Electromagnetism

Objectives

Since magnetic fields are primarily the result of electrons moving around, there is a close relationship between electricity and magnetism.

So close, in fact, that electricity and magnetism are combined into a single phenomenon known as electromagnetism.

In this lesson we will examine how an electric current can be used to generate a magnetic field, and how a magnetic field can be used to generate an electric current.

It would be a good idea to first be familiar with the basics of magnetism before diving into electromagnetism.

Electromagnetism Basics

When an electric current flows through a wire, a magnetic field is generated around the wire.

This magnetic field is circular, with the wire at its center.

Current flowing toward the viewer

Now this tends to create a point of confusion. If the field wraps around the wire, where are the poles?

It turns out it is the direction that the flux wraps around the wire that acts as the magnetic poles. One direction is north, the opposite is south.

If a compass's is placed by the wire, the compasses north pole points in the direction the flux curls.

The direction that the flux wraps around the wire is dictated by which direction the current is flowing through the wire.

This is where the "Right Hand Rule" (RHR) comes in to play.

You take your right hand and point your thumb in the direction of the current. The direction you can curl your fingers is the same as the direction the flux wraps around the wire.

Flipping the direction of current, means flipping your hand, showing that now the field curls the other direction.

The intensity of the magnetic field generated around a wire is directly related to the strength of the electricity running through the wire.

Increasing either the current or the voltage will increase the intensity of the field.

All that being said, the field around a straight wire is not very strong.

We can do better!

If a second wire, with a current flowing in the same direction, is placed next to the original wire, their magnetic fields will reinforce each other.

The more wires which are added, the stronger the magnetic field which can be generated.

How big this field can get is limited by things like the current available, the heat being generated, or the physical space available.

Though if you build something specifically to overcome those limitations, you can build a electromagnet so powerful it will tear itself apart.

It only works once though.

Now, rather than doing this using many separate strands of wire, we can get the same effect using a single wire, with a single current, and wrapping it into a coil loop.

By wrapping the wire into a coil, the magnetic field reinforces itself, becoming stronger without needing to increase the current.

The magnetic field lines running through the center of the loop become tightly packed while those around the outside of the loop are spread out.

This makes the magnetic field strength within the loop far stronger than that outside.

A magnetic field created by running a current through a coil of wire like this is known as a solenoid.

The field can be even further strengthened by placing a high magnetically permeable material within the coil.

Magnetic permeability is a whole thing in and of itself. But, in short, it is the measure of how well a material becomes magnetized when placed within a magnetic field.

It is similar to what conductivity is for electricity.

A highly magnetically permeable material, when placed in a magnetic field, is better able to "conduct" the magnetic field lines, allowing them be more closely packed together.