CLASS XII PHYSICS — CURRENT ELECTRICITY
JNV Digital Library — Concepts Learning Card

SOURCE FOUNDATION
Existing DSpace source: PHYSICS PART I, handle 123456789/478.
Do not duplicate CURRENT ELECTRICITY.pdf.

02 — CONCEPTS: CORE UNDERSTANDING

Electric current — Rate of net charge crossing a cross-section.
Steady: I = Q/t; general: I = dQ/dt.
Common trap: Do not confuse current with charge.

Conventional current — Direction of positive-charge motion; in metals it is opposite to electron drift.
Common trap: Electron flow and conventional current have opposite directions.

Resistance — Opposition of a conductor to current under specified conditions; V = IR for an ohmic conductor.
Common trap: R changes with geometry and physical conditions.

Resistivity — Material property appearing in R = ρl/A; it depends on material and temperature.
Common trap: Resistance is not the same thing as resistivity.

Current density — Current per unit normal area: j = I/A. Microscopically j = nqvd.
Common trap: Use area perpendicular to current.

Drift velocity — Small average directed electron velocity caused by an applied electric field.
Common trap: It is superposed on much larger random thermal motion.

Relaxation time — Average time scale between successive collisions in the simple microscopic model.
Common trap: It connects microscopic motion to resistivity.

Mobility — Drift-speed magnitude per unit field: μ = |vd|/E = eτ/m.
Common trap: Mobility is positive.

Ohmic behaviour — V is proportional to I when physical conditions are fixed.
Common trap: A device may be non-ohmic even though V/I can be defined.

Temperature coefficient — α describes the approximate fractional change of resistivity per degree near a reference temperature.
Common trap: The linear formula is a limited-range approximation.

Electrical power — Rate of electrical energy transfer: P = VI; for a resistor P = I²R = V²/R.
Common trap: Choose the form that matches the known quantities.

EMF — Open-circuit potential difference supplied by a source.
Common trap: Despite its name, emf is not a force.

Internal resistance — Resistance inside a real cell causing internal potential drop Ir.
Common trap: Terminal voltage while supplying current: V = ε − Ir.

Kirchhoff junction rule — Sum entering = sum leaving.
Common trap: Conservation of charge.

Kirchhoff loop rule — Net algebraic potential change around a closed loop is zero.
Common trap: Conservation of energy.

Balanced Wheatstone bridge — Galvanometer current is zero and the arm resistances satisfy the balance ratio.
Common trap: Balance gives a null measurement.

SOURCE COVERAGE
This Concepts card is based on the completed JNV Core Learning Pack and the Class XII Physics Part I, Chapter 3 foundation. It is intended to sit around the existing textbook resource rather than reproduce the textbook PDF.