Electricity and Magnetism

1
Electricity and Magnetism

• 1 Static electricity
• 2 Electric Circuits and Electric Current
• 3 Ohms Law and Resistance
• 4 Series and Parallel Circuits
• Electric Energy and Power
• Alternating currents and Household Current
• Electromagnetic Induction

2
Lightning
3
Atom
4
Charging Ebonite Rod Fur
5
Charging an Object
Ebonite rod Fur ? Negatively charged ebonite
rod Glass rod Silk ? Positively charged
glass rod
6
LAW OF CONSERVATION OF ELECTRIC CHARGE
During any process, the net electric charge of an
isolated system remains constant (is conserved).
7
Like charges repel and unlike charges attract
each other.
8
Conductors and Insulators
Substances that readily conduct electric charge
are called electrical conductors. Conductors have
free electrons, which conduct the electricity.
Examples Metals such as copper, aluminum,
silver, and gold. Materials that conduct electric
charge poorly are known as electrical insulators.
Examples Rubber, plastics, and wood.
9
Charging by Contact and by Induction
An object can be charged by two methods -By
contact. -By induction.
10
Charging By Contact
11
Charging By Induction
12
How to Get the Bulb to Light?
13
How to Get the Bulb to Light?
14
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.
15
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.
16
Direction of Current Flow
17
What Limits the Flow of Current?
18
What Limits the Flow of Current?A Resistance
19
Electric Current Is Analogous to Water Flow
20
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 voltage (V), current
(I), and resistance (R) as follows V I R
21
What Is the Current?
22
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.
23
Circuits
24
Series Circuit
25
Parallel Circuit
26
Electrical Energy
27
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,
28
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,
29
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.
30
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?
31
Electrical Power Transmission
32
AC adapter
INPUT AC 120 V, 60 Hz, 15 W OUTPUT DC 9V, 1A
33
Alternating Current
34
Alternating Voltage
Effective voltage 115 V
35
Household Circuits
36
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
37
Faraday's Law of Electromagnetic Induction
Michael Faraday found experimentally that the
magnitude of the induced emf is proportional to
the rate at which the magnetic flux changed.
Faradays law can be written as,
where N is the number of turns in the loops, A is
the area of one loop, ? is the induced emf, and
B- is the perpendicular component of the magnetic
field.
38
Lenzs Law
The SI unit for the induced emf is the volt, V.
The minus sign in the above Faradays law of
induction is due to the fact that the induced emf
will always oppose the change. It is also known
as the Lenzs law and it is stated as
follows, The current from the induced emf will
produce a magnetic field, which will always
oppose the original change in the magnetic flux.