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How Magnetic Fields Are Created

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  • 0:05 The Source of Magnetic Fields
  • 1:09 Current-Carrying Conductors
  • 1:53 Electromagnets
  • 3:54 Electromagnets in Action
  • 5:01 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.

Have you ever wondered how magnetic fields are created? In this lesson, we'll discuss the very simple answer to this question and look at how electric current can be used to make electromagnets, which have many everyday applications.

The Source of Magnetic Fields

A magnetic field can be created by running electricity through a wire.
Electricity Creates Magnetic Field

All magnetic fields are created by moving charged particles. Even the magnet on your fridge is magnetic because it contains electrons that are constantly moving around inside. The first indication that moving electric charges cause magnetic fields was discovered in the early 19th century. During an experiment, it was observed that when an electric current flowed through a wire, a nearby compass would change direction.

When the current was turned off, the compass would go back to its original north/south alignment with the Earth's magnetic field. The conclusion from this observation was that the electrons moving through the wire were creating a magnetic field that didn't exist when the current was turned off. An important thing to note is that charged particles create magnetic fields only when they are moving. This means that we effectively have a magnet that can be turned on and off with the flick of a switch. This has huge implications when it comes to practical applications, as we'll discuss later.

Current Carrying Conductors

The 19th century experiment showed that a wire carrying an electric current is a magnet. Since the electrons all move through the wire in one direction, there is a well-defined magnetic field surrounding the wire. The strength of the magnetic field is proportional to the amount of current flowing through the wire. In other words, increasing the current increases the strength of the magnetic field. So, if wires are so magnetic, why don't we see paperclips and forks flying across the room and sticking to them? The reason is that the magnetic field just isn't very strong at normal, everyday levels of electric current. We'll need to do a bit more work to turn a wire into a useful magnet.

Electromagnets

By placing ferromagnetic materials in an electrical coil, you can strengthen the electromagnet.
Ferromagnetic Material in Coil

An electromagnet is a magnet that uses an electric current to generate its magnetic field. This differs from permanent magnets, like the ones on your refrigerator, which rely on the magnetic properties of the atoms in the material to create a magnetic field. At this point, our electromagnet is just a wire, but the magnetic field is too weak to do anything practical. However, if we bend the wire around and around to form a coil, the magnetic fields of the loops will concentrate in the center. To further enhance this effect, we can wrap multiple layers of wire on top of each other. Using more turns of wire increases the strength of the magnetic field. This is a definite improvement over our single wire from before, but it's still not strong enough to be really practical.

We can make our electromagnet several thousand times stronger by putting a core of ferromagnetic material, such as iron, in the center of the coil. Ferromagnetic materials contain something called magnetic domains, which are areas in the material that act like tiny magnets. Normally, the domains are randomly configured and the material does not exhibit any magnetism. However, when exposed to a magnetic field, like the one created by our coil of wire, the domains start to align and the individual magnetic fields unify into a bigger field.

The degree of domain alignment depends on the strength of the magnetic field generated by the coil, which as we learned earlier can be controlled by the amount of current flowing through the wire. Just as importantly, when the current is turned off, the magnetic domains go back to their random configuration and the electromagnet loses nearly all of its magnetism. The ability to control a very powerful magnet with a switch has many practical applications.

Electromagnets in Action

We use electromagnets every day without even realizing it. They can literally be found in thousands of different devices because they're so useful. For example, they can be used to lift steel in a scrap yard, to ring a school bell for recess, and even to levitate high-speed trains. Did you know that speakers use electromagnets? Speakers take advantage of the fact that the strength of an electromagnet can be controlled by adjusting the electric current.

The audio source sends a current to the electromagnet in the speaker that controls the sound we hear.
Electromagnet in Speakers

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