One of the most effective ways to detect and separate metals like copper and aluminum from iron is by using a magnet. However, there is a curious interaction that occurs when a significant amount of copper comes into contact with a very powerful magnet, say, a neodymium one. From a certain point of view, the magnet seems "trapped" in a viscous material when placed on the copper surface, and with the help of another magnet, it can even float temporarily.
A while ago we came across several projects involving magnets, including their reaction with iron in the blood and the possibility of seeing their magnetic field with the help of ferrofluids. Neodymium magnets are relatively inexpensive and have a privileged place among home experiments and the DIY world, but we must not forget that they demand respect.
The most powerful magnets can cause serious injuries, and we don't want anyone to end up in the hospital. That said, if you have a neodymium magnet nearby and a thick copper plate (aluminum also works), you'll notice a rather curious behavior...
Neodymium Magnets and Copper
Yes, it's as if the copper surface were covered with a transparent viscous layer that slows down the magnet. Even if we throw it directly, the magnet does not suffer a full impact, but instead brakes at the last moment.
When a strong magnetic field moves through copper, it causes a rearrangement of the electrons that spin in a circular pattern, which is perpendicular to the direction of the approaching magnetic field. The point is that the electrons resist this sudden change, generating their own magnetic field.
Now, here there is no attraction or repulsion (only resistance), and it's also not a property of copper. This last point is proven by passing a neodymium magnet through the center of an open copper coil. If copper were responsible for that "delay", the magnet would react in all cases, but with the open coil it falls like a rock.
When closing the coil, the magnet's momentum is converted into electric current, and an excellent way to visualize the process is to use a simple LED as a bridge. At the same time, the resistance caused by electrical inductance in copper allows a magnet to levitate when a second magnet is placed on the other side of the plate.
All the energy is dissipated by the copper as heat, a loss that is generally not desired, but has real-world applications, for example, the brakes used in high-speed trains.
