PHYS 1112 Module 9.3

Magnetic Field and Force

Magnetic Field Lines

First, let’s define something. You know how we call electric fields ? Well, magnetic fields get their own symbol too, and it’s . I know, it’s not M, for magnetic? It’s not. This is due to historical reasons that I won’t go too deep into here. But when James Clerk Maxwell was developing his theory of electromagnetism, he was naming the vector fields and he did so alphabetically. There’s an field, a field, and so on. It just so happens that the electric field was conveniently named the field and the magnetic field got stuck with the name: the field.

We know that permanent magnets exist in nature, and we also now know from Oersted’s experiments that an electric current is a source of magnetic field too. In this image you can see the magnetic field lines (blue lines with arrows labeled ) for different magnets.

One thing to note about magnetic field lines is that they are always closed loops. This is very different from electric field lines (E-fields start at positive charge and end at negative charge) because B-field lines don’t start or end, they are continuous. Look at the bar magnet (a) in the figure above. Outside the magnet, the B-field lines go from N to S and inside the magnet the B-field lines go from S to N. You can see a similar pattern in the other B-fields too.

---

Magnetic Force (Lorentz Force)

There are three experimental observations about the magnetic force, which are essentially rules about the force:

1. An electric charge (q) can experience a magnetic force only if it is moving in a magnetic field.
2. If the electric charge is moving parallel to the B-field, the charge will not experience a magnetic force.
3. The magnetic force on an electric charge moving through a B-field is always perpendicular to both the velocity of the charge and the B-field vector.