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Magnetic Poles: Definition & Shifts

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  • 0:00 What are Magnetic Poles?
  • 1:37 Earth's Magnetic Field
  • 2:32 Earth's Magnetic Poles
  • 4:06 Geomagnetic Poles
  • 4:57 Magnetic Drift
  • 5:31 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

You probably know that magnets have poles, labeled 'North' and 'South.' But do you know why? And did you know that the earth's magnetic poles are not permanent? You will learn about both topics in this lesson.

What Are Magnetic Poles?

A magnetic pole is a point at either end of a magnetic field where the magnetic force is concentrated and strongest.

The easiest way to visualize the poles of a magnet is by using a bar magnet, the kind you probably came across in a science class in school. That type of magnet is called a permanent magnet because its magnetic properties are constant. The two ends of a permanent magnet are its poles. The magnetic field surrounding the magnet and the force it exerts are depicted using curved lines with arrows called magnetic field lines. The arrows are needed because magnetic force has a direction associated with it, from the North pole to the South pole of the magnet.

One question you might ask is: How did someone decide which end of the magnet was the North pole and which the South? The answer lies with the earth's magnetic field. A compass needle is just a narrow, thin bar magnet balanced on a pin. When you hold the compass level, the needle swings back and forth but finally stops moving.

The end of the compass needle that points north is called the North pole of the magnet. And that's where the names come from. The 'North pole' of any magnet is the one that would be attracted to the earth's North magnetic pole. In that sense, they are just labels that identify the opposite magnetic force produced at each end of the magnet.

The earth's magnetic field also has poles, North and South. But unlike the magnetic poles of a magnet, Earth's magnetic poles are constantly moving around. Why? And what does it mean?

Earth's Magnetic Field

At one time, scientists considered the possibility that the planet actually contained a huge bar magnet, resulting from alignment of iron atoms at the earth's solid inner core. But as our understanding of the earth's internal structure improved, it became obvious that the magnetic field was in fact generated in the molten outer core.

The mechanism is known as the dynamo, and the magnetic field is produced by electrical currents that result from the rotation and churning of the molten iron and nickel in the outer core. (You might already know that electrons moving through a wire also produce a magnetic field).

The earth's magnetic field varies in strength and orientation because it is produced by those constantly changing currents. And it is not symmetrical like the field of a bar magnet; the magnetic lines of force are not the same all around the axis of the magnetic field.

Earth's Magnetic Poles

Even given its variability, the simplest way to approximate the shape and behavior of Earth's magnetic field is still to visualize that it actually is produced by a bar magnet inside the planet. The North magnetic pole and the South magnetic pole are locations on the surface of the earth that represent the opposite ends of the field (although they are not exactly opposite each other due to the field's asymmetry).

Magnetic field

Because the dynamo currents are producing a field that does line up with the earth's axis of rotation, neither magnetic pole is located at the geographic poles, which are points on the surface marking the location of the planet's rotational axis (90 degrees north and south latitude).

The magnetic poles are the places on the earth's surface where the magnetic field force is acting perpendicularly to the surface. The North magnetic pole is where the field is acting straight down; at the South magnetic pole, it is acting straight up.

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