Current Electricity is one of the most important chapters of Class 12 Physics because it is directly connected to daily life electrical devices and also has high weightage in CBSE Board Exams + Competitive Exams (JEE/NEET level basics).

In this chapter, we study how electric charges flow through conductors, what causes this flow, how resistance affects current, and how different circuit elements behave. You will also learn important laws like Ohm’s Law, Kirchhoff’s Laws, and concepts like drift velocity, resistivity, EMF, internal resistance, and Wheatstone bridge.

These Current Electricity Class 12 Physics Notes cover every topic in detail, with clear explanations, formulas, and exam-focused points that are very helpful for students.

Current Electricity Class 12 Physics Notes | Formulas, Numericals, MCQ & Important Questions

1. Electric Current

Definition

Electric current is the rate of flow of electric charge through any cross-section of a conductor.

I = \frac{Q}{t}

Where:

• = current (Ampere, A)
• = charge (Coulomb, C)
• = time (seconds, s)

SI Unit of Current

• Ampere (A)
  1 Ampere = 1 Coulomb per second

1A = 1C/s

Types of Current

1. Direct Current (DC): Constant magnitude and direction (cell, battery)
2. Alternating Current (AC): Changes magnitude and direction periodically (house supply)

2. Current Density (J)

Current density is the current flowing per unit area of cross-section.

J = \frac{I}{A}

Where:

• = current density (A/m²)
• = current (A)
• = area (m²)

Vector Form

\vec{J} = n e \vec{v_d}

Where:

• = number of free electrons per unit volume
• = charge of electron
• = drift velocity

3. Drift Velocity (vd)

In a conductor, free electrons move randomly in all directions. But when an electric field is applied, electrons start drifting slowly in a particular direction. This average velocity is called drift velocity.

Formula of Drift Velocity

v_d = \frac{I}{nAe}

Where:

• = current
• = number density of electrons
• = cross-sectional area
• = charge of electron

Relation Between Current and Drift Velocity

I = nAe v_d

Important Notes

✅ Drift velocity is very small (around )
✅ Current in conductor is fast because electric field spreads quickly

4. Mobility (μ)

Mobility is defined as the drift velocity per unit electric field.

\mu = \frac{v_d}{E}

Where:

• = mobility (m²/Vs)
• = drift velocity
• = electric field

5. Ohm’s Law

Ohm’s law states that at constant temperature, the current through a conductor is directly proportional to the potential difference across its ends.

V \propto I

V = IR 

Where:

• = potential difference (Volt)
• = current (Ampere)
• = resistance (Ohm)

V-I Graph

• For an ohmic conductor: straight line passing through origin
• Slope gives resistance:

R = \frac{V}{I}

Limitations of Ohm’s Law

Ohm’s law is not valid for:
❌ Diodes
❌ Transistors
❌ Thermistors
❌ Electrolytes
❌ Vacuum tubes

6. Resistance (R)

Resistance is the property of a conductor which opposes the flow of current.

R = \frac{V}{I}

SI Unit

• Ohm (Ω)

1Ω = 1V/A

Factors Affecting Resistance

Resistance depends on:

1. Length (L):
2. Area (A):
3. Nature of material: depends on resistivity
4. Temperature

Resistance Formula

R = \rho \frac{L}{A}

Where:

• = resistivity (Ωm)

7. Resistivity (ρ) and Conductivity (σ)

Resistivity (ρ)

Resistivity is the resistance of a material of unit length and unit area.

\rho = \frac{RA}{L}

Unit: Ωm

Conductivity (σ)

Conductivity is reciprocal of resistivity.

\sigma = \frac{1}{\rho}

Unit: S/m (Siemens per meter)

Important Points

✅ Metals have low resistivity (good conductors)
✅ Insulators have high resistivity
✅ Semiconductors have moderate resistivity

8. Temperature Dependence of Resistance

For metallic conductors, resistance increases with temperature.

