At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. If she/he is carrying anything made of metal, the metal detector emits a sound. On what principle does this detector work?
The metal detector works on the principle of resonance in ac circuits. When a person walks through the doorway, they are essentially walking through a coil of many turns connected to a capacitor. The circuit is tuned such that it is in resonance.
If the person is carrying anything made of metal, the impedance of the circuit changes. This alteration leads to a significant change in the current flowing through the circuit. The electronic circuitry detects this change in current and causes a sound to be emitted as an alarm.
Explanation
The provided textbook solution explicitly states that the metal detector operates on the principle of resonance in AC circuits. It describes the setup where the person walks through a coil (inductor) connected to a capacitor. The context highlights that the presence of metal alters the impedance, causing a change in current which is detected to trigger the alarm.