1

Introduction to Electricity

2

The Atom Electric Charge

Conductor Insulator

Potential Voltage

Current Resistance

3

Lighting an Electric Bulb

Light Bulb

Switch

Battery

Electron Flow

+

-

4

Atoms

• Building blocks for all matter

• Composed of three particle types:– Protons are Positively Charged– Electrons are Negatively Charged – Neutrons are Neutral

5

Atoms

• In the Bohr theory of the atom (named after Niels Bohr, 1885–1962), electrons orbit a nucleus containing neutrons and protons.

• Attraction between the opposite charges of electrons and protons keeps atoms together.

6

The Atom

• Electrons orbit around nucleus in shells

• Set number (2n2) of electrons allowed in each shell (n)

7

8

• Valence shell that is not full encourages electron movement to surrounding atoms.

• This movement, when it occurs in the same direction, is electric current.

9

Electric Charge

• Atoms can lose electrons or gain electrons due to friction, heat, light, or magnetic energy.

10

CONDUCTORS

• Materials that loose electrons with very little applied energy are good conductors (copper, salt water).

• Additional energy causes electrons to flow from atom to atom (electric current).

INSULATORS

• Takes large amount of energy to conduct a very slight electric current.

11

Potential, Potential Difference, Voltage• There is a force of attraction between

negative and positively charged atoms.

• Pressure to equalize their charge is known as potential, potential difference or voltage.

• Voltage also known as electromotive force (EMF).

• Voltage has a polarity associated with it.

• The symbol is E or V.

• It is measured in the units of Volts (v).

12

potential energy

• The attraction of opposite charges means that energy is required to pull them apart, and that energy can be recovered when they come together again.

• In between, we say that the energy is kept as potential energy.

13

potential energy

• The most familiar form of potential energy is gravitational potential energy.

• Because of the gravitational attraction between the earth and the objects on it, lifting objects gives them potential energy.

• When an object is dropped, that potential energy is rapidly converted into kinetic energy, which is even more rapidly converted into other forms of energy when the object hits something (e.g., sound, heat, kinetic energy of broken pieces).

• The more mass something has and the higher we lift it, the more potential energy it has.

14

Potential and Potential Energy • In electricity, the quantity analogous to height is

voltage, usually indicated by the symbol V.

• Increasing the voltage of one coulomb of charge by one volt gives it one joule (J) of electrical potential energy, named after James P. Joule (1818–1889).

• 1 volt = 1 J / 1 C

• There is a special unit for the amount of energy one electron gets when its potential is reduced by one volt, an electron-volt (symbol: eV), which equals 1.6 x 10-19 J.

15

Electrostatic Field

Positive Charge

+ -Negative Charge

Lines of force,like magnetic lines of force

16

Current• The flow of electrons through a conductor.

• Symbol for current is I

• Measured in Amperes, Amps, or A

17

Electric Current

I, Current in Ampere = Coulomb of charge / sec

1 A = 1 Coulomb of charge / sec

= 6.24 x 1018 electrons / sec

18

Resistance• Resistance is the opposition to the electron flow.

• Amount of current through a material depends on type and physical dimensions of the material.

• Resistance is measured in ohms ().

• Symbol is R.

• If 1 volt is applied across a resistor and causes a current of 1 A, then the resistor has a resistance of 1 .

R = V / I = 1v / 1A = 1

19

Ohm’s Law

• Mathematical relationship between voltage, current, and resistance

R = V / I

V = I x R

I = V / R

20

Symbols

21

Fundamental electrical symbols

V

+

-

A

+

-

RESISTOR

CAPACITOR

AMMETERVOLTMETER

FUSE

POTENTIOMETER(VARIABLE RESISTOR)

BATTERY ORDC POWER SUPPLY

SWITCH

LIGHT BULB

+

22

Electricity On Human Body

Electrical factors determining shock level:– Resistance– Voltage– Current

• Lower the body resistance, greater the shock hazard

23

SKIN CONDITION RESISTANCE

DRY 100 - 600 K

WET 1 K

INTERNAL BODY

HAND TO FOOT 400 - 600

EAR TO EAR 100

24

SHOCKS

• Any voltage above 30V considered dangerous

• Any current above 5mA considered dangerous

• Danger of shock increases as voltage increases

25

SHOCKS

0 - 1mA - no sensation

0 - 5mA - Safe current values

20mA - Painful shock and inability to let go

50mA - Heart convulsions, usually fatal

1000mA - Current in 100W lamp can kill up to 20 adults