition of electrical power: P = IVUse Ohm’s law: V = IRResult: P = IV = I (IR) P = I2R

R >> r

Electrical power transmission is at high voltage

earth

powerline

Vrr: unavoidable resistance

in the transmission line

Gabrielse

I

R in town

Power lost in the transmission line: I2r (becomes heat) minimize I

Power used in town IV make V big to get same IV

Definition of electrical power: P = IVUse Ohm’s law: V = IRResult: P = IV = I (IR) P = I2R

R >> r

Transformer on telephone poleGabrielse

Transformer on “telephone” pole in Burlington, MA.

High voltage transmission lineGabrielse

Photo by G. Gabrielse.

Notes

Gabrielse

6. Alternating and Direct Current a. Direct: proceeding in a straight line b. Direct Current (DC)

i. Current always goes in a straight(constant current) line. ii. Batteries provide DC voltage

c. Alternate: to take turns, to go back and forth i. Current goes back and forth between positive and negative directions.

ii. Wall outlets provide AC voltage

iii. AC voltage and current is measured using root, mean, square (RMS)

1. RMS voltage gives you the “effective” voltage d. Power (P)

i. P = IV = I2R

Neutral Hot

Ground