Every section of the chapter split into study-sized topics, ranked by how often NEET actually asks it — with the syllabus gaps flagged so nothing is studied blind or skipped by accident.
Typical yield · 2–4 questions per paper · 8–16 marks
Scope check before you start
In both
Current, drift & mobility, Ohm's law, resistance & resistivity, V–I characteristics, energy & power, temperature dependence, emf & internal resistance, cells in series/parallel, Kirchhoff's rules, Wheatstone bridge. Present in the rationalised NCERT text and in the NTA syllabus. Study fully.
Gap content
Series & parallel combination of resistors and metre bridge are named in the NTA syllabus but no longer derived anywhere in this chapter's text. NEET asks both. They must be learnt from outside the book — treat them as first-class topics, not optional extras.
Dropped
Carbon resistors & colour code and potentiometer (comparison of emf, internal resistance, measuring p.d.) are out of the rationalised NCERT and out of the current NTA syllabus. Do not spend time here. Old question banks are still full of them — skip those questions when you meet them.
Resistivity & R = ρl/A (stretching, reshaping)Tier 1
Electrical power & heating effectTier 2
Kirchhoff's rules on multi-loop circuitsTier 2
Cells in series & parallelTier 2
Wheatstone bridge & metre bridgeTier 2
Temperature dependence of resistanceTier 3
Conductors, semiconductors, insulatorsTier 3
V–I characteristics & limits of Ohm's lawTier 3
Electric current: definition & scalar natureTier 4
Tier 1 — Do these first
Four topics that between them account for the majority of Current Electricity questions across recent papers. If time runs out, this is the chapter.
1. Resistor networks & equivalent resistance
Gap content · §3.4 basis
Asked almost every year, often as the chapter's one guaranteed numerical
What NEET asks
Reduce a ladder / cube / symmetric mesh to a single R between two given points.
Current or p.d. in one named branch after reduction.
A wire of resistance R bent into a circle or square — find R across two points on it.
n identical resistors: ratio of maximum to minimum obtainable resistance (answer n²).
Balanced-bridge spotting: a bridge arm carrying zero current gets deleted before reducing.
Must-know
Series: R = R₁ + R₂ + … same I, V divides
Parallel: 1/R = 1/R₁ + 1/R₂ + … same V, I divides
Two in parallel: R = R₁R₂/(R₁+R₂)
n equal in parallel: R/n · Max/Min for n resistors = n²
Current divider: I₁ = I·R₂/(R₁+R₂)
Why it's not in your bookThe rationalised chapter dropped the series–parallel derivation, so this must come from the NTA syllabus list, not the NCERT text. Everything downstream — Kirchhoff, bridges, power — depends on it.
TrapPoints joined by a plain wire are the same node. Redraw the circuit with nodes labelled before writing any formula; most lost marks here are drawing errors, not algebra errors.
2. Cells: emf, internal resistance, terminal voltage
NCERT §3.10
Very high — appears as both a numerical and a concept statement
What NEET asks
Given ε, r and R: find I, terminal voltage V, or the lost volts Ir.
Two readings (V₁ at R₁, V₂ at R₂) → solve for ε and r.
Charging a battery: terminal voltage V = ε + Ir, and why a series resistor is used.
Maximum current Imax = ε/r when R = 0 (short circuit).
Condition for maximum power in the external resistor: R = r, with Pmax = ε²/4r.
Must-know
I = ε/(R + r)
Discharging: V = ε − Ir (V < ε)
Charging: V = ε + Ir (V > ε)
r = R(ε − V)/V · Open circuit (I = 0): V = ε
Trapemf is a potential difference, not a force, and it is measured only in open circuit. A voltmeter across a working cell reads V, never ε — questions rely on students confusing the two.
3. Drift velocity, current density, mobility
NCERT §3.5, §3.5.1
High — a near-fixed one-mark formula question
What NEET asks
Direct substitution into vd = I/(neA) — the single most common form.
Time taken by an electron to cross a wire of length l: t = l/vd (comes out enormous).
How vd changes when the wire is stretched, or when radius is halved at constant I.
Order-of-magnitude comparisons: drift speed vs thermal speed vs signal speed (Example 3.1).
