Current Electricity Notes Class 12th Pdf
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• 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 α 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 = ρl/A= ml/ne²τAwhere ρ 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 ρ = m/ne²τ, Where m, n, e are mass, number density and charge of electron respectively, τ-relaxation time of electrons. ρ 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 σ=1/ρ

• Drift velocity is the average velocity of all electrons in the conductor which drift in opposite direction to the applied electric field. Drift velocity V_d = (eE/m)τ also I = neAvd

• Mobility (μ) 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 R_T = R_o (1+aT) , where α is the temperature coefficient of resistance of the conductor. α 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%)Ω. 

• 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 α 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

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