Electricity

Electricity is a form of energy that can be easily changed to other forms.

Electric current is the flow of electrons through a conductor.

The device which causes the flow of electrons through a conductor is

called a cell.

Electrons flow from the negative terminal to the positive terminal.

Electric current flows from the positive terminal to the negative terminal.

This is called conventional current.

Electric current is expressed as :-

The rate of flow of charges through a

conductor or the quantity of charges flowing through a conductor in

unit time.

I = Q/t

I – current, Q – quantity of charge, t – time

• Electric Circuit is a continuous and closed path of an

electric current.

A schematic diagram of an electric circuit comprising

of a cell, electric bulb, ammeter and plug key.

A

+

-

+-

bulbcell

ammeter

plug key

Electric circuits are made up of electrical components

These components must be joined together without any gap in between to form a closed circuit.

connecting wires

electric cell

circuit boardlight bulb

NOTE:

Components refer to the light bulb, wires, battery

• Incomplete circuits are called open circuits.

connecting wire is

missing

no source of

electrical energy

Both the circuits in the diagram are incomplete, hence they are known as “open circuits”.

An electric current flows only when there is:

• a source of electrical energy and

• a closed circuitconnecting wires

electric cell

circuit boardlight bulb

How to draw Circuit DiagramsComponent Symbol Component Symbol

An electric cell Battery

Light bulb (lamp) Switch

Switch (open)

Switch (closed)

Connecting wires (not joined)

Connecting wires (joined)

+ +

Symbols are used to represent the various electrical components in circuits.

Symbols of components used in electric circuits :-

An electric cell A battery or combination of cells

Plug key or switch Plug key or switch (open) (closed)

Electric bulb A resistor ofresistance R

Variable resistance oror rheostat

Ammeter Voltmeter

A wire joint A wire crossing over without joining

A v+ - -+

+ - + -

There are 2 ways in which an electric circuit can be arranged:

1. Series2. Parallel

Series Circuit

• A series circuit connects the components one after the other

• A single loop is formed

• A break in any part of a series circuit stops the flowof current in the whole circuit.

Resistors in Series

• When resistors are connected in series, the resistance will add up and increase.

• R = R1 + R2 + R3…2 3

The total resistance for this circuit is:

2 + 3 = 5

Parallel Circuit• A parallel circuit divides into two or more branches.

• The current divides and flows through each parallel branch.

• If a component breaks or is removed, the other components remain on.

• When resistors are connected in parallel, the resistance will decrease

• This is because they will provide alternate routes for the current to flow.

Resistors in Parallel

S/E: Resistors in Parallel

6

6

1

R=

1

R

1

R

1

R+ +

1 2 3

What is the resistance of the circuit?

1

R=

1

6

1

6+

1

R=

2

6

1

3= R = 3 The final resistance

is smaller

• When an electric current flows through a wire, the wire heats up. Electrical energy has been converted into heat energy.

• The greater the resistance of the wire, the greater the amount of heat produced. This heating effect is used in common electrical appliances.

Iron Kettle Hair Dryer

A kettle uses both copper and nichrome wires.Copper has low resistance while nichrome has high resistance.

Which material, copper or nichrome, should be used for the heating element, and for the external wire?

Use copper wire for the external wire as it has low resistance & produces less heat

Use nichrome wire for the heating element as it has high resistance& produces a lot of heat

filament wire produces

heat and light

In a light bulb, the heated filamentwhich is also a resistance wire, becomes so hot that light is also emitted.

ARGON (inert/unreactive gas)

If a current I flows through a resistor of resistance R and t be the time for which a charge Q flows through it, then the work done to move the charge through potential difference V

W = Q X VP = W = Q X V Q = I or P = V X I

t t tor Heat energy supplied = P X t = V X I X t

According to Ohm’s law V = IRHeat produced H = I2Rt

( ) A

V

RA B

I I

+ -

+ -

+-

When electric charges move through a wire, they lose some of the energy to the atoms in the wire. On receiving the energy, The atoms vibrate more and more causing the wire to heat up. Some of the Electric Energy is changed to Heat Energy. The higher the resistance the more the heat energy.

The amount of heat generated is governed by Joule's first law: Q = I2·R·t

In industry Soldering, Welding, Cutting, Drilling and Working of Electric Furnaces are based on Heating of Electric Current.