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 $$F = BIl sin theta$$, where $$B$$ is the magnetic flux density, $$I$$ is the current within the wire, $$l$$ is the length of the wire and $$theta$$ 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: