Electromagnetic Induction — Previous Year Questions (Class 12 Physics)
Electromagnetic induction extends the concepts from Class 10, introducing Faraday's and Lenz's laws in detail. It is crucial for understanding transformers
TL;DR: Electromagnetic induction extends the concepts from Class 10, introducing Faraday's and Lenz's laws in detail. It is crucial for understanding transfo…
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Electromagnetic induction extends the concepts from Class 10, introducing Faraday's and Lenz's laws in detail. It is crucial for understanding transformers
Electromagnetic Induction — Previous Year Questions with Solutions
Q (2023, 3 marks): Define magnetic flux. Derive Faraday's law of electromagnetic induction.
Answer: Magnetic flux (Φ) is the measure of the number of magnetic field lines passing through a surface.
Φ = B·A = BA cos(θ)
Where B is magnetic field, A is area, and θ is angle between B and normal to surface.
Faraday's law: The induced EMF in a circuit equals the negative rate of change of magnetic flux:
ε = -N(dΦ/dt)
Derivation (conceptual):
1. When a magnet moves toward a coil, the magnetic flux through the coil increases.
2. By Lenz's law, the induced current opposes this change.
3. The induced EMF is proportional to how fast the flux changes.
4. For N turns: ε = -N(dΦ/dt)
The negative sign indicates opposition (Lenz's law).
Q (2022, 2 marks): A rectangular coil of area 200 cm^2 and 10 turns is placed perpendicular to a uniform magnetic field of 0.5 T. The field is reduced to zero in 0.1 s. Calculate the induced EMF.
Answer: Given:
A = 200 cm^2 = 200 × 10^-4 m^2 = 0.02 m^2
N = 10 turns
B_initial = 0.5 T, B_final = 0 T
Δt = 0.1 s
Change in magnetic flux:
ΔΦ = A × ΔB = 0.02 × (0 - 0.5) = -0.01 Wb
Rate of change of flux:
dΦ/dt = ΔΦ/Δt = -0.01 / 0.1 = -0.1 Wb/s
Induced EMF:
ε = -N(dΦ/dt) = -10 × (-0.1) = 1 V
Q (2023, 3 marks): Explain Lenz's law and state its significance in energy conservation.
Answer: Lenz's law: The direction of the induced EMF (and current) is such that it opposes the change in magnetic flux that caused it.
Significance in energy conservation:
1. When external work is done to change magnetic flux (e.g., moving a magnet), the induced current opposes this motion.
2. Work must be done against the magnetic force to maintain the motion.
3. This work is converted to electrical energy (and ultimately heat in resistances).
4. Without Lenz's law's opposition, perpetual motion would be possible, violating energy conservation.
5. The mechanical energy input equals the electrical energy output, ensuring energy is conserved.
Q (2022, 3 marks): Describe the phenomenon of self-inductance and mutual inductance with appropriate examples.
Answer: Self-inductance (L):
- When current in a coil changes, the changing magnetic field induces an EMF in the same coil.
- ε = -L(dI/dt)
- L is self-inductance (unit: Henry, H)
- Example: In a circuit with a switch, when switched off, the self-induced EMF opposes the decrease in current, causing a spark.
Mutual inductance (M):
- When current in one coil changes, the changing magnetic field induces an EMF in a nearby coil.
- ε2 = -M(dI1/dt)
- Example: A transformer has two coils. Changing current in primary coil induces EMF in secondary coil.
- Transformer principle: V2/V1 = N2/N1 (for ideal transformers)
Q (2023, 2 marks): A transformer has a primary coil with 500 turns and secondary coil with 50 turns. The primary is connected to a 220 V AC source. Calculate the secondary voltage (assuming 100% efficiency).
Answer: Given:
Np = 500 turns (primary)
Ns = 50 turns (secondary)
Vp = 220 V
For an ideal transformer:
Vs/Vp = Ns/Np
Vs = Vp × (Ns/Np)
Vs = 220 × (50/500)
Vs = 220 × 0.1
Vs = 22 V
The secondary voltage is 22 V (step-down transformer).
Q (2021, 3 marks): Explain the construction and working of a step-up transformer. Why is it used in power transmission?
Answer: Step-up transformer:
- Primary coil has fewer turns than secondary coil (Ns > Np).
- Increases voltage, decreases current: Vs > Vp and Is < Ip
- For ideal transformer: VsIs = VpIp (power is conserved)
Usage in power transmission:
1. Power stations generate electricity at moderate voltage (10-25 kV).
2. Step-up transformer increases voltage to 100-400 kV for long-distance transmission.
3. High voltage reduces current, which minimizes power loss in transmission lines.
4. Power loss = I^2R, so reducing current significantly reduces losses.
5. At the destination, step-down transformers reduce voltage to safer levels for distribution.
This makes long-distance power transmission economical and efficient.
Frequently Asked Questions
What is the relationship between power and energy in electromagnetic induction?
Power is the rate of energy transfer. In electromagnetic induction, P = εI, where ε is induced EMF and I is induced current. The energy dissipated as heat in a resistor is E = I^2Rt. In a transformer, input power equals output power (for ideal transformers): Pp = Ps.
Why do transformers not work with direct current (DC)?
Transformers require a changing magnetic flux to induce EMF. DC produces a constant magnetic field, which does not change with time, so dΦ/dt = 0 and no EMF is induced. Only AC, with its constantly varying current, produces a changing magnetic field that can induce EMF in the secondary coil.
More Class 12 Physics PYQs
- Current Electricity
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Alternating Current
- Atoms and Nuclei
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