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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
How to Get the Bulb to Light?
How to Get the Bulb to Light?
Electric Current
The electric current is the amount of charge per unit time that
passes through a surface that is perpendicular to the motion of
the charges.
q
I .
t
The SI unit of electric current is the ampere (A), after the French
mathematician André Ampére (1775-1836). 1 A = 1 C/s. Ampere is a
large unit for current. In practice milliampere (mA) and microampere
(μA) are used.
Direction of Current Flow
Electric current is a flow of electrons. In a circuit, electrons
actually flow through the metal wires.
Conventional electric current is defined using the flow of
positive charges.
It is customary to use a conventional current I in the
opposite direction to the electron flow.
Direction of Current Flow
What Limits the Flow of Current?
What Limits the Flow of Current?
A: Resistance
Electric Current Is Analogous to Water Flow
Ohm’s Law
Georg Simon Ohm (1787-1854), a German physicist, discovered
Ohm’s law in 1826.
This is an experimental law, valid for both alternating current (ac)
and direct current (dc) circuits.
When you pass an electric current (I) through a resistance (R)
there will be a potential difference or voltage (V) created
across the resistance.
Ohm’s law gives a relationship between the voltage (V), current (I),
and resistance (R) as follows:
V=IR
What Is the Current?
Electromotive Force (emf)
The energy needed to run electrical devices comes from
batteries.
Within a battery, a chemical reaction occurs that transfers
electrons from one terminal (leaving it positively charged) to
another terminal (leaving it negatively charged).
Because of the positive and negative charges on the battery
terminals, an electric potential difference exists between them.
The maximum potential difference is called the electromotive
force* (emf) of the battery.
The electric potential difference is also known as the voltage, V.
The SI unit for voltage is the volt, after Alessandro Volta (17451827) who invented the electric battery. 1 volt = 1 J/C.
Circuits
Series Circuit
Parallel Circuit
Try This Box 13.2
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