I enjoy lecturing on topics like Superposition and Electromagnetism in the GCE A-level syllabus as they lend themselves well to the use of fun demonstrations that I can perform in front of the audience.
One of the recent demonstrations that I did was to demonstrate the measurement of the magnetic force acting on a wire and to show that the force can be inverted when the current is reversed. The magnitude of the force can be shown to be consistent with the relationship , where is the magnetic flux density, is the current within the wire, is the length of the wire and is the angle between the wire and the magnetic field. This can be illustrated by independently varying one of the 4 variables and observing the change in force.
The setup is also a good for a demonstration to illustrate Fleming's Left-Hand Rule.
For more details, visit http://www.scienses.com/measuring-the-force-on-a-current-carrying-conductor/.
Meanwhile, here's a video I made to show what I did: