NEET Electromagnetic Induction & Alternating Currents PYQs | 2013–2025

2025

Q1. Coil of N turns, area A rotates in uniform B with angular speed ω. Induced EMF?

Solution:

• Flux through coil: $\Phi = NBA \cos \theta = NBA \cos (\omega t)$Φ=NBAcosθ=NBAcos(ωt)
• Faraday’s law: $\mathcal{E} = – \frac{d\Phi}{dt} = NBA \omega \sin(\omega t)$E=−dtdΦ​=NBAωsin(ωt)

✅ Answer: $\mathcal{E} = NBA \omega \sin (\omega t)$E=NBAωsin(ωt)

Q2. AC generator principle.

Solution:

• Based on Faraday’s law of induction.
• Coil rotates in magnetic field → changing flux → induced EMF → alternating current.

Q3. Transformer: Primary 500 turns, secondary 100 turns, 220 V input → secondary voltage?

Solution:$$\frac{V_s}{V_p} = \frac{N_s}{N_p} \implies V_s = \frac{100}{500} \times 220 = 44 \text{ V}$$Vp​Vs​​=Np​Ns​​⟹Vs​=500100​×220=44 V

2024

Q1. Faraday’s law: $\mathcal{E} = – \frac{d\Phi}{dt}$E=−dtdΦ​

Q2. Conducting rod moving in B:$$\mathcal{E} = B L v \quad (\text{if velocity perpendicular to B})$$E=BLv(if velocity perpendicular to B)

Q3. Self-induction: Coil induces EMF on itself.
Mutual induction: EMF induced in one coil due to current change in another.

2023

Q1. RMS value of AC:

• Instantaneous: $i = I_0 \sin \omega t$i=I0​sinωt
• RMS: $I_{\text{rms}} = \frac{I_0}{\sqrt{2}}$Irms​=2​I0​​

Q2. Coil of inductance L, current $i = I_0 \sin \omega t$i=I0​sinωt:$$\mathcal{E}_L = L \frac{di}{dt} = L I_0 \omega \cos \omega t$$EL​=Ldtdi​=LI0​ωcosωt

Q3. Step-up & step-down transformer:$$\frac{V_s}{V_p} = \frac{N_s}{N_p}$$Vp​Vs​​=Np​Ns​​

• Step-up → $N_s > N_p$Ns​>Np​, Step-down → $N_s < N_p$Ns​<Np​

2022

Q1. Rectangular coil rotating → maximum EMF:
$\mathcal{E}_{max} = NBA \omega$Emax​=NBAω

Q2. Lenz’s law: Induced current opposes change in flux. Example: Magnet falling through coil → induced current produces upward force opposing motion.

Q3. AC current in L-R circuit:$$i(t) = \frac{V_0}{\sqrt{R^2 + (\omega L)^2}} \sin (\omega t – \phi), \quad \phi = \tan^{-1} (\frac{\omega L}{R})$$i(t)=R2+(ωL)2​V0​​sin(ωt−ϕ),ϕ=tan−1(RωL​)

2021

Q1. Solenoid with AC: $I = I_0 \sin \omega t$I=I0​sinωt
Magnetic flux: $\Phi = \mu_0 n A I_0 \sin \omega t$Φ=μ0​nAI0​sinωt

Q2. Average power in AC, purely resistive:$$P_{avg} = I_{rms}^2 R$$Pavg​=Irms2​R

Q3. Electromagnetic damping in moving coil: Eddy currents induced in coil → oppose motion → damping.

2020

Q1. Rotating coil EMF:
$\mathcal{E} = NBA \omega \sin (\omega t)$E=NBAωsin(ωt), max EMF $\mathcal{E}_{max} = NBA \omega$Emax​=NBAω

Q2. Reactance:

• Inductor: $X_L = \omega L$XL​=ωL
• Capacitor: $X_C = \frac{1}{\omega C}$XC​=ωC1​

Q3. Phase relationships:

• Resistor: current in phase with voltage
• Inductor: current lags voltage by 90°
• Capacitor: current leads voltage by 90°

2019

Q1. Rod moving in B:
$\mathcal{E} = B L v$E=BLv

Q2. AC generator principle → rotating coil → changing flux → induced EMF → AC current

Q3. Step-down transformer efficiency 90%:$$\frac{N_s}{N_p} = \frac{V_s}{V_p} = \frac{110}{220} = \frac{1}{2}$$Np​Ns​​=Vp​Vs​​=220110​=21​

2018

Q1. Faraday’s law: $\mathcal{E} = – d\Phi/dt$E=−dΦ/dt, example: moving magnet through coil

Q2. Instantaneous EMF in rotating coil:
$\mathcal{E} = NBA \omega \sin (\omega t)$E=NBAωsin(ωt)

Q3. Self-inductance: $\mathcal{E}_L = -L \frac{dI}{dt}$EL​=−LdtdI​, energy: $U = \frac{1}{2} L I^2$U=21​LI2

2017

Q1. RMS of sinusoidal AC: $I_{\text{rms}} = I_0 / \sqrt{2}$Irms​=I0​/2​

Q2. Coil in AC: $\mathcal{E} = L \frac{di}{dt} = L I_0 \omega \cos \omega t$E=Ldtdi​=LI0​ωcosωt

Q3. Transformer: $V_s/V_p = N_s/N_p$Vs​/Vp​=Ns​/Np​

2016

Q1. Moving conductor: $\mathcal{E} = B L v$E=BLv

Q2. Self vs mutual induction explained

Q3. Average power in AC (resistive): $P = I_{rms}^2 R$P=Irms2​R

2015

Q1. Circular coil rotation: $\mathcal{E} = NBA \omega \sin \omega t$E=NBAωsinωt

Q2. Lenz’s law: Induced EMF opposes flux change, e.g., falling magnet in coil

Q3. Transformer turns ratio: $N_s/N_p = V_s/V_p = 110/220 = 1/2$Ns​/Np​=Vs​/Vp​=110/220=1/2

2014

Q1. Current in L-R AC:$$i = \frac{V_0}{\sqrt{R^2 + (\omega L)^2}} \sin (\omega t – \phi), \quad \phi = \tan^{-1} (\omega L / R)$$i=R2+(ωL)2​V0​​sin(ωt−ϕ),ϕ=tan−1(ωL/R)

Q2. Phase differences:

• R: in-phase, L: current lags, C: current leads

Q3. Inductor AC voltage: $\mathcal{E}_L = L \frac{di}{dt}$EL​=Ldtdi​

2013

Q1. Coil rotating at frequency f:
$\omega = 2 \pi f$ω=2πf, $\mathcal{E}_{max} = NBA \omega$Emax​=NBAω

Q2. AC generator principle: Coil rotates → changing flux → EMF → AC current

Q3. Mutual inductance: $M = \frac{\mathcal{E}_2}{dI_1/dt}$M=dI1​/dtE2​​