R_T = R_0(1 + \alpha \Delta T)

Where:

• = resistance at temperature T
• = resistance at 0°C
• = temperature coefficient of resistance

For Metals

• (Resistance increases with temperature)

For Semiconductors

• Resistance decreases with temperature (negative coefficient)

9. Electrical Energy and Power

Electrical Power

P = VI

Using Ohm’s law:

P = I^2R

P = \frac{V^2}{R} 

Unit: Watt (W)

Electrical Energy

E = Pt

E = VIt 

Unit: Joule (J)
Commercial unit: kWh

1kWh = 3.6 \times 10^6 J

10. Combination of Resistors

(A) Series Combination

When resistors are connected end to end.

R_s = R_1 + R_2 + R_3 + ...

Key Points

✅ Current is same in all resistors
✅ Voltage divides

(B) Parallel Combination

When resistors are connected across same two points.

\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ...

Key Points

✅ Voltage is same across each resistor
✅ Current divides

11. Cells in Series and Parallel

(A) Cells in Series

Total EMF:

E_{total} = E_1 + E_2 + ...

Total internal resistance:

r_{total} = r_1 + r_2 + ...

(B) Cells in Parallel

Total EMF remains same (for identical cells):

E_{total} = E

Total internal resistance:

r_{total} = \frac{r}{n}

Where = number of cells

12. Electromotive Force (EMF)

EMF is the energy supplied by a source per unit charge to move charge through the circuit.

E = \frac{W}{Q}

Unit: Volt (V)

Difference Between EMF and Potential Difference

• EMF is the maximum voltage of the cell when no current flows
• Potential difference is the voltage across terminals when current flows

13. Internal Resistance (r)

Internal resistance is the resistance inside the cell due to electrolyte and electrodes.

Relation Between EMF and Terminal Voltage

V = E - Ir

Where:

• = terminal voltage
• = EMF
• = current
• = internal resistance

Current in Circuit

I = \frac{E}{R + r}

14. Kirchhoff’s Laws

(A) Kirchhoff’s Current Law (KCL)

At any junction, total current entering equals total current leaving.

\sum I_{in} = \sum I_{out}

(B) Kirchhoff’s Voltage Law (KVL)

In any closed loop, algebraic sum of potential differences is zero.

\sum V = 0

Sign Convention

• Moving from negative to positive terminal of cell → +E
• Moving from positive to negative terminal → -E
• Moving along direction of current in resistor → -IR
• Opposite direction → +IR

15. Wheatstone Bridge

Wheatstone bridge is used to measure unknown resistance accurately.

Condition for balance:

\frac{P}{Q} = \frac{R}{S}

Where:

• P, Q, R, S are resistances in four arms
• If balanced, no current flows through galvanometer

Applications

✅ To find unknown resistance
✅ Used in meter bridge

16. Meter Bridge (Slide Wire Bridge)

It is based on Wheatstone bridge principle.

Balance condition:

\frac{X}{R} = \frac{l_1}{l_2}

Where:

• = unknown resistance
• = known resistance
• = balancing length

17. Potentiometer

Potentiometer is used to measure potential difference accurately without drawing current.

Principle

Potential drop across a wire is proportional to its length if current is constant.

V \propto l

V = k l 

Where:

• = potential gradient

Uses

✅ Compare EMF of two cells
✅ Find internal resistance of a cell
✅ Measure small potential difference

18. Galvanometer

A galvanometer is a device used to detect small current.

Conversion into Ammeter

To measure large current, galvanometer is converted into ammeter by connecting shunt resistance (S) in parallel.

S = \frac{G I_g}{I - I_g}

Where:

• = galvanometer resistance
• = full scale deflection current
• = required range

Conversion into Voltmeter

To measure large voltage, connect high resistance in series.

R = \frac{V}{I_g} - G

19. Important Formulas (Quick Revision)

✅ Wheatstone bridge:

20. Exam Tips for Students

✅ Always write formulas first in numericals
✅ Use Kirchhoff’s laws carefully with sign convention
✅ Learn derivations of drift velocity and resistance formula
✅ Practice meter bridge and potentiometer questions
✅ Revise important graphs (V-I, R-T)

Conclusion-Current Electricity Class 12 Physics Notes

These Current Electricity Class 12 Physics Notes cover the chapter in a complete and student-friendly way. If you understand the concepts like Ohm’s law, resistivity, drift velocity, internal resistance, Kirchhoff’s laws, Wheatstone bridge, potentiometer, you can easily solve both theory and numerical questions in exams.

