My son and I are building an electromagnet for a science project. We know that if we wrap the wire around the nail and connect the battery to the wire…presto, a magnet is born. But what is it about flowing current that allows this to happen? — GG, Westfield, NJ
Moving electric charges are inherently magnetic. That’s because electricity and magnetism are intimately related and aren’t really separate phenomena. To see why this is true, imagine two electrons sitting motionless in front of you—they push one another away with electric forces. But now imagine that you and those two electrons are moving northward in a train and someone standing beside the track is watching all of you pass. From that person’s perspective, the two electrons are moving and they exert both electric and magnetic forces on one another. What appears to you to be a purely electric effect appears to the person near the track to involve both electricity and magnetism. Without the appearance of magnetic effects in moving charges, grave inconsistencies would appear in the dynamics of objects view from different perspectives.
So the current in the wire of your electromagnet is inherently magnetic. The magnetic field it produces aligns the tiny magnetic domains in the steel nail so that the nail’s magnetic field greatly strengthens that of the current in the wire.