Magnetic Field: Definition & Creation

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what a magnetic field is, how they are created, and suggest some uses for magnetic fields. A short quiz will follow.

What is a Magnetic Field?

What is a field? Aside from a place where farmers grow crops, it's a concept in physics that seems awfully mysterious. It's something that permeates an area and surrounds you. There are gravitational fields and electric fields. In Star Trek there are even fictional fields, like tetryon fields, and gravimetric fields. But what ARE they?

Fields aren't as mysterious as they sound. A field is really just a map of a particular variable. A gravitational field is a map of the gravitational forces in an area, showing how they might affect a mass as it passes through. An electric field is a map of the electric forces in an area, showing how it might affect a charge passing through.

But what about magnetic fields? Magnetic fields are regions around a magnet where the force of magnetism acts. That force of magnetism can affect other magnets. But it can also affect metals that have some weaker magnetic properties: for example, when you stick a fridge magnet to your refrigerator, the refrigerator door isn't a magnet. But the metal of the door does have some magnetic properties, which is why a magnet will stick to it. The force of magnetism can also affect moving charges, like electrons or protons.

We draw a magnetic field using diagrams like this one:

The Magnetic Field of a Bar Magnet
The Magnetic Field of a Bar Magnet

Lines that are closer together represent a stronger magnetic field.

How are Magnetic Fields Created?

There are two main ways that magnetic fields can be created: by finding ferromagnetic materials, and by moving charges.

Ferromagnetic materials are things like iron and cobalt, which have really strong magnetic properties. You can literally pick up a piece of iron you dug out of the ground and find that it acts just like a bar magnet. And bar magnets, like all magnets, create magnetic fields. Even if a piece of iron isn't acting quite like a bar magnet, all you have to do is put it near a larger magnet, and it will become stronger.

The second way to create a magnetic field is by moving charges. When charges move, they create magnetic fields. For example, a wire with electricity running through it, will have a magnetic field around it.

We can use this idea to make something called an electromagnet. An electromagnet is a coil of wire with electricity running through it.

An Electromagnet Coil
An Electromagnet Coil

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