Current Electricity Notes Class 12th Pdf Download Physics Tutoria

• Electric current is defined as the amount of charge flowing through any

cross section of the conductor in unit time. The rate of flow of chrge through the

conductor is called electric current. I = Q/t. SI Unit Ampere (A). 

• The electric current flowing through the conductor is said to be one ampere

when one coulomb charge flows through it in one second.

• Current density |J| = I/A.

• Ohm’s law: The electric current passing through a conductor is directly

proportional to the potential difference applied across it provided the physical

conditions such as temperature, pressure etc., remain constant. V ? I i.e. V = IR,

Where R is the resistance of the conductor. Resistance R is the ratio of V & I

• The device which opposes the flow of electric current through it is called

resistor. Resistance is the characteristic property of the conductor which offers

opposition for the flow of electric current. 

• Resistance R = ?l/A= ml/ne2?Awhere ? is the resistivity of the material of the

conductor- length and A area of cross section of the conductor. If l is increased n

times, new resistance becomes n2R. If A is increased n times, new resistance

becomes 

• Resistivity is the characteristic property of the material which is the

resistance of the conductor of unit length and unit area of cross section.

• Resistivity ? = m/ne2?, Where m, n, e are mass, number density and charge

of electron respectively, ?-relaxation time of electrons. ? is independent of

geometric dimensions.

• Relaxation time is the average time interval between two successive

collisions

• Conductance of the material G =1/R and conductivity ?=1/?

• Drift velocity is the average velocity of all electrons in the conductor which

drift in opposite direction to the applied electric field. Drift velocity Vd = (eE/m)?

also I = neAvd

• Mobility (?) of a charge carrier is the ratio of its drift velocity to the applied

electric field 

• Effect of temperature on resistance: Resistance of a conductor increase with

the increase of temperature of conductor RT = Ro (1+aT) , where ? is the temperature

coefficient of resistance of the conductor. ? is slightly positive for metal and

conductor, negative for semiconductors and insulators and highly positive for

alloys. 

  Example:- if colour code of carbon resistor is Red Yellow and Orange with

tolerance colour as silver, the resistance of the given resistor is (24×103 ± 10%)?. 

• Cells:- E.M.F of a cell is defined as the potential difference between its

terminals in an open circuit. Terminal potential difference of a cell is defined as the

potential difference between its ends in a closed circuit.

• Internal resistance r of a cell is defined as the opposition offered by the cell 

• Grouping of cells :

i) In series grouping circuit, current is given by ii) In parallel grouping circuit, current is given by where n, m are number of cells in series and parallel connection respectively.

• Kirchhoff’s Rule:

i) Junction Rule:-The algebraic sum of currents at a junction in a network is

zero.

ii) Loop rule:-The algebraic sum of potential differences and emfs of a closed loop in a network is zero

• Wheatstone bridge is an arrangement of four resistors arranged in four arms

of the bridge and is used to determine the unknown resistance in terms of other

three resistances. For balanced Wheatstone Bridge.

• Wheatstone bridge is most sensitive when the resistance in the four arms are

of the same order

• In the balanced condition of the bridge on interchanging the positions of

galvanometer and battery if there is no effect on the balancing length of the bridge.

• The principle of Metre Bridge:

The resistance of the wire of uniform cross

section and composition is directly proportional to its length. 

• Slide Wire Bridge or Metre Bridge is based on Wheatstone bridge and is

used to measure unknown resistance. If unknown resistance S is in the right gap,

• Potentiometer is considered as an ideal voltmeter of infinite resistance.

• Principle of potentiometer: The potential drop across any portion of the wire

of uniform cross section and uniform composition is proportional to the length of

that portion of the wire provided steady current is maintained in it i.e. v ? l

• Smaller the potential gradient greater will be the sensitivity of potentiometer.

• Potentiometer is used to

(i) compare the e.m.f.s of two cells

(ii) determine

the internal resistance of a cell and

(iii) measure small potential differences.

• Expression for comparison of e.m.f of two cells by using potentiometer,  Where 1 l is the balancing length of potentiometer wire orresponding to

e.m.f of the cell, l2 that of terminal potential difference of the cell when a

resistance R is connected in series with the cell whose internal resistance is to be

determined

• Expression for determination of potential difference Where L is

the length of the potentiometer wire, l is balancing length, r is the resistance of potentiometer wire, R is the resistance included in the primary circuit.

• Joule’s law of heating states that the amount of heat produced in a

conductor is proportional to

(i) square of the current flowing through the

conductor , (ii) resistance of the conductor and

(iii) time for which the current is

passed. Heat produced is given by the relation H=I2Rt • Electric power: It is defined as the rate at which work is done by the source

in maintaining the current in electric circuit. P =VI = I2R =V2/R. Power P is the

product of V & I

• Electrical energy: The total work done by the source in maintaining the

current in an electrical circuit for a given time. Electrical energy = VIt = I2Rt

=(V2/R)t = Pt

• Commercial unit of energy 1KWh= 3.6×106J