MP Board Class 12 Physics Ch 3 Current Electricity
Current Electricity is one of the most important and scoring chapters of MP Board Class 12 Physics. Every year MPBSE asks 5 to 8 marks of questions from this chapter — Ohm’s law, Kirchhoff’s laws, Wheatstone bridge, potentiometer and electrical power. These complete notes with formulas, tables and key points are prepared for the MP Board 2027 exam. Practice free chapter tests and previous year papers at mpboard.ai.
📑 Table of Contents
- 1. Electric Current and Current Density
- 2. Ohm’s Law, Resistance and Resistivity
- 3. Drift Velocity and Mobility
- 4. Combination of Resistances
- 5. EMF, Internal Resistance and Cells
- 6. Kirchhoff’s Laws
- 7. Wheatstone Bridge and Potentiometer
- 8. Electrical Energy and Power
- 9. Quick Formula Sheet
- 10. Key Points for MP Board Exam
- 11. FAQs
⚡ 1. Electric Current and Current Density
Electric current is the rate of flow of electric charge through a conductor. Its SI unit is the ampere (A). Current flows from higher to lower potential, while electrons flow in the opposite direction.
| Quantity | Formula | SI Unit |
|---|---|---|
| Electric current | I = q/t = ne/t | ampere (A) |
| Current density | J = I/A | A/m² |
Key point: Current is a scalar quantity, but current density is a vector. The direction of J is the direction of flow of positive charge. For a steady current, I = nAevd where n is the number of free electrons per unit volume.
📐 2. Ohm’s Law, Resistance and Resistivity
Ohm’s law: At constant temperature, the current through a conductor is directly proportional to the potential difference across its ends. V = IR. The resistance of a conductor depends on its length, area of cross-section and material: R = ρL/A, where ρ is the resistivity.
| Quantity | Formula / Value |
|---|---|
| Resistance | R = V/I = ρL/A |
| Resistivity (copper) | 1.72 × 10⁻⁸ Ω m |
| Conductance | G = 1/R (siemens) |
| Conductivity | σ = 1/ρ |
Temperature dependence: For metals, Rt = R₀(1 + αΔt), where α is the temperature coefficient of resistance. Resistivity of metals increases with temperature, while semiconductors show a decrease.
🏃 3. Drift Velocity and Mobility
Drift velocity is the average velocity acquired by free electrons under an applied electric field: vd = eEτ/m, where τ is the relaxation time. The relation between current and drift velocity is I = nAevd. Mobility is drift velocity per unit electric field: μ = vd/E, with SI unit m²/Vs.
🔗 4. Combination of Resistances
| Combination | Equivalent Resistance |
|---|---|
| Series | Rs = R₁ + R₂ + R₃ + … |
| Parallel | 1/Rp = 1/R₁ + 1/R₂ + 1/R₃ + … |
In series, current is the same and potential divides; in parallel, potential is the same and current divides. Series increases resistance; parallel decreases it.
🔋 5. EMF, Internal Resistance and Cells
EMF (ε) is the work done per unit charge by the cell; terminal voltage is the potential difference across the cell when current flows: V = ε − Ir, where r is internal resistance. Cells in series add emfs; cells in parallel increase current capacity. Grouping of cells is a favourite MPBSE numerical topic.
🧮 6. Kirchhoff’s Laws
Junction rule (KCL): The algebraic sum of currents meeting at a junction is zero — charge is conserved. Loop rule (KVL): The algebraic sum of potential differences around any closed loop is zero — energy is conserved. Sign convention practice is essential for solving complex circuits.
⚖️ 7. Wheatstone Bridge and Potentiometer
Wheatstone bridge is balanced when P/Q = R/S, and no current flows through the galvanometer. The meter bridge uses this principle to measure unknown resistance. The potentiometer compares emfs and measures internal resistance of a cell because its potential gradient is uniform: ε₁/ε₂ = l₁/l₂.
💡 8. Electrical Energy and Power
Joule’s law of heating: Heat produced H = I²Rt (in joules). Electric power is the rate of doing work: P = VI = I²R = V²/R. Commercial unit of energy is the kilowatt-hour (kWh): 1 kWh = 3.6 × 10⁶ J. Electrical appliances and fuse rating questions are common.
📋 9. Quick Formula Sheet
| No. | Quantity | Formula |
|---|---|---|
| 1 | Current | I = q/t = nAevd |
| 2 | Ohm’s law | V = IR |
| 3 | Resistance | R = ρL/A |
| 4 | Drift velocity | vd = eEτ/m |
| 5 | Mobility | μ = vd/E |
| 6 | Terminal voltage | V = ε − Ir |
| 7 | Wheatstone balance | P/Q = R/S |
| 8 | Power | P = VI = I²R = V²/R |
| 9 | Joule heat | H = I²Rt |
| 10 | Energy | 1 kWh = 3.6 × 10⁶ J |
🎯 10. Key Points for MP Board Exam
- Numericals: Combination of resistances, cells in series/parallel, and power calculations carry 2–5 marks each.
- Derivations: Drift velocity expression, Wheatstone bridge balance condition, and potentiometer comparisons are board favourites.
- Concepts: Difference between emf and terminal voltage, temperature dependence of resistivity, and Ohm’s law limitations are frequently asked.
- Diagrams: Label Wheatstone bridge, potentiometer and meter bridge circuits carefully — diagram marks are easy to secure.
- For 2027: Expect at least one numerical on Joule heating or cell grouping. Practise MP Board previous year papers to spot repeated patterns.
❓ 11. Frequently Asked Questions
Q1. What is Ohm’s law? At constant temperature, the current through a conductor is directly proportional to the potential difference across its ends, V = IR.
Q2. What is the difference between emf and terminal voltage? EMF is the total work done per unit charge by the cell (ε), while terminal voltage is the potential difference across the cell terminals when current flows (V = ε − Ir).
Q3. Why does resistance increase with temperature in metals? On heating, lattice ions vibrate more, increasing collisions with free electrons and reducing relaxation time, so resistivity and resistance increase.
Q4. State Kirchhoff’s two laws. Junction rule: sum of currents at a junction is zero. Loop rule: sum of potential differences around a closed loop is zero.
Q5. What is the condition for a balanced Wheatstone bridge? The bridge is balanced when P/Q = R/S, and no current flows through the galvanometer.
🎯 Get exam-ready for MP Board 2027
Practice with chapter-wise notes, PYQs and mock tests — all free at mpboard.ai.
Conclusion: Current Electricity is a highly scoring chapter — master the formulas, practise Kirchhoff’s law circuits and cell-grouping numericals, and revise the key points above before the 2027 exam. Also revise Electric Charges and Fields and Electrostatic Potential and Capacitance notes. For complete notes, solved PYQs and free mock tests, stay with mpboard.ai — your one-stop resource for MP Board Class 12 Physics preparation.