What is a Magnetic Field?

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  • 0:05 Detecting a Magnetic…
  • 3:27 Ferromagnetic Materials
  • 5:15 Lesson Summary
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Lesson Transcript
Instructor: Jim Heald

Jim has taught undergraduate engineering courses and has a master's degree in mechanical engineering.

Magnetic fields fill the space around all magnets, but they're impossible to detect with our own senses. We'll use a common tool to map out a magnetic field and then discuss ferromagnetic materials to see how a magnetic field can be used to create new magnets.

Detecting a Magnetic Field with a Compass

It's a common misconception that a compass is simply a tool for showing you the direction you're headed. A compass is really a magnetic field direction detector. Now, doesn't that sound fancy? The confusion stems from the fact that we almost always use a compass to detect the direction of Earth's magnetic field as a way to know which way we're headed on the earth's surface. While this is a great practical use for a compass, we're going to conduct a hypothetical experiment using the compass to diagram the magnetic field lines of a gigantic bar magnet.

The term 'magnetic field' basically refers to the space around a magnet where other magnets will experience a force. The problem is we can't detect the magnetic field with our own senses, so we need to use a compass to help us 'see' the field. A compass is nothing more than a tiny magnet suspended such that it can freely rotate in response to a magnetic field. Like all magnets, the needle has a north pole and a south pole that are attracted and repelled by the poles of other magnets. When the compass is placed in a strong magnetic field, the forces of attraction and repulsion turn the needle until it is perfectly aligned with the direction of the field.

In a magnetic field, the needle of a compass will run parallel to the direction of the field.
Compass in Magnetic Field

For our experiment, we're going to imagine that we have a bar magnet the size of a school bus sitting in a wide open space. This should help you visualize walking around the magnet and should convince you that we're dealing with a very strong magnetic field! With the compass in hand, we'll start out next to the north pole and observe the orientation of the needle. What we would see is that the needle points straight out and away from the magnet. If we started walking in the direction that the needle was pointing, we'd find that as we got farther away from the pole, the needle would start to turn to the side. Continuing to follow the needle, we would eventually walk all the way around the magnet and arrive at the south pole. Here, the needle would point directly into the magnet. Drawing the path that we walked would give us a diagram that looked something like this:

Diagram of a single field line
field line around magnet

If we repeated this experiment several more times, but starting from slightly different locations, our diagram would eventually look like this:

The spacing of the field lines is related to the strength of the magnetic field.
Field Line Magnetic Field

Each one of the lines is called a field line, and it shows the direction of the magnetic field at various locations around the magnet. This diagram tells us a couple of different things. First, it shows us that the direction of the magnetic field is always considered to be coming out of the north pole and going into the south pole. This is really just a convention, but one that is universally followed. The second thing to notice is that the spacing of the lines indicates the strength of the magnetic field. We can see that the field lines are most closely spaced near the poles of the magnet (where the field is strongest) and spread farther apart as we move away from the poles. If you've ever played around with magnets, you've probably felt how much the force between two magnets changes as the poles get closer and closer together.

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