An airport security scanner uses electromagnetic induction to detect metallic objects inside luggage. The scanner consists of a large rectangular frame cont...
Question 1 Report
An airport security scanner uses electromagnetic induction to detect metallic objects inside luggage. The scanner consists of a large rectangular frame containing a transmitter coil connected to an alternating current supply and a separate receiver coil connected to a sensitive detector. When a suitcase containing metal passes through the frame, the changing magnetic field from the transmitter coil induces eddy currents in the metal object. These eddy currents produce their own magnetic field, which is picked up by the receiver coil. Fig. 4.1 shows a simplified cross-section of the scanner frame.
(a) State what is meant by electromagnetic induction. [1]
(b) Explain why the transmitter coil must carry an alternating current, rather than a steady direct current, for the scanner to detect metal objects. [2]
(c) Explain why eddy currents are induced in the metal object as it passes through the scanner. [2]
(d) Suggest why the scanner does not produce a signal when a suitcase containing only plastic and fabric items passes through the frame. [2]
Answer Details
(a) Electromagnetic induction is the generation of an e.m.f. (voltage) in a conductor when the magnetic flux through it changes, or when a conductor cuts magnetic field lines. [1]
This is a fundamental definition: whenever a conductor experiences a changing magnetic environment, a voltage is induced across it. If the conductor forms a complete circuit, this voltage drives a current.
(b) An alternating current produces a continuously changing magnetic field. [1] A steady direct current would produce a constant (unchanging) magnetic field, which would not induce any e.m.f. or current in the metal object, since a changing field is required for induction. [1]
Electromagnetic induction requires a changing flux. An a.c. supply reverses direction many thousands of times per second (at the supply frequency), ensuring the field is always changing and can therefore induce eddy currents in any metal passing through it.
(c) The changing magnetic field from the transmitter coil passes through the metal object. [1] This changing flux induces eddy currents (circulating currents) in the metal because metals are electrical conductors. [1]
Faraday's law applies: the changing flux through the metal induces an e.m.f. Because the metal is a conductor with low resistance, this e.m.f. drives circulating (eddy) currents within the body of the metal. These eddy currents then produce their own magnetic field, which the receiver coil detects.
(d) Plastic and fabric are electrical insulators (non-conductors). [1] Since no eddy currents can be induced in them, no secondary magnetic field is produced for the receiver coil to detect. [1]
Electromagnetic induction can generate an e.m.f. in any material experiencing a changing flux, but only conductors allow currents to flow in response. Insulators like plastic and fabric have extremely high resistance, so even though the changing flux passes through them, no significant currents flow and no detectable secondary field is created.
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