Every magnet, and every moving electric charge, is surrounded by an invisible area of influence. This region is called a magnetic field. It's a force field that can attract or repel other magnetic materials or moving charges without touching them.

While we can't see the field itself, we can see its effects. If you've ever sprinkled iron filings on a piece of paper with a magnet underneath, you've seen a map of this field. The filings arrange themselves into a distinct pattern of lines.

These patterns reveal what scientists call magnetic field lines. They aren't real lines, of course, but a tool to help us visualize the invisible. They tell us two important things about the magnetic field at any point: its direction and its strength.

The strength of the field is shown by how close the lines are to each other. Where the lines are packed tightly together, the magnetic field is strong. Where they are spread far apart, the field is weak. You can see in the image above that the field is strongest near the magnet's poles.

An interesting property of magnetic field lines is that they always form closed loops. They don't start or end anywhere. They travel from the north pole to the south pole outside the magnet, and then continue through the magnet from south back to north, completing the loop.

While field lines give us a good picture of the magnetic field, sometimes we need to quantify how much of the field is passing through a particular area. This measurement is called magnetic flux.

Imagine holding a hoop in a rainstorm. The total amount of rain passing through your hoop is like the magnetic flux. It depends on how hard it's raining (the field strength), the size of your hoop (the area), and how you angle the hoop relative to the rain (the orientation).

A stronger magnetic field or a larger area will result in a greater magnetic flux. The angle is also key. If you hold the hoop parallel to the rain, no water passes through. Similarly, if a surface is parallel to the magnetic field lines, the magnetic flux through it is zero. Flux is maximized when the surface is perpendicular to the field lines, catching them all head-on.

The standard unit for magnetic flux is the weber, abbreviated as Wb.

Understanding magnetic flux is crucial because, as we'll see later, a change in flux is what generates electric currents in a process called electromagnetic induction. This principle is the foundation for electric generators and transformers.

Ready to check your understanding?

We've covered the fundamentals of what magnetic fields are and how we describe them. Next, we'll look at where these fields come from.