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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

Lightning

Atom

Charging Ebonite Rod Fur

Charging an Object

Ebonite rod Fur ? Negatively charged ebonite

rod Glass rod Silk ? Positively charged

glass rod

LAW OF CONSERVATION OF ELECTRIC CHARGE

During any process, the net electric charge of an

isolated system remains constant (is conserved).

Like charges repel and unlike charges attract

each other.

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.

Charging by Contact and by Induction

An object can be charged by two methods -By

contact. -By induction.

Charging By Contact

Charging By Induction

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.

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

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

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 (1745-1827) who invented

the electric battery. 1 volt 1 J/C.

Circuits

Series Circuit

Parallel Circuit

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,

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,

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.

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?

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 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

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.

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.