✅ Current Electricity Class 12 Physics Notes +Important Questions (Repeated)

🔥 1 Mark Very Short Questions (VSA)

1. Electric current ka SI unit kya hai?
2. Current density ka formula likho.
3. Drift velocity kya hoti hai?
4. Ohm’s law ka statement likho.
5. Resistance ka unit kya hota hai?
6. Resistivity ka unit kya hota hai?
7. Conductivity aur resistivity ka relation likho.
8. Temperature badhne par metallic conductor ka resistance kaise change hota hai?
9. EMF aur potential difference me ek difference likho.
10. Internal resistance kya hoti hai?
11. Kirchhoff’s first law kya kehta hai?
12. Kirchhoff’s second law kya kehta hai?
13. Series combination me current same hota hai ya voltage?
14. Parallel combination me voltage same hota hai ya current?
15. 1 kWh kitne Joule ke barabar hota hai?

⭐ 2 Marks Important Questions (Short Answer)

1. Drift velocity aur current ke beech relation derive karo.
2. Resistance kis-kis factor par depend karta hai?
3. Resistivity kya hai? Ye length aur area par depend karti hai ya nahi?
4. Ohm’s law ki limitations likho (2 points).
5. Series aur parallel combination me resistors ke 2 differences likho.
6. EMF aur terminal voltage ka relation likho aur explain karo.
7. Internal resistance ka effect current par kya hota hai?
8. Kirchhoff’s laws ka use kyu karte hain?
9. Joule’s law of heating ka statement aur formula likho.
10. Ammeter aur voltmeter me basic difference likho.

✅ 3 Marks Important Questions (Most Repeated)

1. Resistivity (ρ) aur resistance (R) me difference explain karo.
2. R = ρL/A formula derive karo.
3. Power in electric circuit ke formulas derive karo:
4. Cell ke internal resistance ke saath current ka formula derive karo:

   I = \frac{E}{R+r}

6. Wheatstone bridge balance condition derive karo:

   \frac{P}{Q}=\frac{R}{S}

   v_d = \frac{I}{nAe}

🔥 5 Marks Long Answer Questions (Board Exam Favourite)

1. Kirchhoff’s laws state karo aur ek circuit solve karke current find karo. (Most repeated)
2. Wheatstone bridge ka principle + balance condition derive + applications.
3. Meter bridge ka working explain karo aur unknown resistance find karne ka formula derive karo.
4. Potentiometer ka principle explain karo aur why potentiometer is better than voltmeter.
5. Galvanometer ko Ammeter aur Voltmeter me convert karne ka method + formulas.
6. Temperature dependence of resistance explain karo aur formula:

   R_T = R_0(1+\alpha T)

📌 Numericals (Most Repeated Problems)

1. Ek wire ka length double kar diya aur area half kar diya, resistance kitna change hoga?
2. 2Ω, 3Ω, 6Ω resistors ko parallel me connect karo, equivalent resistance find karo.
3. 4Ω aur 6Ω ko series me connect karo, total resistance aur current find karo (given V).
4. EMF = 2V aur internal resistance = 0.5Ω hai, external resistance 4.5Ω ho to current find karo.
5. Power dissipated in resistor: V aur R given ho, power calculate karo.
6. Drift velocity nikalna (I, n, A given).
7. Wheatstone bridge balance condition me missing resistance calculate karo.
8. Meter bridge me balancing length given ho, unknown resistance find karo.

✅ Most Repeated Derivations (Must Learn)

1. Drift velocity derivation
2. R = ρL/A derivation
3. Power formulas derivation
4. Current with internal resistance
5. Wheatstone bridge balance condition
6. Galvanometer conversion formulas

🎯 Assertion-Reason

(Repeated Pattern)

1. A: Metals ka resistance temperature badhne par badhta hai.
   R: Temperature badhne par electron collisions increase hote hain.
2. A: Parallel combination me equivalent resistance kam hota hai.
   R: Total current divide hota hai aur effective conducting area badh jata hai.
3. A: Potentiometer is more accurate than voltmeter.
   R: Potentiometer current draw nahi karta.

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