# Silly putty & electromag in action

I am teaching part of an electromag course this semester but I am doing the maths bit of the course, so I’m not doing much electromag, apart from doing stuff like getting the students to calculate the electrostatic potential inside a spherical cavity. This is a bit of a shame as this means I don’t get to show this to the students:

I think it is a lovely illustration of both energy and forces, and of magnetism. The silly putty has very small magnetic particles in it and these particles have a lower energy in the strong magnetic field of the magnet than they do outside it.

Now, forces are just the rate of energy of energies with position. So because as the magnet moves toward the magnetic silly putty the energy goes down, this necessarily means that there is a force pulling the putty and the magnet together. As the putty is soft this results in the putty engulfing the magnet. The minimum energy configuration is probably with the magnet in the centre of the silly putty, so that is where the magnet goes. Simple.