Transcript Electric Power

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
Voltage
Time
Voltage
Time
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
Voltage
Time
Voltage
Time
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 Electrons
 The 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 Electrons
 The 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 Electrons
 The 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 Power
 The 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)
W
P= t
Electric Power
 Electric Power = Energy / Time
= (Charge/Time) x (Energy/Charge)
= Current x Voltage
P=
W
t
=
W
q
q
t
=V I = IV
P = IV
 Unit: 1 watt = (1 ampere) x (1 volt)
Electric Power
 Derive the formulas of
1. P, I, R,
2. P, V, R
Electric Power
P = I V = I (I R) = I 2 R
V
V2
P = IV = (
)V =
R
R
2
V
P = IV = I2R =
R
Electric Power
 Since Energy / Time = Power,
Energy = Power xTime
W=Pt
 Derive the formulas of
1. W, I, V, and t
2. W, I, R, and t
3. W, V, R and t
Electric Power
 Since Energy/Time = Power, so
Energy = Power xTime
W = P t = IV t = I2R t
V2
V2
W = P t = IV t = (
)t=
t
R
R
2
V
W = Pt = IVt = I2Rt =
t
R
Electric Power
 Energy can be represented in units of
kilowatt-hours (kW·h)
 1 kW·h = 3.6 x 106 J
 A kilowatt is 1000 watt, and a kilowatthour is the energy consumed in 1 hour at
the rate of 1 kilowatt
Electric Power Example
 A 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 Example
 A 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=IV
= (0.7 A)(120 V)
= 84 W
Electric Power Example
 A 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 Example
 A 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=IVt
= (21 A)(110 V)(13 hr)
= 30030 W-hr
= 30.03 kW-hr
Cost = ($ 0.1 /kW-hr)(30.03 kW-hr) = $3.00
Electric Power Exercise
 A 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 Exercise
 Alice 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