Chapter Hub
Current Electricity
Current Electricity studies the steady flow of charge through conductors and circuits. For Class 12 Physics, the chapter begins with microscopic ideas such as drift velocity and then connects them to measurable quantities such as current, potential difference, resistance, and resistivity. The chapter is formula-rich, but marks are usually earned by using the correct physical condition: Ohm's law applies only for ohmic conductors at constant physical conditions, Kirchhoff's rules require a consistent sign convention, and bridge balance requires zero current through the galvanometer branch. Circuit diagrams are central to this chapter. You should practise identifying series and parallel parts, battery polarity, internal resistance, loop direction, junction currents, and Wheatstone bridge arms before substituting values. Numericals often test unit discipline and interpretation, not only calculation. Final answers should include SI units and a short physical meaning, such as whether terminal voltage is less than emf during discharge or why equivalent resistance decreases in a parallel combination.
Difficulty
Medium
Study time
70-90 min
Plan by time
Pick the window that matches what you have right now.
If you have 15 min
Last-pass revision
Skim the Quick Revision table — definitions, formulas, and the traps board examiners reuse.
Open Quick RevisionIf you have 45 min
Targeted practice
Read the high-priority concepts, then drill the common-trap list before moving on.
Open Key ConceptsIf you have 70 min
First full pass
Walk every concept in chapter order, then revise and quiz. Best for the first time you study this chapter.
Open Key ConceptsChapter Learning Map
Start with one of the buckets below, then open the full map when you want the complete concept roadmap.
Key Concepts
Work through the concepts in order. Each one has an explanation, a worked example, a self-check and an exam answer.
All Concepts
All 6 concepts of the chapter, in the order the NCERT chapter teaches them.
Electric Current and Drift Velocity
Electric current is the rate of flow of electric charge through a cross-section of a conductor. Drift velocity is the small average velocity with which free electrons move opposite to the applied electric field inside a conductor.
Ohm's Law and Resistivity
Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided temperature and other physical conditions remain constant. Resistivity is the material property that measures how strongly a substance opposes current flow.
Series and Parallel Combination of Resistors
A combination of resistors is an arrangement of two or more resistors connected in a circuit so that they can be replaced by a single equivalent resistance producing the same current-voltage effect between two terminals.
EMF, Internal Resistance and Terminal Voltage
EMF of a cell is the work done by the source per unit charge in driving charge around the complete circuit. Internal resistance is the opposition to current within the cell, and terminal voltage is the potential difference available across the cell terminals when current flows.
Kirchhoff's Junction and Loop Rules
Kirchhoff's junction rule states that the algebraic sum of currents at a junction is zero, based on conservation of charge. Kirchhoff's loop rule states that the algebraic sum of potential changes around any closed loop is zero, based on conservation of energy.
Wheatstone Bridge and Balance Condition
A Wheatstone bridge is a four-resistor network used to compare resistances. It is balanced when no current flows through the galvanometer branch, giving the condition P/Q = R/S.
Exam Intelligence
The traps that cost marks in this chapter, and how its main concepts turn up in the paper.
Common Traps
- Confusing electron drift direction with conventional current direction.
- Using Ohm's law without checking constant temperature or ohmic behaviour.
- Adding resistances directly in a parallel combination.
- Forgetting internal resistance while calculating terminal voltage.
- Changing Kirchhoff sign convention midway through an equation.
- Applying Wheatstone balance formula when galvanometer current is not zero.
- Leaving numerical answers without SI units or physical interpretation.
How CBSE Asks This Chapter
- Electric Current and Drift Velocity
Questions may ask for the relation between current and drift velocity, the direction of electron drift, a derivation using relaxation time, or a numerical calculation of current, area, or drift speed.
- Ohm's Law and Resistivity
It is commonly asked as a statement of Ohm's law with conditions, V-I graph interpretation, resistance-resistivity distinction, or numerical problems involving R = ρL/A and V = IR.
- Series and Parallel Combination of Resistors
Questions may ask for equivalent resistance of mixed circuits, comparison of brightness of bulbs, current division, voltage division, or identifying series-parallel parts from a circuit diagram.
- EMF, Internal Resistance and Terminal Voltage
Exams ask for terminal voltage during discharge, current supplied by a cell with internal resistance, comparison of cells in series or parallel, and reasoning why terminal voltage changes with current.
Quick Revision
Concept, formula or equation to remember, and the trap that loses marks — in one scannable view.
Electric Current and Drift Velocity
Remember
Key formulae: I = dq/dt, where I is current in ampere, dq is charge in coulomb, and dt is time in second; vd = eEτ/m, where e is electronic charge in coulomb, E is electric field in volt per metre, τ is relaxation time in second, and m is electron mass in kilogram; I = neAvd, where n is number density in m−3 and A is area in m2.Trap to avoid
Students often write that electrons move in the same direction as conventional current.
Remember
Ohm's law: V = IR, where V is potential difference in volt, I is current in ampere, and R is resistance in ohm.Trap to avoid
Students often treat resistivity and resistance as the same quantity.
Series and Parallel Combination of Resistors
Remember
Series: Rs = R1 + R2 + R3 + ..., same current through all resistors, voltage divides.Trap to avoid
Students sometimes add resistances directly in a parallel combination.
EMF, Internal Resistance and Terminal Voltage
Remember
For a discharging cell: V = ε - Ir.Trap to avoid
Students often assume terminal voltage is always equal to emf.
Kirchhoff's Junction and Loop Rules
Remember
Junction rule: ΣI = 0, or total current entering a junction equals total current leaving.Trap to avoid
A common error is changing the sign convention halfway through the loop equation.
Wheatstone Bridge and Balance Condition
Remember
Balance condition: P/Q = R/S, where P and Q are resistances in one arm and R and S are corresponding resistances in the other arm, all measured in ohm.Trap to avoid
Students sometimes use P/R = Q/S without checking the actual arm positions.
| Concept | Remember | Trap to avoid |
|---|---|---|
| Electric Current and Drift Velocity | Key formulae: I = dq/dt, where I is current in ampere, dq is charge in coulomb, and dt is time in second; vd = eEτ/m, where e is electronic charge in coulomb, E is electric field in volt per metre, τ is relaxation time in second, and m is electron mass in kilogram; I = neAvd, where n is number density in m−3 and A is area in m2. | Students often write that electrons move in the same direction as conventional current. |
| Ohm's Law and Resistivity | Ohm's law: V = IR, where V is potential difference in volt, I is current in ampere, and R is resistance in ohm. | Students often treat resistivity and resistance as the same quantity. |
| Series and Parallel Combination of Resistors | Series: Rs = R1 + R2 + R3 + ..., same current through all resistors, voltage divides. | Students sometimes add resistances directly in a parallel combination. |
| EMF, Internal Resistance and Terminal Voltage | For a discharging cell: V = ε - Ir. | Students often assume terminal voltage is always equal to emf. |
| Kirchhoff's Junction and Loop Rules | Junction rule: ΣI = 0, or total current entering a junction equals total current leaving. | A common error is changing the sign convention halfway through the loop equation. |
| Wheatstone Bridge and Balance Condition | Balance condition: P/Q = R/S, where P and Q are resistances in one arm and R and S are corresponding resistances in the other arm, all measured in ohm. | Students sometimes use P/R = Q/S without checking the actual arm positions. |
Practice
Use short concept checks first, then move into the full chapter test.
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