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NCERT Class 12 Physics Chapter 3 Notes, Current Electricity Class 12 Chapter 3 Notes

NCERT Class 12 Physics Chapter 3 Notes, Current Electricity Class 12 Chapter 3 Notes

Edited By Vishal kumar | Updated on Jul 08, 2025 01:26 PM IST

Think of your phone charger- that moment when you switch it on, and there is an electric flow to charge your phone, that is a real-life example of the concepts in Class 12 Physics Chapter 3: Current Electricity. We explore the underlying concepts of electric currents, resistances, circuits, and flow of charge in conductive materials a very important concept in the way electronic devices operate in this chapter of NCERT Notes.

NCERT Class 12 Physics Chapter 3 Notes, Current Electricity Class 12 Chapter 3 Notes
NCERT Class 12 Physics Chapter 3 Notes, Current Electricity Class 12 Chapter 3 Notes

This chapter is an essential part of the CBSE Class 12 Physics curriculum, and at the same time, essential to JEE, NEET, and other competitive exam candidates because it reinforces the conceptual understanding of concepts such as Ohm Law, Kirchoff Law, internal resistance of cells, Wheatstone bridge, meter bridge, and resistivity and conductivity. Being good at these concepts will lay a foundation in solving theoretical and numerical problems in exams. In order to facilitate students in revising the topics well, Careers360 provides well-organised NCERT Notes of Class 12 Chapter 3 Current Electricity, developed through the efforts of experienced Physics teachers. This revision notes include summaries of the main concepts with fast learning, an extended list of significant formulas, neatly labelled diagrams to support visual understanding, solved sample questions to improve problem-solving ability

Background wave

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NCERT Class 12 Physics Chapter 3 Notes: Download PDF

Download the PDF version of Current Electricity NCERT Notes to view them offline. Using these notes, students can revise anytime without the internet and is most suitable for fast and easy access.

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NCERT Class 12 Physics Chapter 3 Notes

The NCERT Class 12 Physics Chapter 3 Notes – Current Electricity provide a clear and concise summary of essential concepts like Ohm’s Law, Kirchhoff’s Laws, and circuit analysis. These notes are ideal for quick revision and exam preparation for CBSE, JEE, and NEET.

Electric Current:

  • The flow of charge through a conductor per unit of time is defined as electric current. It is measured in amperes (A) and is essential for understanding electrical circuits and electromagnetism.

I=qt

Where, i is the current, q is the charge and t is the time.

  • If the rate of flow of charge is variable, the current at any time is i=dqdt
  • Current Density: The amount of electric current flowing through a material per unit cross-sectional area is referred to as its current density. It is a vector quantity denoted by J and can be written as,
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j¯=ΔiΔA

  • If the cross-sectional area is not perpendicular to the current but forms an angle θ with the current direction, then

Jav=didAcosθdi=JdAcosθ=JdA

  • Relation between current density and electric field

J=σE

J=Eρ

Where, σ is conductivity and ρ is resistivity or specific resistance of the substance

μ=vdE

Where, μ is mobility and vd is drift velocity

Ohm’s Law:

  • Ohm's Law states that in a conductor, under constant external conditions such as temperature and pressure, the current flowing through the conductor is directly proportional to the potential difference across its two ends.

VIV=IR

R- Electric Resistance

R=ρlA

where ρ is resistivity / specific resistance, l is the length of the conductor and A is the area of the cross-section of the conductor

  • Ohmic Substance: An Ohmic substance is a substance that obeys Ohm's Law. It has a linear I-V graph, and the slope gives the conductance, which is the reciprocal of resistance.
  • Non-ohmic Substances: Non-ohmic or non-linear conductors are substances that do not obey Ohm's Law, such as gases and crystal rectifiers.
  • Superconductor: Superconductors are materials that have zero resistivity below a critical temperature. Electrical resistance is zero in this state
  • In superconductor,s resistivity is zero

,

Resistivity:

ρ=mne2τ

Where, m is the mass, n is the number of electrons per unit volume, e is the charge of the electron, and τ is the relaxation time.

