Formula CheatsheetCBSE & ICSEClass 12PhysicsCh-3Verified Faculty Notes & Solutions
Current Electricity- Class 12 Physics Chapter 3
Formula sheet
2 students readUpdated 30 September 2026
Formula Sheet
Class 12 Physics Formula Sheet
Current Electricity — Complete Revision Guide
1. Basic Current & Charge Relations
- Electric Current: I = Q / t or I = dQ / dt
- Current Density (J): J = I / A = nqv(d) (Vector form: J = sigma * E)
- Drift Velocity (v_d): v(d) = I / (nAe) = (e * E * tau) / m
- Mobility (mu): mu = v(d) / E = (e * tau) / m
- Relation (Current & Drift Velocity): I = neA * v(d)
2. Ohm's Law, Resistance & Resistivity
- Ohm's Law: V = IR or R = V / I
- Resistance (R): R = rho * (L / A) rho depends only on material and temperature, not dimensions.
- Conductance & Conductivity: G = 1 / R (Siemens), sigma = 1 / rho = (n * e^2 * tau) / m
- Temperature Dependence: R(T) = R(0) * [1 + alpha * (T - T(0))]
3. Electrical Energy & Power
- Electric Power (P): P = VI = I^2 * R = V^2 / R
- Electrical Energy (W): W = VIt = I^2 * R * t = (V^2 * t) / R
- Heat Produced: H = I^2 * R * t joules = (I^2 * R * t) / 4.18 calories
4. Cells, EMF, and Internal Resistance
- Terminal Potential Difference: Discharging: V = E - Ir | Charging: V = E + Ir
- Internal Resistance (r): r = R * [(E / V) - 1]
- Cells Combinations: Series: I = (nE) / (R + nr) | Parallel: I = E / (R + r/m)
5. Kirchhoff's Laws & Circuit Analysis
- KCL (Junction Rule): Sum of I(in) = Sum of I(out) (Conservation of Charge)
- KVL (Loop Rule): Sum of Delta(V) = 0 (Conservation of Energy)
- Wheatstone Bridge: P / Q = R / S (Balanced Condition)
- Meter Bridge: X = R * [(100 - l) / l]
6. Potentiometer Measurements
- Potential Gradient (k): k = V / L = (I * rho) / A
- Comparing EMFs: E1 / E2 = l1 / l2
- Internal Resistance: r = R * [(l1 / l2) - 1]
Key Formulas (Most Frequently Used)
- Drift & Current: I = neAv(d) and v(d) = (e * E * tau) / m
- Ohm's & Resistance: V = IR and R = rho * (L / A)
- Power: P = VI = I^2 * R = V^2 / R
- Terminal Voltage: V = E +/- Ir
- Wheatstone & Potentiometer: P/Q = R/S and E1/E2 = l1/l2
Important Conversions & Units
- Current: 1 A = 1 C/s
- Resistivity: Ohm * meter (Ohm-m)
- Power: 1 W = 1 J/s = 1 V*A
- Energy: 1 kWh = 3.6 * 10^6 Joules
Board Important Questions & Derivations
- Derivation 1: Derive relation between current and drift velocity (I = neAv_d).
- Derivation 2: Deduce Ohm's law from drift velocity and relaxation time.
- Derivation 3: Maximum current condition in mixed grouping (nR = mr).
- Derivation 4: Balanced condition of a Wheatstone bridge using Kirchhoff's laws.
- Conceptual Q: Why is a potentiometer preferred over a voltmeter? (Draws zero current at null point).
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