Current Electricity Formula Sheet — NEET Physics

Q: What are the most important Current Electricity formulas for NEET?

This Current Electricity formula sheet covers all the high-yield Physics formulas, definitions and facts you need for NEET, across Electric current, drift velocity, Ohm’s law, resistance, V-I characteristics, electrical energy & power, resistivity, Series & parallel resistors, temperature dependence of resistance, EMF & internal resistance, combination of cells, Kirchhoff’s laws, Wheatstone bridge, metre bridge — each shown with the key result and, where useful, a worked example.

Q: Is this Current Electricity formula sheet free?

Yes — the full chapter formula sheet is free to read online, no login or payment required.

Q: How should I revise Current Electricity for NEET?

Blurt the Current Electricity key points from memory, then check against this sheet to find your gaps — and practise a few previous-year questions on the chapter to make sure you can apply them under time pressure.

Every key Current Electricity formula, definition and fact for NEET Physics in one place — with common examiner traps and worked examples. Free to read; blurt from memory, then check your gaps.

Syllabus — topics coveredNCERT · 5 sub-topics

Electric current, drift velocity, Ohm’s law, resistance
V-I characteristics, electrical energy & power, resistivity
Series & parallel resistors, temperature dependence of resistance
EMF & internal resistance, combination of cells
Kirchhoff’s laws, Wheatstone bridge, metre bridge

Electric Current & Drift Velocity

Electric current

I = \frac{q}{t} = \frac{d q}{d t}

conventional current is

opposite

to electron flow; unit ampere (A)

Drift velocity & current

v_{d} = \frac{e E τ}{m}; I = neAv_{d}

τ =

relaxation time,

n =

free-electron density,

A =

area,

e =

electron charge

Electrons in a conductor drift opposite to the applied field E with drift velocity v_d producing current I=neAv_d — Electrons drift opposite to $E$ .

Comparative: the three transport quantities

Quantity	Formula	Meaning
Drift velocity $v_{d}$	$e E τ / m$	mean electron speed
Current density $J$	$I / A = n e v_{d} = σ E$	current per area
Mobility $μ$	$v_{d} / E = e τ / m$	drift speed per field

🚫 Examiner Trap · Current & drift

(1) Conventional current is

opposite

to electron motion. (2) Drift speed is tiny (

\sim

mm/s) yet the bulb lights instantly — the field propagates at

\sim c

, not the electrons. (3)

v_{d} \propto E \propto V

, so current

\propto V

(Ohm). (4) Current is a

scalar

but

J

is a vector.

Ohm's Law, Resistance & Resistivity

Ohm's law

V = I R; J = σ E (microscopic)

R =

resistance,

σ =

conductivity; holds for

ohmic

conductors at fixed T

Resistance & resistivity

R = \frac{ρl}{A}; ρ = \frac{m}{n e ^{2} τ} = \frac{1}{σ}

ρ =

resistivity (material + T only, NOT shape);

l =

length,

A =

area

V-I graph: straight line for an ohmic conductor and a curve for a non-ohmic device — Ohmic $=$ linear; non-ohmic (diode, GaAs) $=$ curved.

Temperature dependence

ρ_{T} = ρ_{0} [1 + α (T - T_{0})]; R_{T} = R_{0} [1 + α Δ T]

α =

temperature coefficient of resistivity (

K^{- 1}

)

Comparative:

ρ

vs temperature

Material	$α$	$ρ$ as $T ↑$
Metal	$> 0$	rises
Semiconductor / insulator	$< 0$	falls ( $n ↑$ )
Alloy (manganin)	$\approx 0$	nearly constant
Superconductor	—	$0$ below $T_{c}$

🚫 Examiner Trap · Ohm, resistance & resistivity

(1)

ρ

depends on material & T

only

— stretching a wire changes R (via l,A) but not

ρ

. (2) Stretch to

n \times

length (volume fixed)

\Rightarrow R \to n^{2} R

. (3) Metals:

ρ ↑

with T (

τ ↓

); semiconductors:

ρ ↓

(

n ↑

). (4) Ohm's law is

not

universal — diodes/GaAs are non-ohmic.

