Magnetism

Outcomes

Know gravity and magnetism are examples of non-contact forces.
Describe examples of gravity and magnetism.
Identify when a magnet attract another magnet.
Identify when a magnet repels another magnet.
Be able to draw magnetic field lines on a bar magnets.
Identify uses of magnets and electromagnets in everyday life.

Magnets

The train in this photo is called a maglev train. The word maglev stands for “magnetic levitation.” Magnets push the train upward so it hovers, or levitates, above the track without actually touching it. This eliminates most of the friction (Links to an external site.)Links to an external site. acting against the train when it moves. Other magnets pull the train forward along the track. Because of all the magnets, the train can go very fast. It can fly over the tracks at speeds up to 480 kilometers (300 miles) per hour! What are magnets and how do they exert such force? In this article, you’ll find out.

Magnetic Poles

A magnet is an object that attracts certain materials such as iron. You’re probably familiar with common bar magnets, like the one shown in the Figure below (Links to an external site.)Links to an external site.. Like all magnets, this bar magnet has north and south magnetic poles. The red end of the magnet is the north pole and the blue end is the south pole. The poles are regions where the magnet is strongest. The poles are called north and south because they always line up with Earth’s north-south axis if the magnet is allowed to move freely. (Earth’s axis is the imaginary line around which the planet rotates.)

Q: What do you suppose would happen if you cut the bar magnet pictured in the Figure above (Links to an external site.)Links to an external site.along the line between the north and south poles?

A: Both halves of the magnet would also have north and south poles. If you cut each of the halves in half, all those pieces would have north and south poles as well. Pieces of a magnet always have both north and south poles no matter how many times you cut the magnet.

Magnetic Force and Magnetic Field

The force that a magnet exerts on certain materials, including other magnets, is called magnetic force. The force is exerted over a distance (Links to an external site.)Links to an external site. and includes forces of attraction and repulsion. North and south poles of two magnets attract each other, while two north poles or two south poles repel each other. A magnet can exert force over a distance because the magnet is surrounded by a magnetic field. In the Figure below (Links to an external site.)Links to an external site., you can see the magnetic field (Links to an external site.)Links to an external site. surrounding a bar magnet. Tiny bits of iron, called iron filings, were placed under a sheet of glass. When the magnet was placed on the glass, it attracted the iron filings. The pattern of the iron filings shows the lines of force that make up the magnetic field of the magnet. The concentration (Links to an external site.)Links to an external site. of iron filings near the poles indicates that these areas exert the strongest force. You can also see how the magnetic field affects the compasses placed above the magnet.

When two magnets are brought close together, their magnetic fields interact. You can see how they interact in the Figure below (Links to an external site.)Links to an external site.. The lines of force of north and south poles attract each other whereas those of two north poles repel each other.

Summary

A magnet is an object that attracts certain materials such as iron. All magnets have north and south magnetic poles. The poles are regions where the magnet is strongest.
The force that a magnet exerts is called magnetic force. The force is exerted over a distance (Links to an external site.)Links to an external site. and includes forces of attraction and repulsion. A magnet can exert force over a distance because the magnet is surrounded by a magnetic field (Links to an external site.)Links to an external site.

Electromagnetism

What a clever dog! He wants to go inside to get out of the rain, so he’s ringing the doorbell. Does your home have a doorbell? Did you ever wonder how a doorbell works? The answer is electromagnetism.

What Is Electromagnetism?

Electromagnetism is magnetism produced by an electric current (Links to an external site.)Links to an external site.. When electric current flows through a wire, it creates a magnetic field that surrounds the wire in circles. You can see this in the diagram below. Note that electric current is conventionally shown moving from positive to negative electric potential, as in this diagram. However, electrons in current actually flow in the opposite direction, from negative to positive potential.

Q: If more current (Links to an external site.)Links to an external site. flows through a wire, how might this affect the magnetic field surrounding the wire?

A: With more current (Links to an external site.)Links to an external site., the magnetic field is stronger.

Uses of Electromagnetism

Electromagnetism is used not only in a doorbells but in many other electric devices as well, such as electric motors and loudspeakers. It is also used to store information on computer disks. An important medical use of electromagnetism is magnetic resonance (Links to an external site.)Links to an external site. imaging (MRI). This is a technique for making images of the inside of the body in order to diagnose diseases or injuries. Magnetism created with electric current is so useful because it can be turned on or off simply by turning the current on or off. The strength of the magnetic field is also easy to control by changing the amount of current. You can’t do either of these things with a regular magnet (Links to an external site.)Links to an external site..

Summary

Electromagnetism is magnetism produced by an electric current. Current flowing through a wire creates a magnetic field that surrounds the wire in concentric circles.
Electromagnetism is very useful because it can be turned on or off by turning the current on or off, and it can be made stronger or weaker by using more or less current.

Activities:

Review

What is a magnet?
Describe the magnetic poles of a bar magnet.
Explain why a magnet can exert force over a distance (Links to an external site.)Links to an external site..
Sketch two bar magnets that are arranged so their magnetic fields attract each other. Label the magnetic poles, and add arrows to represent lines of force between the two magnets.