Mobility definition and its unit; relation μ = eτ/m.
Trap · directionDrift velocity of electrons is opposite to E, i.e. towards increasing potential; conventional current and j point along E. Any question phrased "in which direction do the electrons move" is testing exactly this reversal.
4. Resistivity and R = ρl/A
NCERT §3.4, §3.7
High — usually as a "wire is stretched / recast" numerical
What NEET asks
A wire is stretched to n times its length: new R = n²R (volume constant).
A wire is drawn to half its radius or folded in half: find the new resistance.
Two wires of same material, different l and A: ratio of R, or of ρ (ρ is unchanged).
Compute ρ from measured R, l and A.
Conductivity σ = 1/ρ and its unit (S m⁻¹).
Must-know
R = ρl/A ρ = RA/l σ = 1/ρ
Stretched to n×length (volume fixed): R′ = n²R
Radius halved (volume fixed): R′ = 16R
Folded in half: R′ = R/4
Trapρ is a material property — stretching, bending or cutting the wire never changes it. Only temperature (and material) does. Questions ask for ρ hoping you'll change it along with R.
Tier 2 — Regular scorers
Not guaranteed every year, but frequent enough that leaving any of them out is a real risk. These are also where the longer numericals live.
5. Electrical energy, power & heating effect
NCERT §3.9
Frequent — bulbs in series/parallel is the signature question
What NEET asks
Two bulbs of different wattage in series or parallel — which glows brighter?
Bulb rated (P, V) connected to a different supply: find actual power drawn.
Heat produced H = I²Rt; comparison of heat in series vs parallel branches.
Energy in kWh and cost of running an appliance.
Why power is transmitted at high voltage — loss Pc = P²Rc/V².
Must-know
P = VI = I²R = V²/R H = I²Rt
Bulb resistance from rating: R = V²_rated/P_rated
Series (same I): P ∝ R → lower-wattage bulb is brighter
Parallel (same V): P ∝ 1/R → higher-wattage bulb is brighter
Trap"Brighter" always means more power dissipated in that arrangement, not the printed rating. Decide which quantity is common (I in series, V in parallel) before choosing between P = I²R and P = V²/R.
6. Kirchhoff's rules
NCERT §3.12
Frequent — often the paper's longest circuit numerical
What NEET asks
Two-loop circuit with two cells: find the current in a named branch.
Junction rule as a one-liner: unknown current at a node given three others.
Which conservation law each rule expresses (charge for junction, energy for loop).
Sign of a current that comes out negative — meaning it flows opposite to the assumed arrow.
Must-know
Junction: ΣI_in = ΣI_out (charge conservation)
Loop: ΣΔV = 0 (energy conservation)
Resistor: −IR along the direction of travel
Cell: −→+ gives +ε ; +→− gives −ε
Trap · signsFix one traversal direction per loop and keep it for the whole loop. Losing a sign on a cell is the commonest error; the answer often still looks plausible, so check it by substituting into a second loop.
7. Cells in series and in parallel
NCERT §3.11
Moderate — usually a clean substitution question
What NEET asks
n identical cells in series or parallel across R: find the current.
Two unequal cells in parallel: equivalent emf and equivalent internal resistance.
One cell reversed in a series chain: εeq = ε₁ − ε₂.
Mixed grouping — when series beats parallel (series when R ≫ r, parallel when R ≪ r).
TrapA cell connected with reversed polarity enters εeq with a minus sign but its r still adds. Internal resistances never subtract.
8. Wheatstone bridge & metre bridge
NCERT §3.13 · metre bridge is gap content
Moderate — one balanced-bridge or balance-point question
What NEET asks
Apply the balance condition to find the unknown fourth resistance.
Recognise a balanced bridge inside a larger network so the galvanometer arm can be removed.
Metre bridge: unknown R from balance length l, using R/S = l/(100 − l).
Which way the balance point shifts when a resistance is increased, or when the gaps are interchanged.
Why the balance point is best kept near the middle of the wire (least fractional error).
