Electromagnetic Induction & Alternating Currents Formula Sheet — NEET Physics

Every key Electromagnetic Induction & Alternating Currents 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 · 4 sub-topics

Faraday’s law, Lenz’s law, eddy currents, self & mutual inductance
Alternating currents, peak & RMS values, reactance & impedance
LCR series circuit, resonance, power & wattless current
AC generator & transformer

Faraday's & Lenz's Laws

Magnetic flux

Φ = B \cdot A = B A cos θ

θ =

angle between

B

and the area normal; unit weber (Wb);

Φ = 0

when plane

∥ B

Faraday's law

ε = - N \frac{d Φ}{d t}

flux can change via B, area A, OR orientation

θ

;

N =

turns

A bar magnet approaching a coil induces a current that opposes the magnet's motion — Lenz: the induced current opposes the change.

Induced charge

q = \frac{N ΔΦ}{R}

depends ONLY on the net flux change,

not

on how fast it changes

Three ways to change flux

Change	Example
$B$ varies	magnet moves toward coil
Area $A$ varies	sliding rod on rails
Angle $θ$ varies	rotating coil (generator)

🚫 Examiner Trap · Faraday & Lenz

(1) Lenz's law

=

energy conservation; the induced current

opposes the change

, not the flux itself. (2) Induced

emf

is independent of R, but induced

current

(

ε / R

) is not. (3) Induced charge

q = N ΔΦ/ R

ignores the time taken. (4) A changing B makes a

non-conservative

E-field (

\oint E \cdot d l \neq = 0

Motional EMF & Eddy Currents

Motional emf (rod)

ε = Blv

l =

rod length,

v =

speed

⊥ B

; from Lorentz force qvB on free charges

Rotating rod / disc

ε = \frac{1}{2} B ω l^{2} = \frac{1}{2} B ω R^{2}

ω =

angular speed,

R =

rod length; one end at the axis

A conducting rod sliding on rails in a magnetic field induces an emf Blv — Sliding rod on rails: $ε = Blv$ .

Retarding force & power

F = \frac{B ^{2} l ^{2} v}{R}; P = \frac{B ^{2} l ^{2} v ^{2}}{R}

external work done

=

Joule heat dissipated (energy conservation)

⚡ Shortcut · Eddy currents

Swirling currents in bulk conductors that

oppose

the motion (Lenz)

\Rightarrow

magnetic braking, induction furnace, damping. Reduced by

laminating

the core.

🚫 Examiner Trap · Motional emf

(1)

ε = Blv

needs

v

B

and the rod mutually

⊥

— use components otherwise. (2) Rotating rod is

\frac{1}{2} B ω l^{2}

(the

\frac{1}{2}

is forgotten constantly). (3) The magnetic force

opposes

the motion, so you must do work to keep v constant. (4) Eddy losses scale with thicknes

s^{2}

— lamination cuts them.

Self & Mutual Inductance, Energy

Self-inductance

Φ = L I, ε = - L \frac{d I}{d t}; L_{so l} = \frac{μ _{0} μ _{r} N ^{2} A}{l}

L =

inductance (henry); depends only on geometry & core

Mutual inductance

ε_{2} = - M \frac{d I _{1}}{d t}; M = k L_{1} L_{2}

coupling

k \leq 1

(

k = 1

ideal);

M = μ_{0} N_{1} N_{2} A / l

for coaxial solenoids

Current growth and decay in an LR circuit with time constant tau = L/R — LR circuit: exponential growth & decay.

LR growth / decay

I = \frac{E}{R} (1 - e^{- t / τ}); I = I_{0} e^{- t / τ}, τ = \frac{L}{R}

reaches

63%

(growth) / falls to

37%

(decay) in one

τ

Energy stored

U = \frac{1}{2} L I^{2}; u = \frac{B ^{2}}{2 μ _{0}} (density)

analogous to

\frac{1}{2} C V^{2}

for a capacitor

🚫 Examiner Trap · Inductance

(1) An inductor opposes

change

in current: at

t = 0

it blocks current (acts open), at

t = \infty

it's a wire (short). (2) Series with coupling:

L = L_{1} + L_{2} \pm 2 M

(sign from aiding/opposing fluxes). (3) L depends on geometry/core

only

, not on I. (4) Energy density

B^{2} /2 μ_{0}

mirrors

\frac{1}{2} ε_{0} E^{2}

Alternating Current — RMS & R, L, C

AC quantities

i = i_{0} sin ω t; i_{r m s} = \frac{i _{0}}{2}, i_{m e an} = \frac{2 i _{0}}{π} (half cycle)

AC meters read

rms

; full-cycle mean

= 0

Phase relationship of current and voltage for pure R (in phase), L (current lags) and C (current leads) — Phase of $I$ relative to $V$ for R, L, C.

Comparative: single elements

Element	Opposition	Phase of $I$	Power
R	$R$	in phase	$i_{r m s}^{2} R$
L	$X_{L} = ω L$	lags $9 0^{\circ}$	$0$
C	$X_{C} = \frac{1}{ω C}$	leads $9 0^{\circ}$	$0$

🚫 Examiner Trap · AC & reactance

(1) Pure L or C consumes

zero

average power (I is

9 0^{\circ}

out of phase). (2)

X_{L} = ω L \propto f

(blocks high f);

X_{C} = 1/ ω C \propto 1/ f

(blocks low f / DC). (3) Meters read rms (

i_{0} / 2

), not peak. (4) At DC (

f = 0

X_{L} = 0

(wire),

X_{C} = \infty

(open).

