JEE Main 2022 · 25 July · evening · Single correct · +4 / −1
When you walk through a metal detector carrying a metal object in your pocket, it raises an alarm. This phenomenon works on :
- (a) Electromagnetic induction
- (b) Resonance in ac circuits
- (c) Mutual induction in ac circuits
- (d) Interference of electromagnetic waves
Answer and solution
Answer (b) Resonance in ac circuits
Solution
Physics sir · 4 steps
The detector is a tuned LC circuit: metal changes the coil's inductance and knocks it off resonance.
- What is inside. The doorway frame hides a coil, which acts as an inductor, joined to a capacitor. The circuit is tuned so it sits exactly at resonance and draws a large current.
ω₀ = 1/√LC
- Metal walks in. A metal object near the coil changes its effective inductance L, and with it the impedance. The resonant frequency shifts, so the circuit is no longer at resonance.
- Why it is sensitive. Near resonance the current is very sharp in frequency, so even a small change in L makes the current drop a lot. The detector senses this drop and sounds the alarm.
- So. A metal detector works on resonance in ac circuits, option (b).
The idea to keep. A sharply tuned resonant circuit acts as a sensitive detector: a tiny change in L or C moves it off resonance, and the current changes a lot. That big change is what you measure.
Answer from the NTA final answer key. Solution by PrepWiser.