IIT JEE Physics Practice Paper – Electromagnetic Induction & AC (Set 18)
Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Electromagnetic Induction & Alternating Current – IIT JEE Notes (Set 18)
Electromagnetic Induction
Definition
Electromagnetic induction is the phenomenon of production of induced emf and current in a conductor whenever magnetic flux linked with it changes.
Discovery
Michael Faraday discovered electromagnetic induction experimentally.
Magnetic Flux
Definition
Magnetic flux is the total magnetic field passing through a surface.
Formula
Φ = BA cosθ
Unit
The SI unit of magnetic flux is Weber (Wb).
Faraday’s Laws of Electromagnetic Induction
First Law
Whenever magnetic flux linked with a circuit changes, an emf is induced in the circuit.
Second Law
The magnitude of induced emf is proportional to the rate of change of magnetic flux.
Formula
E = – dΦ/dt
Key Insight
The negative sign represents Lenz’s law.
Lenz’s Law
Statement
The direction of induced current is such that it opposes the cause producing it.
Importance
Lenz’s law is based on conservation of energy.
Motional EMF
Formula
E = Blv
Concept
When a conductor moves in a magnetic field, emf is induced across its ends.
Self Induction
Definition
The phenomenon in which changing current in a coil induces emf in the same coil.
Self Inductance
L = Φ/I
Unit
Henry (H)
Energy Stored in an Inductor
Formula
U = ½LI²
Key Insight
Inductors store energy in magnetic field.
Mutual Induction
Definition
Changing current in one coil induces emf in nearby coil.
Application
Used in transformers.
Transformer
Working Principle
Transformer works on mutual induction.
Turns Ratio
Vp/Vs = Np/Ns
Step-Up Transformer
Increases voltage and decreases current.
Step-Down Transformer
Decreases voltage and increases current.
Eddy Currents
Definition
Eddy currents are circulating currents induced in bulk conductors.
Reduction Method
Reduced using laminated iron cores.
Applications
Induction furnace, magnetic braking, speedometers.
Alternating Current (AC)
Definition
Alternating current changes magnitude and direction periodically.
AC Frequency in India
50 Hz
AC Voltage Equation
Formula
V = V₀ sinωt
Current Equation
I = I₀ sinωt
RMS Value
Definition
RMS value is the effective value of AC equivalent to DC producing same heating effect.
Formula
Irms = I₀/√2
Vrms = V₀/√2
Average Value of AC
Important Point
Average value of alternating current over complete cycle is zero.
AC Circuits
Pure Resistor Circuit
Voltage and current are in phase.
Pure Inductor Circuit
Current lags voltage by 90°.
Pure Capacitor Circuit
Current leads voltage by 90°.
Reactance
Inductive Reactance
XL = ωL
Capacitive Reactance
XC = 1/ωC
Key Insight
Inductive reactance increases with frequency, while capacitive reactance decreases.
LCR Circuit
Resonance Condition
XL = XC
Resonance Frequency
f = 1/2π√LC
Key Insight
At resonance, impedance becomes minimum and current becomes maximum.
Power in AC Circuit
Formula
P = Vrms Irms cosφ
Power Factor
cosφ
Importance
Higher power factor means efficient power transmission.
AC Generator
Working Principle
Based on electromagnetic induction.
Function
Converts mechanical energy into electrical energy.
Back EMF
Concept
In electric motors, induced emf opposes applied voltage and is called back emf.
Importance
Protects motor from excessive current.
Conceptual Insights
Key Understanding
Electromagnetic induction always opposes change in magnetic flux.
Common Mistakes
Students often confuse RMS value with average value and forget phase relations in AC circuits.
Important Exam Concepts
Conceptual Traps
Transformer cannot work on DC because magnetic flux must change continuously.
JEE Strategy
Practice formulas, phase diagrams, and resonance problems thoroughly. Focus on Faraday’s law, transformers, and AC circuit analysis.