Goal: Bring together information about direction and strength to achieve a vector representation. Develop Cross-Product Formalism.
The big question that is before us is how to quantify the observations we have made. As alluded to in the beginning of this unit, the magnetic field is a vector quantity. As we have seen explicitly, the magnetic field has a direction which tends to curve. That is, magnetic field lines cannot be expected to be straight lines.
Further, the magnetic field lines seem to circle around and reconnect to themselves. That is, they form closed loops. This can be established by cutting a magnet into smaller and smaller pieces. Each piece will be have opposite poles, and the opposite poles always go next to each other if the pieces are returned to the original arrangement in the "uncut' magnet. By an argument of extension to the limit of cutting the magnets into infinitely small pieces, combined with never seeing like poles attract, we can reach the 'accepted' conclusion that the magnetic fields lines are closed circles.
Finally, we seem to see that lines of current produce magnetic fields but that the fields are never parallel to nor completely in the plane of the current.
Ask the students to form the simplest possible rule they can which allows them to predict the shape of a magnetic field if they are told a long straight current exists. Ask them to apply the rule to a circular current to test their model.
Discuss the right hand rule for finding the field direction for a given
current flow along a particular line.