Combination of Resistors

Comparative: series vs parallel

	Series	Parallel
Effective $R$	$\sum R_{i}$	$\frac{1}{R} = \sum \frac{1}{R _{i}}$
Same for all	current $I$	voltage $V$
Divides	$V \propto R$	$I \propto 1/ R$
$R_{e ff}$ vs members	$>$ largest	$<$ smallest

Series / Parallel

R_{s} = R_{1} + R_{2} + \dots; \frac{1}{R _{p}} = \frac{1}{R _{1}} + \frac{1}{R _{2}} + \dots

two in parallel:

R = \frac{R _{1} R _{2}}{R _{1} + R _{2}}

Three resistors in series and three in parallel with their equivalent-resistance formulas — Series (same $I$ ) vs parallel (same $V$ ).

Useful results

▸n equal resistors R: series nR, parallel $R / n$ (ratio $n^{2}$ ).
▸Stretching a wire to $n \times$ length: $R \to n^{2} R$ (volume constant).
▸Balanced Wheatstone arm carries no current $\Rightarrow$ remove it when reducing.

🚫 Examiner Trap · Resistor combinations

(1) Parallel R is

less

than the smallest member — opposite to series. (2) Identify nodes first; resistors between the SAME two nodes are in parallel even if drawn far apart. (3) In a balanced bridge the central arm has

I = 0

— short/remove it to simplify. (4) Symmetry (Wheatstone, cube of resistors) collapses many networks fast.

Cells: EMF, Internal Resistance & Grouping

EMF

ε

: Work done per unit charge by the cell; equals the terminal voltage on

open

circuit (

I = 0

). Internal resistance r models energy lost inside.

Terminal voltage

V = ε - I r (discharge); I = \frac{ε}{R + r}

charging:

V = ε + I r

; short circuit

I_{ma x} = ε / r

;

V < ε

under load

A cell of emf and internal resistance r driving an external resistor R, with terminal voltage V=epsilon-Ir — Internal resistance drops the terminal voltage.

Comparative: grouping

n

identical cells

Grouping	Current $I$	Best when
Series	$\frac{n ε}{R + n r}$	$R ≫ r$
Parallel	$\frac{ε}{R + r / n}$	$R ≪ r$
Mixed ( $m$ rows)	$\frac{nm ε}{m R + n r}$	$R = n r / m$

🚫 Examiner Trap · Cells & grouping

(1) Terminal

V = ε - I r

on discharge but

ε + I r

while

charging

— sign flips. (2)

V = ε

only at

I = 0

(open circuit); under load

V < ε

. (3) Mixed grouping gives max current when external

R =

total internal

n r / m

. (4) Identical cells in parallel: emf stays

ε

, internal resistance becomes

r / n

Kirchhoff's Laws & Electrical Power

Kirchhoff's laws

KCL: \sum I = 0 (junction); KVL: \sum I R = \sum ε (loop)

KCL

=

charge conservation; KVL

=

energy conservation

Junction rule with currents in and out, and a loop with a resistor and a cell — Junction (KCL) and loop (KVL) rules.

Power & energy

P = V I = I^{2} R = \frac{V ^{2}}{R}; H = I^{2} Rt

Joule heating;

1 kWh = 3.6 \times 1 0^{6}

J (commercial unit)

Maximum power transfer

P_{ma x} = \frac{ε ^{2}}{4 r} when R = r

efficiency only

50%

at max power

Comparative: bulbs (rated

P

V

)

	Series	Parallel
Combined $P$	$\frac{1}{P} = \sum \frac{1}{P _{i}}$	$P = \sum P_{i}$
Brighter bulb	$lower$ -watt	higher-watt
Reason	same I, $P = I^{2} R$	same V, $P = V^{2} / R$