Must-know
Balanced bridge: R₁/R₂ = R₃/R₄ ⇒ I_g = 0
Metre bridge: R/S = l/(100 − l)
At balance, the galvanometer branch carries no current
→ it may be removed (open) or replaced by a wire (short)
GapThe metre bridge derivation was removed from this chapter but the topic remains on the NTA list. Learn it as an application of the balance condition, where the two ratio arms are the two parts of a uniform 1 m wire.
TrapAt balance the bridge is balanced both ways: swapping the cell and the galvanometer changes nothing. And balance is independent of the cell's emf — a question that changes the battery is testing whether you know the balance point doesn't move.
Tier 3 — Theory questions, cheap to secure
Rarely numerical. These show up as statement, assertion–reason or graph-reading questions, so a single careful reading plus the graphs is usually enough.
9. Temperature dependence of resistance
NCERT §3.8
Moderate — one numerical or one graph question
What NEET asks
Heating-element numericals: R₁ at room temperature, R₂ = V/I at steady state, find T₂ (Examples 3.3, 3.6, Exercise 3.6).
Find α from two resistance readings at two temperatures.
Sign of α for metals (+), semiconductors and insulators (−), and alloys (≈ 0).
Why nichrome, manganin and constantan are used in standard resistors.
Must-know
ρ_T = ρ₀[1 + α(T − T₀)] R_T = R₀[1 + α(T − T₀)]
α = (R₂ − R₁)/(R₁(T₂ − T₁)) unit: °C⁻¹ or K⁻¹
Metals: τ falls with T → ρ rises
Semiconductors: n rises sharply with T → ρ falls
TrapThe reason ρ rises for a metal is the fall in relaxation time τ, not a change in electron number density n. For semiconductors it is the opposite — n dominates. Assertion–reason questions live on this distinction.
10. Conductors, semiconductors, insulators
NCERT §3.7
Low–moderate — a statement or ordering question
What NEET asks
Order the three classes by resistivity, or place a given ρ value in the right class.
Metal range 10⁻⁸ to 10⁻⁶ Ω m; insulators about 10¹⁸ times higher.
Effect of doping on a semiconductor's resistivity (it falls).
Which carriers conduct: electrons in metals, both ions in electrolytes.
TrapResistivity is the property being classified, not resistance. A thick insulator rod and a thin copper wire can have the same R while their ρ differ by twenty orders of magnitude.
11. V–I characteristics & limitations of Ohm's law
NCERT §3.6 · Points to Ponder 2
Low–moderate — graph identification
What NEET asks
Identify ohmic vs non-ohmic from a V–I graph (straight line through origin = ohmic).
Slope questions: on a V–I plot slope is R; on an I–V plot slope is 1/R.
Which device shows each failure — diode (asymmetric), GaAs (non-unique V for one I).
The statement that V = IR defines resistance for any device; Ohm's law is the extra claim that R is independent of V.
Trap · graphsCheck the axes before quoting a slope. A curve bending towards the V-axis means R is increasing with current, which is the usual clue for a heated filament.
Tier 4 — Read once, don't drill
Background sections. Worth a single careful pass because a definition question is possible, but no practice set is needed.
12. Electric current: definition and scalar nature
NCERT §3.2, §3.3
Occasional — one definition or one integration question
I = q/t for steady current; I = dq/dt for varying current, so q = ∫I dt (area under an I–t graph).
Current is a scalar even though it is drawn with an arrow: I = j · ΔS.
Orders of magnitude: nerve currents in μA, appliances in A, lightning in tens of kA.
Why there is no net current without a field — random thermal velocities average to zero.
The ampere is defined through magnetic effects, which is Chapter 4's business.
TrapCurrent density j is a vector; current is not. Do not add currents at a junction as vectors.
Suggested order of attack
Start with Tier 1 topic 4 (R = ρl/A), then topic 1 (networks). Networks are the load-bearing skill for the whole chapter, and they need the resistance formula first.
Then topic 2 (cells and internal resistance), then topic 5 (power). Both reuse network reduction immediately, so the practice compounds.
Then topic 3 (drift velocity). Independent of the circuit work — a good change of gear, and quick marks.
Then Kirchhoff and the bridges (6, 7, 8) as one block. Bridges are a special case of Kirchhoff; learning them together halves the work.
Finish with Tier 3 in a single reading session, graphs open alongside. Then a mixed 45-question test on the whole chapter.