question about metal detected in Physics?

Question

3)Land mines made of metal can be detected by sending in a pulse of magnetic filed but plastic land mines cannot be detected in this way. Why?

Answer 1

It is a property of magnetic materials that the field applied across them is compounded to a great extent. A metal detector is 2 sets of concentric wires. There is current in both of them. So when we move the detector in such a way that the inner ring of wire contains the magnetic material, the field is compounded to a great extent. This sets up a reverse current in the outer coil, in accordance with Lenz's law. This change in the current is detected by the metal detector. So, it gives the signal.

Plastic on the other hand is not a ferro-magnetic material. So, there is no increase in the field, and hence plastic landmines cannot be detected by metal detectors.