Chapter-13 Outline

1
Chapter-13 Outline

• Electric Circuits and Electric Current, I
• 2 Ohms Law, ?V I R and Resistance, R.
• 3 Series and Parallel Circuits
• 4 Electric Energy and Power,
• 5 Alternating currents and Household Current

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How to Get the Bulb to Light?
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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.
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.
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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.
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Electric Current Is Analogous to Water Flow
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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 (1745-1827) who invented
the electric battery. 1 volt 1 J/C.
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Ohms Law
Georg Simon Ohm (1787-1854), a German physicist,
discovered Ohms 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. Ohms law gives a
relationship between the potential difference
(?V), current (I), and resistance (R) as
follows ?V I R
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What Is the Current?
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Circuits
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Series Circuit
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Parallel Circuit
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Example Box 13.2
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Use of Voltmeter and Ammeter
Voltmeter is connected across the resistance.
Ammeter is inserted into the circuit to measure
current.
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Electrical Energy
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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,
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Electric Power
Since the electrical energy is charge times
voltage (QV), the above equation becomes,
Since the current is charge flow per unit time
(Q/t), the above equation becomes,
Since V IR, the above equation can also be
written as,
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Killowatt-hour (kWh)
The SI unit of power is watt, after James Watt
(1736-1819), who developed steam engines.
Utility companies use the unit kilowatt-hour to
measure the electrical energy used by customers.
One kilowatt-hour, kWh is the energy consumed for
one hour at a power rate of 1 kW.
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Exercises
1. State Ohms 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?
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Electrical Power Transmission
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Power and Current Ratings of some common
Appliances
Appliance Power (W) Current (A)
Stove 6000 (220V) 27
Clothes dryer 5400 (220V) 25
Water heater 4500 (220V) 20
Clothes washer 1200 10
Dishwasher 1200 10
Iron 1100 9
Coffeemaker 1000 8
TV 100 0.8
AC adapter
INPUT AC 120 V, 60 Hz, 15 W OUTPUT DC 9V, 1A
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Alternating Current
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Alternating Voltage
Effective voltage 115 V
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Household Circuits
SP5 A 600-W toaster, a 1200-W iron, and a 500-W
food processor are all connected to the same
115-V household circuit, fused at 15 A.a. What
is the current drawn by each of these
appliances?b. If these appliances are all turned
on at the same time, will there be a problem.
Explain. c. What is the resistance of the
heating element in the iron?