ELECTRICITY

The term“ELECTRICITY “came from the classical Latin“Electrum(Amber )“, from the Greek"Elektron “ and also from the Modern Latin“Electricus(literally resembling amber)"

Two types of electricity

1. Static electricity :- An electric charge that collects or builds up on the surface of an object
• eg :- Lightening , Shocking
2. Dynamic electricity :-The flow of charges carried through a material such as wire or other conductors
• eg :- Power lines , Computers

ELECTRIC CURRENT

An electric current is a flow of electric charge

Electric current is denoted by letter “I” .Electric current is expressed by the rate of flow of electric charges.Rate of flow means the amount of charge flowing through a particular area in unit time.

If a net charge (Q) flows through a cross section of any matter in time (t)

•         Electric current = Net charge/Time

                        I = Q/t

•  Unit :- Ampere (A)
• One Ampere :- It is constituted by the flow of one coulomb of charge per one second                                    

                        1 A=1 C/1 S

• Small quantities of current are expressed in Milli Ampere (1 ma=10^-3A) or in Micro Ampere (1 μA= 10^-6A)

• Ammeter is used to measure electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured.

ELECTRIC POTENTIAL  &  POTENTIAL DIFFERENCE

                  The amount of work needed to move a unit charge from a reference point to a specific point against an electric field ( or we can say infinity to that point ).Typically , the reference point is   earth,although any point beyond the influence of the electric field charge can be used.

• Electric potential is a scalar quantity

      Potential difference (v) between to points = Workdone (w) / Charge (Q)

                                                                  V=W/Q

• The SI unit of electric potential difference is Volt (V)
• One volt :- It is the potential difference between two points in a current carrying conductor when one joule of work is done to move a charge of 1 coulomb from one point to the other.

                                                  1 V=1 Joule/1 Coulomb

                                                  1 V = 1 J/ 1 C = 1 JC^-1

• Measurement of potential difference is Volt meter .Volt meter is always connected in parallel across the points between which the potential difference is measured.

ELECTRIC CIRCUITS

      An electric circuit is a path in which electrons from a voltage or current source flow.The point where those electrons enter an electrical circuit is called the "source".The point where the electrons leave an electrical circuit is called the "return" or "earth ground".

• SYMBOLS OF SOME COMMONLY USED COMPONENTS IN CIRCUIT DIAGRAMS :-
  

OHM'S LAW

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

i e, V ∝ I

       V / I = a  constant = R

i e , V=IR

        R is a constant for the given metallic wire at a given temperature and is called its resistance. It is the property of a conductor to resist the flow of charges through it. Its SI unit is ohm, represented by the Greek letter Ω. According to Ohm’s law, R = V/I
If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω. 
That is, 1 ohm = 1 Volt /1 Ampere
Also we get I = V/R

Graphical verification of ohm's law

            The graph between the current due to a potential difference across the conductor is straight line passing through the origin .Greater the resistance , lesser will be the conductance of current

fig :- ohm's law circuit diagramfig :- ohm's law graph

• Factors on which resistance depends 

   

Resistance of a conductor depends up on the length of the conductor L, 

and the cross sectional area of the conductor A

R   ∝  L
R   ∝  1/A
R    ∝  L/A  Or   R = ρ L/A

Where , ρ - Proportionality constant called resistivity of the material of the conductor

RESISTIVITY

•              The resistivity of a given material is defined as the resistance offered by a cube of that  material of side 1m,when the current flows perpendicular to the opposite faces 
• Substance having low resistivities are good conductors as they allow electric current to flow   through them.

ρ=RA/L

unit :- ohm metere (Ω⋅m)

     It only depends the nature of the material . 

     eg :- copper-1.6 x 10^-8 ,gold - 2.4 x 10^-8

CLASSIFICATION OF MATERIALS ON THE BASIS OF RESISTIVITY

1 - Good Conductors 

• Substance having low resistivities are good conductors as they allow electric current to flow   through them .
•  Metals having very low resistivity in the range of ρ  10^-2 to 10^-8 Ω⋅m  conductivity :- σ   10 ^2 to 10 ^8 sm^-1
•  Resistiivity of pure metals is very low - Eg :- Copper,Nickel etc

2 - Resistors

• These substance have moderate resistivity since they offer significant resistance to the flow of electric current and produce heat.
• Alloys have resistivity in the range of 10^-8 Ω⋅m -  Eg :- Nichrome, Tungsten
• Tungsten is used for filaments in electric bulbs because its melting point is very high

3 - Insulators

• The substances having very high resistivities and negligible conductivities are called insulators.
• They don't allow electric current to flow through them.
• σ = 10^-11   -  10^19 sm^-1
• ρ = 10^11    -   10^19 Ω⋅m
• Due to this property ,rubber and other insulators are generally used by the electricans while working with live wires to protect them selves from electric shocks.

COMBINATION OF RESISTANCE

• In an electric circuit, some times we require less current and some times more current.To obtain this requirement resistance to may be connected in series and parallel arrangement. 
• In case the current required is less we need the total resistance to be greater and for this resistance are connected in series. 
• For more current the resistance has to be decreased and hence,individual resistances can be connected in parallel.

Resistance in series

• When individual resistances are connected end to end consecutively, they are said to be in series.
• In series combination of resistances ,
• Current flowing through all resistances is same
• The total voltage across the combination is equal to the sum of the voltage drops across the individual resistance
• The total resistance of the combination is equal to the sum of the individual resistances
• If R1,R2, and R3 are connected in series then the equivalent resistance R will be R1+R2+R3

Resistance in parallel

• When individual resistances are connected between the same 2 points they are said to be connected in parallel.
• In parallel combination of resistances,
• The voltage across each resistor is same as the voltage across the whole combination
• The current through different resistances will be different and the total current is the sum of current through different resistances
•  The sum of the reciprocals of the separate resistances is equal to the reciprocal of the equivalent resistance
•  If 3 resistance connected parallel then equivalent resistance R will be given by,1/R=1/R1 +1/R2 +1/R3

HEATING EFFECT OF ELECTRIC CURRENT (JOULE'S LAW)

When a current I flow through a resistor of electric resistance R,heat produced H depends directly on the square of the current,resistance and time T for which the current is allowed to pass.

H=I^2RT. 

SI unit of heat energy is Joule.

Application of heating effect:- The heating effect of current is used in the electrical heating applicances such as electric iorn,room heater ,geysers,bulb and safety device fuse etc.. 

ELECTRIC POWER

The rate at which electric energy is consumed is called electric power and is expressed as follows 

Power = Work done / Time = W/t  , 

P = VI Or P= I^2R  , P =V^2/R .

Unit -Watt(W).
           where,P=Power,V= Potential difference (voltage),I=Current ,R=Resistance,W=Workdone,t=time taken for the work to be done. 

It is the power consumed by a device that carries 1A of current when operated at a potential difference of 1V.Thus 1W=1V X 1A.

Unit of watt is very small.In actual practice we use a much larger unit is called kilowatt.It is equal to 1000 watts. 

Since electrical energy is the product of power and time ,the unit of electric energy is ,ie, watt hour (Wh).One watt hour is the energy consumed when one watt of power is used for 1 hour. 

The commercial unit of electric energy is kilowatt hour (KWh) ,commonly known as unit  1KWh=1000watt x 3600 second =3.6 x 10^6 watt second =3.6 x 10^6 Joule (J)

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