Physics: Turning Magnetism On and Off With Current

Electromagnetism is the study of how electric currents generate magnetic fields, and how those fields can be controlled. Unlike a permanent magnet, whose field is always present, an electromagnet produces magnetism only while current flows through its coil. This makes its field switchable: turn the current on, and the magnet acts; turn it off, and the field disappears. The strength of the field also depends on factors such as the current and the number of turns in the coil, giving further control. This controllability matters in real applications where magnetism must be engaged and disengaged on demand. In a scrapyard crane, an electromagnet can lift metal while current flows and release it simply by switching the current off. A permanent magnet cannot do this, since its field is always on and cannot be turned off, so objects cannot be released at will. The key relationship is therefore between current and magnetic field: controlling one controls the other.