NEET · Physics
Teach charge flow from microscopic drift to macroscopic circuit laws, while making the validity limits of Ohm's law and each network formula visible. The page owns the full Unit 12 current scope, including cells, Kirchhoff laws, Wheatstone bridge and metre bridge.
In short
The 2026 NEET Current Electricity unit connects microscopic carrier drift with current, resistance and circuit behavior. It explicitly includes Ohmic and non-Ohmic V-I characteristics, resistivity/conductivity and temperature dependence, cell emf/internal resistance, resistor and cell combinations, Kirchhoff laws, Wheatstone bridge and metre bridge.
Subject faculty
Also in Physics: Kumar Gandharva Saxena
Define conventional current as the direction of positive-charge flow. In a metallic conductor, electron drift is opposite conventional current. Use the drift relation only with a clearly defined carrier density, charge magnitude and cross-sectional area.
Do not write "V = IR always." Ohm's law is a material/device behavior under fixed relevant physical conditions when current is proportional to applied potential difference. The official syllabus explicitly includes both Ohmic and non-Ohmic V-I characteristics, so the page should use graphs to distinguish constant-resistance behavior from nonlinear devices.
Separate geometry (R) from material property (rho). The relation R = rho L/A assumes a uniform conductor of length L, constant cross-section A and a defined temperature/state. Temperature dependence should be shown as an approximation over the range where a constant temperature coefficient is reasonable.
Emf is energy supplied per unit charge by the source, not simply "voltage flowing." Terminal voltage depends on current direction and internal resistance. The sign convention must distinguish a discharging cell from a cell being charged.
Kirchhoff's junction rule follows charge conservation; loop rule follows energy conservation around a closed circuit under the lumped-circuit model. A bridge formula is valid only for the matching circuit topology and balance condition. The metre-bridge length ratio must use the same left/right resistor assignment shown in the diagram.
The official 2026 Unit 20 includes measuring a wire's resistivity with a metre bridge and resistance using Ohm's law. Surface this as a short "syllabus practical connection" rather than as invented separate weightage.
Current: I = dQ/dt. Conventional current direction. Unit A.
Current
Use when — Conventional current direction. Unit A.
Drift-current magnitude: I = n q A v_d. q is carrier charge magnitude; uniform carrier density and cross-section.
Drift-current magnitude
Use when — q is carrier charge magnitude; uniform carrier density and cross-section.
Mobility: v_d = mu E in magnitude form. Low-field linear transport model used at NCERT/NEET depth. mu in m^2 V^-1 s^-1.
Mobility
Use when — Low-field linear transport model used at NCERT/NEET depth. mu in m^2 V^-1 s^-1.
Ohm law: V = IR. Ohmic conductor under fixed physical conditions where V-I is linear.
Ohm law
Use when — Ohmic conductor under fixed physical conditions where V-I is linear.
Uniform wire: R = rho L/A. Uniform material/cross-section at specified temperature. rho in ohm m.
Uniform wire
Use when — Uniform material/cross-section at specified temperature. rho in ohm m.
Conductivity: sigma = 1/rho. Linear homogeneous material model. Unit S/m.
Conductivity
Use when — Linear homogeneous material model. Unit S/m.
Temperature approximation: R = R0[1 + alpha(T-T0)]. Approximate over a range where alpha is effectively constant.
Temperature approximation
Use when — Approximate over a range where alpha is effectively constant.
Electrical power: P = VI; resistor forms I^2R, V^2/R. Steady DC resistor for derived forms. Unit W.
Electrical power
Use when — Steady DC resistor for derived forms. Unit W.
Series resistors: R_eq = sum R_i. Same current in ideal series branch.
Series resistors
Use when — Same current in ideal series branch.
Parallel resistors: 1/R_eq = sum 1/R_i. Same potential difference across branches.
Parallel resistors
Use when — Same potential difference across branches.
Discharging cell terminal voltage: V = E - Ir. Current leaves positive terminal; E emf, r internal resistance.
Discharging cell terminal voltage
Use when — Current leaves positive terminal; E emf, r internal resistance.
Kirchhoff junction: sum I_in = sum I_out. Lumped steady circuit.
Kirchhoff junction
Use when — Lumped steady circuit.
Kirchhoff loop: algebraic sum of potential changes = 0. Use one traversal direction and consistent source/resistor signs.
Kirchhoff loop
Use when — Use one traversal direction and consistent source/resistor signs.
Wheatstone balance: P/Q = R/S. Balance condition with named four-arm layout and zero galvanometer current.
Wheatstone balance
Use when — Balance condition with named four-arm layout and zero galvanometer current.
Metre bridge: R_left/R_right = l/(100-l). At balance for uniform bridge wire; exact ratio depends on diagram assignment.
Metre bridge
Use when — At balance for uniform bridge wire; exact ratio depends on diagram assignment.
Target concept: terminal voltage and internal resistance.
Condition: a cell of emf 12 V and internal resistance 1 ohm supplies 2 A.
Method selection: use the discharging-cell terminal relation because current leaves the positive terminal.
Reasoning/calculation: V = E - Ir = 12 - (2)(1) = 10 V.
Units/sign check: Ir is a voltage drop in volts; terminal voltage is lower than emf during discharge.
PI category: Decision / Selection Error if the charging sign is chosen; Execution Error for arithmetic/sign loss.
Treats electron drift direction as conventional current direction
Knowledge gapWhy it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Redraw carrier and conventional-current arrows.
Uses V=IR with one constant R for a nonlinear V-I curve
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
First classify the device as Ohmic/non-Ohmic.
Uses R=rho L/A while temperature/cross-section is changing
Execution errorWhy it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Recheck model assumptions before substitution.
Gets Kirchhoff loop sign wrong
Execution errorWhy it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
Fix a loop direction and annotate each rise/drop before writing equations.
Bridge ratio does not match diagram labels
Needs reviewWhy it happens
Preparation Intelligence classification: Needs Review.
How it is corrected
Re-derive from balance rather than relying on memorized orientation.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
No. The 2026 syllabus explicitly includes Ohmic and non-Ohmic V-I characteristics. V=IR with constant R describes the Ohmic linear case under fixed physical conditions.
Resistance depends on material and geometry; resistivity is a material property at a specified physical state/temperature.
Energy supplied by the source per unit charge. Terminal voltage can differ from emf when current flows through internal resistance.
Yes. Unit 12 explicitly lists metre bridge, and Unit 20 also links resistivity measurement to the metre-bridge experiment.
Official scope is taken from the NEET (UG) 2026 syllabus and official NTA documents; concept structure follows current NCERT contents. No performance, weightage or question-frequency data is asserted.
