Transcript 13 Electric Circuits

Chapter-13 Outline
1 Electric Circuits and Electric Current, I
2 Ohm’s Law, V = I R; and Resistance, R.
3 Series and Parallel Circuits
4 Electric Energy and Power, P = IV
5 Alternating currents and Household Current
q
I .
t
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.
Electromotive Force (emf) or
Voltage
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) or simple the voltage, V of the battery.
The SI unit for voltage is the volt, after Alessandro Volta (17451827) who invented the electric battery. 1 volt = 1 J/C.
Electrical Resistance
When electric current flows through a metal wire there exists a
hindrance to the flow, known as electrical resistance.
This is because as the electrons move through they will collide
with the atoms of the conductor.
The SI unit of resistance is the ohm (Ω), after Georg Simon Ohm
(1787-1854), a German physicist, who discovered Ohm’s law.
A resistor is a material that provides a specified resistance in an
electric circuit.
Resistance is the primary component in many electrical devices.
Examples: Toaster, Heater, Iron, incandescent light bulb, etc...
Electrical Quantities and their Units
Quantity
Symbol
Unit
Name
Unit
Abbreviation
Current
I
ampere
A
Voltage or
emf
V
volt
V
Resistance
R
ohm
Ω
Power
P
watt
W
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 (ΔV) or voltage (V) created
across the resistance.
Ohm’s law gives a relationship between the potential difference
(ΔV=V), current (I), and resistance (R) as follows:
V=IR
What Is the Current?
Circuits
Series Circuit and Parallel Circuit
Use of Voltmeter and Ammeter
Voltmeter is connected across the resistance.
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?
Electricity: Generation and Transmission
http://www.duke-energy.com/about-energy/generatingelectricity/nuclear.asp
Transformers
A transformer is a device for increasing or decreasing an ac
voltage.
A transformer consists of a primary coil and a secondary coil, both
wound on an iron core. The changing magnetic flux produced by
the current in the primary coil induces an emf in the secondary
coil. At the far right is the symbol for a transformer.
Power and Current Ratings of
some common Appliances
AC adapter
INPUT: AC
120 V, 60 Hz,
15 W
OUTPUT: DC
9V, 1A
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
Alternating Current and Voltage
Effective voltage
= 115 V
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?