Current Electricity – Class 12 Physics Notes

 🔌 Chapter 3: Current Electricity – Class 12 Physics Notes

Label: Physics, Class 12, Current Electricity

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🔹 Introduction

Deals with the flow of electric charge through conductors, mainly metals.

Covers Ohm’s law, resistivity, combination of resistors, emf, Kirchhoff’s laws, and cells.

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⚡ Electric Current

Electric current (I) is the rate of flow of electric charge:

I = Q / t

Unit: Ampere (A)

Direction: From positive to negative terminal (conventional direction)

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🔋 Electric Current in Conductors

Carried by free electrons in metals

Flow due to potential difference (voltage)

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⚙️ Drift Velocity

Average velocity of electrons due to electric field

vd = eEτ / m

Where:

e = electron charge

E = electric field

τ = relaxation time

m = mass of electron

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🔌 Current and Drift Velocity Relation

I = neAvd

Where:

n = number of electrons per unit volume

A = cross-sectional area

vₑ = drift velocity

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🧪 Ohm’s Law

Voltage is directly proportional to current:

V = IR

Where:

R = resistance (Ω)

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🧬 Resistivity and Conductivity

Resistivity (ρ) is resistance per unit length and area:

R = ρ × (l / A)

Conductivity (σ) is reciprocal of resistivity:

σ = 1 / ρ

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🔁 Effect of Temperature

Material Resistance with Temperature

Metals Increases

Alloys Slightly increases

Semiconductors Decreases

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🧲 Combination of Resistors

🔗 In Series:

Total resistance:

R_total = R₁ + R₂ + R₃ + ...

🔀 In Parallel:

Total resistance:

1 / R_total = 1 / R₁ + 1 / R₂ + ...

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🔋 EMF and Internal Resistance of Cell

EMF (ε): Max potential difference when no current flows

Terminal voltage:

V = ε - Ir

Where r = internal resistance of cell

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🧮 Kirchhoff’s Laws

1. Junction Rule (KCL):

Total current entering a junction = Total current leaving

2. Loop Rule (KVL):

Sum of all voltages in a loop = 0

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🔄 Wheatstone Bridge

Used to measure unknown resistance:

(P / Q) = (R / S)

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🧠 Meter Bridge

Practical application of Wheatstone Bridge

(R₁ / R₂) = (L₁ / L₂)

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🔍 Potentiometer

Used to measure EMF of cell and internal resistance

Works on null point method

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📝 Quick Revision Table

Concept Formula

Current I = Q / t

Drift Velocity vd = eEτ / m

Current–Drift Velocity I = neAvd

Ohm’s Law V = IR

Resistance R = ρ (l / A)

Conductivity σ = 1 / ρ

Series Resistance R_total = R₁ + R₂ + ...

Parallel Resistance 1/R_total = 1/R₁ + 1/R₂ + ...

Terminal Voltage V = ε - Ir

Kirchhoff's Loop Rule ΣV = 0

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✅ Summary

Current is flow of charges

Ohm’s Law relates current and voltage

Resistance depends on material and temperature

Resistors combine i

n series and parallel

Kirchhoff’s laws help analyze circuits

Wheatstone Bridge and potentiometer are precise instruments

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Guys, do the derivation from your side using the formula.

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