  • Resistivity is a material's intrinsic property, and its value tends to increase with the presence of impurities and mechanical stress.
  • The reciprocal of resistivity is called conductivity.
  • The reciprocal of resistance is termed conductance, and its SI unit is either Ω-1 or Siemens.

Temperature-Dependent Resistivity:

ρ=ρo(1+α(T−To))

ρ: Resistivity at temperature T

ρo: Resistivity at temperature To

  • Temperature Coefficient of Resistance

RT=RT0[1+α[TT0]]

RT - Resistance at temperature T
R0 - Resistance at temperature To
α - temperature coefficient of resistance

α=RTRoRo(TTo)


Where the value of α is different at different temperatures

Colour Coding of Resistance:

The carbon resistance typically consists of four coloured ring bands labelled as A, B, C, and D.

1625645382707

  • Colour bands A and B indicate the significant digits of the resistance value.
  • Colour band C represents the decimal multiplier.
  • Colour band D indicates the tolerance, expressed as a percentage, around the specified resistance value.
  • May be remembered as BBROY Great Britain Very Good Wife.

Tolerance of Gold is ±5%
Tolerance of Silver is ±10%
Tolerance if no colour ±20%

1625645386796

Grouping of Resistance:

Series Grouping of resistance

Req=R1+R2+R3++Rn

ReqEquivalent Resistance 
For nidentical resistance: Req=nR

V=Vn

Parallel Grouping of Resistance:

1Req=1R1+1R2++1Rn


If two resistances are in Parallel:

Req=R1R2R1+R2

Heat developed in a resistor:

  • When a constant current flows through a resistance R for t seconds, the loss in electric potential energy appears itself as increased thermal energy (heat H) in the resistor. This relationship is expressed mathematically as,

= energy  time =i2R=iR=V2R

  • The power developed = energy/time =i2R=V2/R (from Ohm's law)
  • The unit of heat is the joule (J), and the unit of power is the watt (W).

Cell:

  • The device which converts Chemical energy into electrical energy is known as an electric cell.
  • Internal resistance: In the case of a cell the opposition of electrolyte to the flow of current through it. It is shown by r.
  • The internal resistance of a cell depends on the distance between electrodes.

r∝d

  • The internal resistance of a cell depends on the area of the electrodes

r∝A

  • The internal resistance of a cell concentration of electrolyte

r∝c

  • The internal resistance of a cell temperature of the electrolyte

r∝1/temp

  • Emf of a cell: The electromotive force (emf) of a cell is defined as the work done or energy carried by unit charge when it completes one full cycle within the circuit.
  • Potential difference: The potential difference, also known as voltage, is the electrical pressure across the terminals of a cell when it is actively supplying current to an external resistance in the circuit.
  • Equation of cell:
  1. When supplying the current: E=V+iR
  2. When the cell is being charged: E=V−iR
  • Current supplied by the cell:

1625645385158

Cell supplies a constant current in the circuit.

i=ER+r

Where, R- External resistance and r- internal resistance

  • Potential drop inside the cell=ir
  • The internal resistance of the cell, r=(EV1)R
  • The power dissipated in external resistance, P=(ER+r)2R
  • Maximum power is obtained when the resistance value of the load is equal in value to that of the voltage source's internal resistance.

