Transcript 13 Electric Circuits

13 Electric Circuits
Chapter Outline
1 Electric Circuits and Electric Current
2 Ohm’s Law and Resistance
3 Series and Parallel Circuits
4 Electric Energy and Power
5 Alternating currents and Household Current
Everyday Phenomenon:
The Hidden Switch in Your Toaster
Use of Voltmeter
Voltmeter is connected across the resistance.
Use of Ammeter
Ammeter is inserted into the circuit to measure current.
Electrical Energy
Electrical Energy and Power
Our daily life depends on electrical energy. We use many
electrical devices that transform electrical energy into other
forms of energy. For example, a light bulb transforms electrical
energy into light and heat. Electrical devices have various power
requirements. Electrical power, P is defined as the electrical
energy transfer per unit time,
Energy
P
.
time
Electric Power:
Energy
P
.
time
Since the electrical energy is charge times voltage (QV), the
above equation becomes,
QV
P
.
t
Since the current is charge flow per unit time (Q/t), the
above equation becomes,
QV Q
P
 V  I V .
t
t
Since V = IR, the above equation can also be written as,
V2
P  IV  I R 
.
R
2
Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt (17361819), who developed steam engines.
joule
J
watt  W 
 .
sec ond s
Utility companies use the unit kilowatt-hour to measure
the electrical energy used by customers. One kilowatthour, kWh is the energy consumed for one hour at a
power rate of 1 kW.
Exercises
1. State Ohm’s law in an equation form in terms of voltage
and current.
2. Define power in an equation form in terms of voltage and
current.
3. When an appliance is plugged in a 120-volt outlet, it
draws a current of 8 amperes. Calculate the power of the
appliance.
4. If the above appliance is used 10 hours a day for 28 days
per month, and if the cost of electricity is 12 cents per
kilowatt-hour, how much does it cost to operate the
appliance for a year?
Electrical Power Transmission
AC adapter
INPUT: AC 120 V, 60 Hz, 15 W
OUTPUT: DC 9V, 1A
Alternating Current
Alternating Voltage
Effective voltage = 115 V
Household Circuits
Power and Current Ratings of
some common Appliances
Appliance
Stove
Clothes dryer
Water heater
Clothes washer
Dishwasher
Iron
Coffeemaker
TV
Power (W)
6000 (220V)
5400 (220V)
4500 (220V)
1200
1200
1100
1000
100
Current (A)
27
25
20
10
10
9
8
0.8
Everyday Phenomenon
Bimetallic Thermostat
CP5