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.
The force acting on a current-carrying conductor is given by where B is the magnetic flux density, I is the current, l is the length of the wire and is the angle between the current and the magnetic field. It can be measured in directly using a weighing scale as described below:
An electronic weighing scale
Two neodymium magnets
40 cm long wire
Place some plasticine between two neodymium magnets to hold them together with each magnet having a different pole facing up (north for one and south for the other).
Place the magnets on the weighing scale.
Connect the 40 cm wire across the opposite terminals of the batteries.
Hold the wire horizontally and place it between the two magnets so that the current runs perpendicularly to the magnetic field lines.
Using Fleming's left-hand rule, one can predict whether the measured weight will increase or decrease. If the magnetic force acting on the wire is upward, by Newton's 3rd law, the reaction force acting on the magnets is downward and the measured weight will increase, and vice versa.
Flipping the current around in the opposite direction will yield opposite results.