Pmax=E24r

  • Series grouping of cells:

In series grouping anode of one cell is connected to the cathode of other cells

1625645390724

n= identical cells which are connected in series, then
- Equivalent e.m.f of combination is Eeq=nE
- Equivalent internal resistance req=nr
- Main current / current from each cell i=nER+nr
- Power dissipated in the external circuit is (nER+nr)2R
- Conditions for Maximum Power is R=nr
- Pmax =n(E24r) when nr<R

  • Parallel grouping of cells:

In parallel grouping, all anodes are connected to one point and all cathode together at other points

1625645392618

For n cells connected in parallel
Equivalent e.m.f Eeq=E
Equivalent internal resistance Req=rn
The main current is

i=ER+rn
The potential difference across the external resistance

V=iR
Current from each cell

i=in


The power dissipated in the circuit

P=(ER+rn)2R

Condition for Maximum Power

R=rnPmax=n(E24r)when r>>nR

Kirchhoff's first law:

  • In a circuit, at any junction, the sum of the currents entering the junction must equal the sum of the currents leaving the junction. This law is also known as Junction rule or Kirchoff's current law (KCL)

∑i=0

1625645394837

i1+i3=i2+i4

This law is simply based on the conservation of charge.

Kirchhoff's second law:

  • The algebraic sum of all the potentials across a closed loop is zero. This law is also known as Kirchhoff's Voltage law (KVL)
  • This law is based on the conservation of energy.

∑V=0

In closed-loop

1625645395551

i1R1+i2R2E1i3R3+E2+E3i4R4=0

Wheatstone's Bridge:

1625645399491

It is an arrangement of four resistances that can be used to measure one of them in terms of rest

PQ=RSVB=VD

(Balanced condition)
No current will flow through the galvanometer
unbalanced condition: VB>VD

(VAVB)<(VAVD)


Current will flow from A to B

Meter bridge:

  • The meter bridge is used to find the resistance of a wire, enabling the calculation of its specific resistance. Operating on Wheatstone's bridge principle, it provides a precise method for measuring resistance by balancing known and unknown resistances.

1625645400026

PQ=RSS=(100l)lR

Current Electricity Previous year Question and Answer

Q1: n identical cells, each of e.m.f. E and internal resistance r, are connected in series. Later on it was found out that two cells ' X ' and ' Y ' are connected in reverse polarities. Calculate the potential difference across the cell ' X '.

Answer:

Two cells are connected in reverse polarity
So total emf =(n2)E2E

=(n22)E=(n4)E


 Req =nrI=EnetReq=(n4)Enr

potential difference across cell X=E+Ir

=E+(n4)Enr×r=nE+(n4)En=nE+(n4)En=E(n+n+4)n=2E(n+2)n

Q2: Two batteries of emf's 3 V and 6 V and internal resistances 0.2Ω and 0.4Ω are connected in parallel. This combination is connected to a 4Ω resistor. Find the equivalent internal resistance of the combination

Answer:

Given:
Two batteries are connected in parallel, and then connected to an external resistor of R=4Ω.
Battery 1:E1=3 V,r1=0.2Ω
Battery 2:E2=6 V,r2=0.4Ω

The equivalent internal resistance of the combination is calculated using the formula:

req=r1r2r1+r2

Substitute the values into the formula:

req=0.20.40.2+0.4=0.080.6=215Ω0.133Ω

Hence, the equivalent internal resistance of the combination is 0.133 ohm.

Q3: Two wires P and Q are made of the same material. The wire Q has twice the diameter and half the length as that of wire P. If the resistance of wire P is R, the resistance of the wire Q will be:

Answer:

R=ρlA=ρlπr2 Resistance of Q=R ' =ρ(l2)π(2r)2R=ρlπr24×2=ρl8πr2=R8

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Frequently Asked Questions (FAQs)

1. Why is the chapter Current Electricity important for board exams?

Current Electricity carries significant weight in the Physics board exam, accounting for around 10% of the total marks. It covers fundamental concepts that form the base for other chapters and practicals.

2. Is this chapter important for competitive exams like JEE and NEET?

Yes, this chapter is very important for entrance exams such as JEE Main and NEET. Questions based on Ohm’s Law, circuits, and resistivity often appear in these exams.

3. How can I improve in solving numerical problems in this chapter?

Start with simple problems and gradually move to complex circuits. Practice daily, understand formulas, and use Kirchhoff’s and Ohm’s Laws effectively to solve questions step-by-step.

4. What is a Wheatstone Bridge and its application?

A Wheatstone Bridge is a network of four resistors used to precisely measure unknown resistance. It is widely used in sensor applications like strain gauges and temperature measurement.

5. How is resistance affected by temperature?

In most conductors, resistance increases with an increase in temperature due to more frequent collisions between electrons and atoms. However, in semiconductors, resistance decreases with temperature.

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A block of mass 0.50 kg is moving with a speed of 2.00 ms-1 on a smooth surface. It strikes another mass of 1.00 kg and then they move together as a single body. The energy loss during the collision is

Option 1)

0.34\; J

Option 2)

0.16\; J

Option 3)

1.00\; J

Option 4)

0.67\; J

A person trying to lose weight by burning fat lifts a mass of 10 kg upto a height of 1 m 1000 times.  Assume that the potential energy lost each time he lowers the mass is dissipated.  How much fat will he use up considering the work done only when the weight is lifted up ?  Fat supplies 3.8×107 J of energy per kg which is converted to mechanical energy with a 20% efficiency rate.  Take g = 9.8 ms−2 :

Option 1)

2.45×10−3 kg

Option 2)

 6.45×10−3 kg

Option 3)

 9.89×10−3 kg

Option 4)

12.89×10−3 kg

 

An athlete in the olympic games covers a distance of 100 m in 10 s. His kinetic energy can be estimated to be in the range

Option 1)

2,000 \; J - 5,000\; J

Option 2)

200 \, \, J - 500 \, \, J

Option 3)

2\times 10^{5}J-3\times 10^{5}J

Option 4)

20,000 \, \, J - 50,000 \, \, J

A particle is projected at 600   to the horizontal with a kinetic energy K. The kinetic energy at the highest point

Option 1)

K/2\,

Option 2)

\; K\;

Option 3)

zero\;

Option 4)

K/4

In the reaction,

2Al_{(s)}+6HCL_{(aq)}\rightarrow 2Al^{3+}\, _{(aq)}+6Cl^{-}\, _{(aq)}+3H_{2(g)}

Option 1)

11.2\, L\, H_{2(g)}  at STP  is produced for every mole HCL_{(aq)}  consumed

Option 2)

6L\, HCl_{(aq)}  is consumed for ever 3L\, H_{2(g)}      produced

Option 3)

33.6 L\, H_{2(g)} is produced regardless of temperature and pressure for every mole Al that reacts

Option 4)

67.2\, L\, H_{2(g)} at STP is produced for every mole Al that reacts .

How many moles of magnesium phosphate, Mg_{3}(PO_{4})_{2} will contain 0.25 mole of oxygen atoms?

Option 1)

0.02

Option 2)

3.125 × 10-2

Option 3)

1.25 × 10-2

Option 4)

2.5 × 10-2

If we consider that 1/6, in place of 1/12, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will

Option 1)

decrease twice

Option 2)

increase two fold

Option 3)

remain unchanged

Option 4)

be a function of the molecular mass of the substance.

With increase of temperature, which of these changes?

Option 1)

Molality

Option 2)

Weight fraction of solute

Option 3)

Fraction of solute present in water

Option 4)

Mole fraction.

Number of atoms in 558.5 gram Fe (at. wt.of Fe = 55.85 g mol-1) is

Option 1)

twice that in 60 g carbon

Option 2)

6.023 × 1022

Option 3)

half that in 8 g He

Option 4)

558.5 × 6.023 × 1023

A pulley of radius 2 m is rotated about its axis by a force F = (20t - 5t2) newton (where t is measured in seconds) applied tangentially. If the moment of inertia of the pulley about its axis of rotation is 10 kg m2 , the number of rotations made by the pulley before its direction of motion if reversed, is

Option 1)

less than 3

Option 2)

more than 3 but less than 6

Option 3)

more than 6 but less than 9

Option 4)

more than 9

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