First of we must understand about charge which is the fundamental unit of atom . Charge can be  positive due to loss of electron  negative charge due to gain of electron. like charge always repel each other while opposite charge attract each other

unit of charge is coulomb   1 coulomb charge is  charge present in 1.6* 10-18 electrons. Charge on one electron is negative  value 1.6 *10-18 C

Q=ne   Q is the total charge   nis the number of electrons   e is the charge on 1 electron

CURRENT : It is the rate of flow of charge  through the conductor  is called current  flowing through te conductor.

current = charge/time      I=  Q/t

S.I . unit of current  is  Ampere  It is defined  when one coulomb charge flowing through the conductor in  one second than current flowing is one ampere.

1A= 1 C/s       1  mA=  10-3 A       1  micro A =  10-6 A

Current is measured by Ammeter  which is connected  in series.  It is to be noted that direction of flow of current is always opposite to the direction of flow of electrons.

POTENTIAL   : It is the amount of work done in bringing the unit postive charge  from infinity to  a point in the electric field.

POTENTIAL DIFFERENCE  ; It is defined as the work done in taking the unit charge  from one point to another point in  the electric field .

potential difference = work done /charge     V  =W/Q

S.I. unit  of potential difference  is  Volt  It is defined as when one joule of work is done in taking  one coulomb charge from one point to another than potential difference across the two points is  one Volt.

one volt = one joule/ one coulomb

Potential difference is measured by the instrument  Voltmeter which is connected  parallel to the circuit .It has high resistance ,cell is  the simplest device to maintain the potential difference  the current always flow from high potential to low pontential.




OHM’S LAW:  The potential difference across the two points of the metallic conductor is directly proportional to current passing through the circuit provided that the temperature remains constant. V~ I                   ~ =directly proportional

V=IR   R is constant called Resistance for the metal

V-I graph.

 ohm graph


RESISTANCE (R) It is the property of the conductor to resist the flow of charges through conductor S.I. unit of resistance is ohm (n) .

one ohm = one volt / one ampere

When potential difference applied across the circuit  is one volt such that current flowing is one ampere than the resistance across the circuit is one ohm.

Rheostat t It is a variable resistance component  used for regulating the current without changing the sources of voltage.

FACTORS OF RESISTANCE  :  Resistance of the conductors depends upon  following factors  ; resistance of conductor is directly proportional to the length of the conductor , temperature of the  conductor, inversely proportional to the aera of cross section , and nature of conductor.

RESISTIVITY  It is defined as  the resistance of the conductor in unit lenght per unit aera  when flow of current is perpendicular  to it’s opposite faces.

S.I: unit ohmmeter let K be the resistivity , R be the resistance,  l be the length and a be the area of cross section.

K=R .l / a   unit ohm -meter

Resistivity does not depend upon length or area of cross section  it changes with the change in temperature, range of resistivity of metal to alloys is 10-8 to10-6 for insulator it is positive and very high.


Resistances in series   when two or more resistance are connected  end to end this arrangement is called series combination.

Effective Resistance is   R= R1+R2+ R3 +——–. Current passing from the each resistance remain same, the equivalent resistance is more than each resistance.

Total Voltage =  Sum of all Voltage        V= V1+ V2+ V3

since  V=IR     V1=I.R1    V2=IR2      V3=IR3

V= IR1 +IR2 +IR3     >>>>>>>>>>  IR =I ( R1+R2+R3)

R= R1+R2 +R3+——–

series combination



RESISTANCES IN PARALLEL. The voltage across each resistance remain same and equal to applied voltage. the total current is the sum of the current passing from each resistance of the circuit.      I= I1+I2+I3 ——  Reciprocal of equivalent resistance is equal to sum of reciprocal of individual resistance.




IN  the series  circuit  if one component fails the circuit  gets broken and no component will going to work ,as different appliance require differnt amount of current  whoch cannot be possible in series more over total resistance  reduced   than in series.

HEATING EFFECT OF CURRENT: If  elecritic  circuit  is having pure resistance ,when current passes through it  which produces energy in the form of heat it is called  heating effect of current of current.

E =P.T     heat produced H=VI.t    since  V=IR     H=IR.I.t    H=I^2Rt

JOULE’S LAW  : heat produced in the resistors are directly to square of the current, resistance of the resistors and time for which current flowing through the conductors

H= I^2Rt

Heating effect is desirable  in devices like   electric heaters,electric iron ,electric bllub etc but  not desirable in computer monitors , TV , computers ,refrigerators, etc.

In electric bulb  most of the power is consumed  by the filament which produce heat and  l a small part of energy  is  radiated to light. The filament of the bulb is of  tungsten  as it is not oxidised at high temperature having high melting point of 3380 deg C  .the bulb is filled with chemically in active gas  Argon , Nitrogen to make the lfe of the bulb longer.

ELECTRIC FUSE : It is a safety device  which  protect  the electric appliances incase of short circuiting or overloading. It is made of pure  tin  or alloy of copper and tin. connected in series. with  live wire  having low melting point.

ELECTRIC POWER The rate at which electric energy is consumed dissipated in electric circuit.

power = potential difference . current

S.I unit of power is Watt  =  Volt . Ampere

1  Kilo watt hour = 3.6 .10 ^6  joule   = 1 unit


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