DC & AC

Direct Current (DC)A flow of charge that flows in one

direction, even if the current moves in unsteady pulses

A battery produces direct current Electrons always move through

the circuit in the same direction from the negative terminal and toward the positive terminal

Direct Current (DC)

Direct Current (DC) Waveform

Time

Voltage

Time

Voltage

Alternating Current (AC)A flow of charge is alternating its

directions This is accomplished by

alternating the polarity of voltage at voltage source

Alternating Current (AC)

Alternating Current (AC) Waveform

Time

Voltage

Time

Voltage

Alternating Current (AC)Nearly all of the commercial AC

circuits in North America involves 120 V and 60 Hz

Europe adopted 220 V as their standard

Alternating Current (AC)The 120 V refers to the

“root-mean-square” (RMS) average of the voltage The actual voltage in a 120 V AC circuit varies between +170 V and – 170 V peaks. It delivers the same power as a 120 V DC circuit

Alternating Current (AC)Because most electric service in the

United States is three-wire: one wire at +120 V, one wire at 0 V (neutral), and the other wire at -120 V

Most of the appliance use +120V/-120 V and the neutral wires, producing 120 V. When use both +120V and -120 V wires, a 240 V is produced

AC-to-DC Conversion

Speed of Electrons

Thermal Speed vs. Drift Speed

Thermal Speed vs. Drift Speed

Thermal motion (random motion) speed inside a metal wire is about 1/200 the speed of light

Under electric field, the Drift Speed (net speed) is only about 0.01 cm/s

Speed of ElectronsThe electrons will collide with the

metallic ions in their path and transfer some kinetic energy to them

The extremely high speed of electricity is not due to the electrons but due to the signal. The signal is traveling at near high speed

Speed of ElectronsThe electrons inside the conductor

will shift forward (DC) or forward and backward (AC)

Why does the electric power company charge you money when they provide you AC electricity which no net electrons enter your home?

Speed of ElectronsThe AC outlets in your home do not

supply you electrons but supply you energy

The source of the electrons is the conducting circuit material itself

Electric Power

Electric PowerThe rate at which electrical energy

is converted into another form (mechanical energy, heat, or light) is called electric power

(Electric Power) = (Electric Energy) / (Time)

Unit: Watts (W)P =

Wt

Electric PowerElectric Power = Energy / Time

= (Charge/Time) x (Energy/Charge) = Current x Voltage

Unit: 1 watt = (1 ampere) x (1 volt)

P = I V

P =

= V I = I V

Wt = W

qqt

Electric PowerDerive the formulas of

1. P, I, R, 2. P, V, R

Electric PowerP = I V = I (I R) = I 2 R P = I V = ( ) V =

VR

V 2

R

P = I V = I

2 R = V 2

R

Electric PowerSince Energy / Time = Power,

Energy = Power x Time

Derive the formulas of 1. W, I, V, and t2. W, I, R, and t3. W, V, R and t

W = P t

Electric PowerSince Energy/Time = Power, so

Energy = Power x Time

W = P t = I V t = I 2 R t W = P t = I V t = ( ) t = t

V 2

R V 2

R

W = Pt = IVt = I2Rt = t

V 2

R

Electric PowerEnergy can be represented in

units of kilowatt-hours (kW·h)1 kW·h = 3.6 x 106 JA kilowatt is 1000 watt, and a

kilowatt-hour is the energy consumed in 1 hour at the rate of 1 kilowatt

Electric Power ExampleA light bulb is plugged into a 120-

volt outlet and has a 0.7 A current in it. What is the power rating of the light bulb?

Electric Power ExampleA light bulb is plugged into a 120-

volt outlet and has a 0.7 A current in it. What is the power rating of the light bulb?

P = I V = (0.7 A)(120 V)

= 84 W

Electric Power ExampleA heater uses 21 A when connected to

a 110-V line. If electric power costs 10 cents per kilowatt-hour in this location, what is the cost of running the heater for 13 hours?

Electric Power ExampleA heater uses 21 A when connected to

a 110-V line. If electric power costs 10 cents per kilowatt-hour in this location, what is the cost of running the heater for 13 hours?W = I V t = (21 A)(110 V)(13 hr)

= 30030 W-hr = 30.03 kW-hrCost = ($ 0.1 /kW-hr)(30.03 kW-hr) = $3.00

Electric Power ExerciseA 120 V outlet in Tony’s house is wired

with a circuit breaker on an 8 A line. a) If Tony tries use his newly-bought 1200-Watt hair dryer, will he trip the circuit breaker? b) What is the resistance of the hair dryer?

Electric Power ExerciseAlice likes to keep her 40-Watt front

porch light on at night time from 10 p.m. to 6 a.m., and Alice pays 8.00¢ per kWh, how much does it cost to run the light for this amount of time each week?

The End