At the security area of an airport you walk through a weak ac magnetic field inside a large coil of wire What is the result of a small piece of metal on your person that slightly alters the magnetic?

Answer 1

Metal detectors at the airport work the same way as metal detectors used to locate treasure and artifacts. The concepts used in metal detectors are hard to understand unless you are familiar with electromagnetism but I'll try.

There's more than one way metal detectors work. The doorway type at the airport you appear to be referring to may be either of two.

1. Electricity sent through a coil creates a magnetic field. When this field passes through an object that conducts electricity this causes the object to create a magnetic field which can be sensed by another coil in the sending unit. So if the sending unit senses a reflected magnetic field it knows a conductive object is within range.
2. The next kind is even harder to understand. The coil is sent pulses of electricitiy, these pulses cause brief magnetic fields the pulses are echoed (reflected) back to the sending device. Non-conductive objects reflect the pulses quickly. Conductive objects cause the echo to be delayed. The delay tells the detector there is a conductive object within it's range.

The hand held wands work yet a third way that depends on radio frequency.

These are simplified explanations and not exact.