Class 12 Physics Chapter 3: Current Electricity

CBSE 2026–27 | NCERT-aligned free study resource

1. Electric Current

Electric current is the rate of flow of charge: I = dQ/dt. Conventional current is taken from higher potential to lower potential in the external circuit, while electrons in a metallic conductor drift in the opposite direction.

2. Drift of Electrons

Free electrons undergo random thermal motion. An applied electric field produces a small average drift velocity. Current density is J = I/A, and microscopic conduction leads to J = σE.

3. Ohm’s Law

For an ohmic conductor under constant physical conditions, V ∝ I, so V = IR. Resistance R = ρL/A, where ρ is resistivity.

4. Resistivity

Resistance depends on dimensions; resistivity is a material property at a specified temperature. Metals generally show increasing resistivity with temperature.

5. Cells, EMF and Internal Resistance

The emf of a cell is the energy supplied per unit charge by the source. If a cell of emf ε and internal resistance r supplies current I through an external resistance R, terminal voltage is V = ε − Ir and I = ε/(R+r).

6. Kirchhoff’s Rules

Junction rule: sum of currents entering a junction equals sum leaving it. Loop rule: algebraic sum of potential changes around a closed loop is zero.

7. Wheatstone Bridge

At balance, no current flows through the galvanometer and P/Q = R/S. This principle is useful for accurate resistance measurement.

8. Meter Bridge

The meter bridge applies the Wheatstone-bridge principle using a uniform resistance wire. At balance, the resistance ratio equals the corresponding wire-length ratio.

9. Electrical Power

P = VI = I²R = V²/R. Electrical energy consumed over time t is W = Pt.

Worked Example

A 12 V source with internal resistance 1 Ω is connected to a 5 Ω resistor. I = 12/(5+1) = 2 A. Terminal voltage = 12 − 2×1 = 10 V.

Common Exam Traps

• Do not confuse emf with terminal voltage when current flows.
• Use consistent sign conventions in Kirchhoff loops.
• Resistance and resistivity are different quantities.
• Power formulas are equivalent only when the relevant V, I and R refer to the same component.

Practice Questions

1. Explain drift velocity and its relation to current.
2. Derive R = ρL/A.
3. Derive the terminal-voltage relation of a cell.
4. Solve a two-loop circuit using Kirchhoff’s rules.
5. Explain the principle and balance condition of a Wheatstone bridge.

Essential Circuit Diagrams & Numericals

Ohmic Conductor

For an ohmic conductor at constant physical conditions, V∝I and the V-I graph is linear. Resistance is R=V/I.

Kirchhoff’s Rules

At a junction, the algebraic sum of currents is zero. Around a closed loop, the algebraic sum of potential changes is zero. Choose current directions consistently and keep signs consistent.

Worked Numerical

A 6 Ω resistor connected to 12 V carries I=V/R=2 A and dissipates P=VI=24 W.

Wheatstone Bridge

CBSE-Style Practice

1. Use Ohm’s law to find an unknown resistance.
2. Calculate drift-related quantities using the prescribed relations.
3. Solve a two-loop circuit using Kirchhoff’s rules.
4. Determine unknown resistance using a balanced Wheatstone bridge.
5. Compare resistivity and resistance for different dimensions.

Common Mistakes to Avoid

• Confusing emf with terminal potential difference.
• Mixing resistance and resistivity.
• Applying Kirchhoff sign conventions inconsistently.