# Current Electricity Notes, Class 12 Physics | SwaVid

Current Electricity, NCERT Class 12 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers.

Canonical: https://www.swavid.com/cbse/class-12/physics/chapter/current-electricity

Source: https://www.swavid.com/cbse/class-12/physics/chapter/current-electricity

# Current Electricity

## Formulas and definitions

## Key ideas

## Common mistakes

## Quick checks

## Keep going

## Questions parents ask

## Class 12 Physics on SwaVid

### Electric current and current density

### Ohm's law and drift velocity

### Resistivity and temperature dependence

### Electromotive force and cells

### Kirchhoff's rules

### Wheatstone bridge

### Next chapter

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SwaVid › CBSE › Class 12 › Physics › Current Electricity

Notes written by SwaVid from the NCERT textbook.

Class 12 Physics, Chapter 3. Key ideas, formulas and common mistakes.

CBSE prints 16 marks for Units I-II together, not for this chapter alone.

Short answer. This chapter covers the flow of electric charges in conductors, electric current, Ohm's law, and resistance. It develops the microscopic picture of drift velocity, rules for combining cells, and Kirchhoff's circuit laws.

Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 3, Current Electricity

Electric current is the net charge flowing across a cross-section per unit time. Current density j is a vector directed along the electric field, defined as current per unit area perpendicular to the flow.

In a conductor, free electrons drift with an average drift velocity under an applied electric field. Ohm's law states that the potential difference across a conductor is directly proportional to the current through it, provided temperature and other physical conditions remain constant.

Resistivity is an intrinsic property of a material determined by electron density and relaxation time. For metallic conductors, resistivity increases with temperature, while for semiconductors, it decreases with temperature.

A cell maintains a potential difference across an external circuit through internal chemical reactions. The potential difference across cell terminals equals its electromotive force minus the internal potential drop due to internal resistance when discharging.

Kirchhoff's junction rule states that the total current entering a node equals the total current leaving it, reflecting charge conservation. Kirchhoff's loop rule states that the algebraic sum of changes in potential around any closed loop is zero, reflecting energy conservation.

A Wheatstone bridge consists of four resistors arranged in four arms with a galvanometer bridging opposite junctions. When the bridge is balanced, no current flows through the galvanometer, satisfying the ratio condition R₁ / R₂ = R₃ / R₄.

Try each question first. Then tap it to see the answer.

v d = I / (n e A)

v d = 2.0 / (8.5 × 10²⁸ × 1.6 × 10⁻¹⁹ × 1.0 × 10⁻⁶)

v d = 2.0 / (1.36 × 10⁴) ≈ 1.47 × 10⁻⁴ m s⁻¹

1.47 × 10⁻⁴ m s⁻¹

ΔT = 127.0 - 27.0 = 100.0 °C

R = R₀ (1 + α ΔT) = 10.0 × (1 + 1.70 × 10⁻³ × 100.0)

R = 10.0 × (1 + 0.170) = 10.0 × 1.170 = 11.7 Ω

11.7 Ω

Total circuit resistance = R + r = 10.0 + 2.0 = 12.0 Ω

Current I = ε / (R + r) = 6.0 / 12.0 = 0.5 A

Terminal potential V = ε - I r = 6.0 - (0.5 × 2.0) = 5.0 V

5.0 V

Moving Charges and Magnetism .

Electrostatic Potential and Capacitance .

The 14 chapters of Class 12 Physics .

CBSE does not give marks to one chapter. It prints 16 marks for Units I-II together (Electrostatics; Current Electricity). Current Electricity is part of that group.

No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed.

Moving Charges and Magnetism.

SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works.

- Ch 3 NCERT book
- 16 marks, Units I-II together
- 3 quick checks
- Treating electric current as a vector quantity because it has a direction . Electric current is a scalar because it does not obey vector addition rules. Current density j is the vector quantity associated with charge flow.
- Using terminal voltage instead of emf when applying Ohm's law to a complete circuit . A cell has internal resistance r, so the total current is I = ε / (R + r), and the terminal voltage across the external resistor is V = ε - I r.
- Assuming drift speed is equal to the speed of the electric signal . Drift speed of electrons is very slow, typically of the order of mm s⁻¹. The electric field establishes across the conductor at almost the speed of light, causing immediate current.
- Applying the same sign convention incorrectly across resistors and cells in Kirchhoff's loop rule . Traversing a resistor in the direction of current gives a potential drop -I R. Traversing a cell from negative to positive terminal gives a potential rise +ε regardless of the current direction.

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