Essential Idea:

Formulas:

$$ \text{}\\ Fq = vB\sin\theta\\ F = BIL\sin\theta\\\\\text{}\\ \text{Magnetic field strength a distance d from a current-carrying wire}\\ B = \frac{\mu_0I}{2\pi d}\\\text{}\\\text{Magnetic field strength in the center of a current-carrying loop of radius R}\\ B= \frac{\mu_0I}{2R} $$

The magnetic force can be demonstrated using two bar magnets, which are metallic bars that have north and south poles:

North-seeking Pole

Because of their historical use for navigation, magnetic poles of detection devices are defined like this:

• The pole labeled "North" is really the north-seeking pole.

<aside> ❗ From the pole law we see that the north geographic pole is actually a south magnetic pole!

</aside>

There is said to be a magnetic field at a point if a force acts on a magnetic pole at that point.

Magnetic fields are visualised through the construction of field lines.

Properties of magnetic field lines

• Magnetic field lines are conventionally drawn from the north-seeking pole to the south-seeking pole.
• The strength of the field is shown by the density of the field lines, closer lines mean a stronger field.
• The field lines never cross
• The field lines act as though made from an elastic thread, they tend to be as short as possible.

Example of a magnet

A bar magnet is a piece of ferrous metal which has a north and a south pole. Looking at the B-field about such a magnet, determine the north and the south poles.