🚫 Examiner Trap · Kirchhoff & power

(1) A bulb's resistance

R = V^{2} / P_{r a t e d}

— use

rated

values to get R, then recompute P in the actual circuit. (2) In

series

, the lower-wattage (higher-R) bulb glows

brighter

— counter-intuitive. (3) Max-power (

R = r

) wastes half the energy in r. (4) Transmit at high V: line loss

P_{c} = P^{2} R_{c} / V^{2}

Wheatstone & Metre Bridge

Wheatstone balance

\frac{P}{Q} = \frac{R}{S} (galvanometer I_{G} = 0)

P, Q, R, S =

the four arm resistances

Wheatstone bridge: four resistors P, Q, R, S in a diamond with a galvanometer across one diagonal and a cell across the other — Balanced when no current flows through G.

Metre bridge

\frac{R}{S} = \frac{l}{100 - l} \Rightarrow S = R \frac{100 - l}{l}

l =

balancing length (cm) from the left end

Why it is accurate

▸A $null$ method — reading is independent of cell EMF and galvanometer calibration.
▸Most sensitive when the four arms are comparable (balance near the middle).
▸Interchanging battery and galvanometer does $not$ change the balance.

🚫 Examiner Trap · Bridges

(1) Balance condition is

P / Q = R / S

— at balance

I_{G} = 0

, so

galvanometer resistance is irrelevant

. (2) Metre-bridge: keep the balance point near the centre to minimise % error from end corrections. (3) Don't take readings with current flowing for long (heats the wire, shifts

ρ

). (4) The bridge measures a

ratio

, so a precise standard R is essential.

Potentiometer & Measuring Instruments

Potentiometer: Measures p.d. / EMF drawing

no current at balance

\Rightarrow

acts as an ideal voltmeter. Potential gradient

K = V / L

is constant on a uniform wire.

Uses

ε = K l; \frac{ε _{1}}{ε _{2}} = \frac{l _{1}}{l _{2}}; r = R (\frac{l _{1} - l _{2}}{l _{2}})

l =

balance length; compare EMFs and find internal resistance

r

Potentiometer: driver cell and rheostat across a uniform wire, with a jockey and galvanometer locating the balance length for an unknown cell — Balance length l sets $ε = K l$ .

Comparative: ammeter vs voltmeter

	Ammeter	Voltmeter
Measures	current	p.d.
Connection	series	parallel
Resistance	low ( $0$ ideal)	high ( $\infty$ ideal)
Made from G by	shunt $S$ (parallel)	series $R$ (high)

Galvanometer conversion

shunt S = \frac{G i _{g}}{I - i _{g}}; series R = \frac{V}{i _{g}} - G

G =

galvo resistance,

i_{g} =

full-scale current

🚫 Examiner Trap · Potentiometer & meters

(1) A potentiometer beats a voltmeter because it draws

zero

current at balance (true EMF, not terminal V). (2) Ammeter in

series

(low R), voltmeter in

parallel

(high R) — swapping them can short the circuit. (3) To measure EMF the driver cell's emf must exceed the cell tested, else

no balance point

. (4) Longer wire

\Rightarrow

smaller K

\Rightarrow

higher sensitivity.

More NEET Physics formula sheets

Frequently Asked Questions

What are the most important Current Electricity formulas for NEET?

This Current Electricity formula sheet covers all the high-yield Physics formulas, definitions and facts you need for NEET, across Electric current, drift velocity, Ohm’s law, resistance, V-I characteristics, electrical energy & power, resistivity, Series & parallel resistors, temperature dependence of resistance, EMF & internal resistance, combination of cells, Kirchhoff’s laws, Wheatstone bridge, metre bridge — each shown with the key result and, where useful, a worked example.

Is this Current Electricity formula sheet free?

Yes — the full chapter formula sheet is free to read online, no login or payment required.

How should I revise Current Electricity for NEET?

Blurt the Current Electricity key points from memory, then check against this sheet to find your gaps — and practise a few previous-year questions on the chapter to make sure you can apply them under time pressure.

Also useful: all formula sheets · NEET previous-year papers · most important chapters.