A security door system uses an electromagnet to hold a heavy metal door shut. When the electromagnet is switched on, it holds the door closed. When a correct code is entered, the electromagnet is switched off and the door can be opened. Explain how the electromagnet works and why the door is attracted to it when the current is switched on.

A solenoid is a coil of wire wound into a tight helix. When electric current flows through the coil, it generates a magnetic field along the axis of the solenoid. The iron core placed inside the coil concentrates the field lines, greatly increasing the magnetic flux density and making the device a strong electromagnet. The iron plate on the door’s inner face is brought into this magnetic field; the field induces a magnetic dipole moment in the plate, giving it north and south poles. The induced pole that faces the electromagnet is always opposite to the nearest pole of the electromagnet, so the attractive force between opposite poles pulls the door toward the core. Thus, when the current is switched on the door is attracted and held shut, and when the current is switched off the magnetic field collapses, the induced magnetism disappears and the door can be opened.