Schooling · Class 12 · Physics · Current Electricity · Chapter on a page
CBSE Class 12 · Physics · NCERT Physics Part-I
Current Electricity: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Electric current I is the net charge…
- In a metallic conductor, current…
- Ohm's law states that, at constant…
- Resistivity ρ of a material is…
- For metals, resistivity increases…
- Resistors in series add directly…
- The emf (ε) of a cell is the work…
- Formulas and facts
- I = Q/t
- I = n A e vd
- V = I R
- R = ρ L/A
- Temperature dependence of resistivity
- Req = R₁ + R₂ + …
- 1/Req = 1/R₁ + 1/R₂ + …
- Worked examples
- Finding drift velocity
- Equivalent resistance and current
- Potentiometer – comparing two cells
- Watch out for
- Confusing emf with terminal voltage
- Adding resistances using the…
- Taking drift velocity to be very…
- Forgetting sign conventions in…
- Using the Wheatstone bridge formula…
What this chapter is about
This chapter moves from electrostatics (charges at rest) to charges in motion — the electric current. You study what happens when a steady potential difference is maintained across a conductor, how free electrons drift under an electric field, and why different materials resist the flow of current to different extents. The ideas here connect the microscopic picture (electrons colliding with ions) to the macroscopic quantities you measure with ammeters and voltmeters.
Key ideas
- Electric current I is the net charge flowing through a cross-section per unit time: I = Q/t. The SI unit is the ampere (A), where 1 A = 1 C/s.
- In a metallic conductor, current arises from the drift of free electrons; drift velocity vd is related to current by I = n A e vd, where n is the number density of free electrons, A is cross-sectional area, and e is the electron charge.
- Ohm's law states that, at constant temperature, the potential difference V across a conductor is directly proportional to the current I through it: V = I R. The constant R is called resistance (unit: ohm, Ω).
- Resistivity ρ of a material is defined by R = ρ L/A, where L is length and A is cross-sectional area. Resistivity depends on the material and temperature, not on the conductor's dimensions.
- For metals, resistivity increases roughly linearly with temperature; for semiconductors and insulators, resistivity decreases as temperature rises.
- Resistors in series add directly (Req = R₁ + R₂ + …); resistors in parallel combine as 1/Req = 1/R₁ + 1/R₂ + ….
- The emf (ε) of a cell is the work done per unit charge by non-electric forces inside the cell; the terminal voltage V = ε − I r, where r is internal resistance.
- Kirchhoff's junction rule (conservation of charge): the algebraic sum of currents at any junction is zero. Kirchhoff's loop rule (conservation of energy): the algebraic sum of potential changes around any closed loop is zero.
Formulas and facts to remember
- Current: I = Q/t (charge per unit time).
- Drift velocity relation: I = n A e vd.
- Ohm's law: V = I R.
- Resistance from resistivity: R = ρ L/A.
- Temperature dependence of resistivity: ρ(T) = ρ₀ [1 + α (T − T₀)], where α is the temperature coefficient.
- Series combination: Req = R₁ + R₂ + …
- Parallel combination: 1/Req = 1/R₁ + 1/R₂ + …
- Terminal voltage of a cell: V = ε − I r.
- Wheatstone bridge balance condition: P/Q = R/S (no current through the galvanometer).
- Potentiometer principle: potential drop along a uniform wire is proportional to length; used to compare emfs or measure internal resistance.
Common mistakes
- Confusing emf with terminal voltage → Remember that terminal voltage equals emf only when no current flows; otherwise V = ε − I r.
- Adding resistances using the parallel formula when they are actually in series → Trace the current path: if the same current passes through both, they are in series.
- Taking drift velocity to be very high because current appears instantaneous → Drift velocity is tiny; what propagates fast is the electric field, not the electrons themselves.
- Forgetting sign conventions in Kirchhoff's loop rule → Assign a direction to the loop; drop potential across a resistor if traversing in the direction of current, rise if opposite. For a cell, rise if going from negative to positive terminal.
- Using the Wheatstone bridge formula when the bridge is not balanced → The relation P/Q = R/S holds only when the galvanometer shows zero deflection.
Quick revision
- Current is rate of flow of charge; drift velocity is very small (order 10⁻⁴ m/s in copper at typical currents).
- Ohm's law: V = I R; valid at constant temperature for ohmic conductors.
- R = ρ L/A — longer wires have more resistance; thicker wires have less.
- Series resistances add; for parallel, reciprocals add.
- Kirchhoff's rules let you solve any circuit: junction rule conserves charge, loop rule conserves energy.
- Potentiometer gives a null method for comparing emfs without drawing current from the cell under test.