LCR Series Circuit — Impedance

Impedance & current

Z = R^{2} + (X_{L} - X_{C})^{2}; i_{0} = \frac{e _{0}}{Z}

Z =

impedance (

Ω

); net reactance

X_{L} - X_{C}

Phase angle

tan ϕ = \frac{X _{L} - X _{C}}{R}

X_{L} > X_{C}

: inductive (I lags);

X_{L} < X_{C}

: capacitive (I leads)

Impedance triangle with R, net reactance X_L minus X_C, and impedance Z as the hypotenuse — Impedance triangle: Z, R, $(X_{L} - X_{C})$ .

Voltages add as phasors

E_{r m s} = V_{R}^{2} + (V_{L} - V_{C})^{2}

V_{R} = I R

V_{L} = I X_{L}

V_{C} = I X_{C}

Comparative: two-element circuits

Circuit	$Z$
RL	$R^{2} + X_{L}^{2}$
RC	$R^{2} + X_{C}^{2}$
LC	$∣ X_{L} - X_{C} ∣$

🚫 Examiner Trap · LCR series

(1)

V_{L}

and

V_{C}

are

18 0^{\circ}

apart

\Rightarrow

they

subtract

; add voltages as

phasors

, never algebraically (

V_{R} + V_{L} + V_{C} \neq = E

). (2)

V_{L}

V_{C}

alone can

exceed

the source voltage (at resonance). (3) Same current flows through all series elements; it's the voltages that differ in phase. (4) Use the SAME

ω

for

X_{L}

and

X_{C}

Resonance & Power in AC

Series resonance

ω_{0} = \frac{1}{L C}, f_{0} = \frac{1}{2 π L C}

ω_{0}

X_{L} = X_{C}

Z = R

(min), I max,

cos ϕ = 1

Current versus frequency peaking at the resonant frequency, sharper for smaller resistance — Sharper peak $\Rightarrow$ higher Q (smaller R).

Quality factor

Q = \frac{ω _{0} L}{R} = \frac{1}{ω _{0} CR} = \frac{1}{R} \frac{L}{C}

bandwidth

Δ ω = R / L

; high Q

=

sharp/selective

Power in AC

P = E_{r m s} I_{r m s} cos ϕ; cos ϕ = \frac{R}{Z}

cos ϕ =

power factor; only R dissipates power

Comparative: across resonance

$ω$	Nature	$cos ϕ$
$< ω_{0}$	capacitive (I leads)	$< 1$
$= ω_{0}$	resistive, $I$ max	$1$
$> ω_{0}$	inductive (I lags)	$< 1$

🚫 Examiner Trap · Resonance & power

(1) At resonance

Z = R

(minimum), so current is

maximum

— series resonance is an 'acceptor'. (2) Wattless current (

I sin ϕ

) dissipates

no

power; a

choke

(high L, low R) limits AC almost loss-free (

cos ϕ \approx 0

). (3)

V_{L} = V_{C}

at resonance (they cancel)

\Rightarrow E = V_{R}

. (4) Power uses rms values

\times cos ϕ

, not peak.

AC Generator & Transformer

AC generator

ε = NBA ω sin ω t = ε_{0} sin ω t

peak emf

ε_{0} = NBA ω

; converts mechanical

\to

electrical energy

Transformer (ideal)

\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}} = \frac{I _{p}}{I _{s}}

ideal:

V_{p} I_{p} = V_{s} I_{s}

(power conserved); mutual induction,

AC only

Transformer with primary and secondary coils wound on a laminated soft-iron core — Turns ratio sets the voltage ratio.

Comparative: step-up vs step-down

	Step-up	Step-down
Turns	$N_{s} > N_{p}$	$N_{s} < N_{p}$
Voltage	$V_{s} > V_{p}$	$V_{s} < V_{p}$
Current	$I_{s} < I_{p}$	$I_{s} > I_{p}$

🚫 Examiner Trap · Generator & transformer

(1) A transformer works on

AC only

— DC gives no changing flux, no output (and burns the primary). (2) It steps voltage and current

oppositely

(power conserved), never both up. (3) Losses: copper

I^{2} R

, eddy (laminate core), hysteresis (soft iron), leakage. (4) Transmit at

high V / low I

to cut

I^{2} R

line loss.

More NEET Physics formula sheets

Frequently Asked Questions

What are the most important Electromagnetic Induction & Alternating Currents formulas for NEET?

This Electromagnetic Induction & Alternating Currents formula sheet covers all the high-yield Physics formulas, definitions and facts you need for NEET, across Faraday’s law, Lenz’s law, eddy currents, self & mutual inductance, Alternating currents, peak & RMS values, reactance & impedance, LCR series circuit, resonance, power & wattless current, AC generator & transformer — each shown with the key result and, where useful, a worked example.

Is this Electromagnetic Induction & Alternating Currents formula sheet free?

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

How should I revise Electromagnetic Induction & Alternating Currents for NEET?

Blurt the Electromagnetic Induction & Alternating Currents 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.