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5.78: Magnets

Difficulty Level: At Grade Created by: CK-12
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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 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. You can also watch a video introduction to magnets at this URL: http://www.youtube.com/watch?v=8Y4JSp5U82I

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 . 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 above 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 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 , you can see the magnetic field 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 of iron filings near the poles indicates that these areas exert the strongest force. To see an animated magnetic field of a bar magnet, go to this URL: http://elgg.norfolk.e2bn.org/jsmith112/files/68/149/Bar+magnet.swf

When two magnets are brought close together, their magnetic fields interact. You can see how they interact in the Figure below . The lines of force of north and south poles attract each other whereas those of two north poles repel each other. The animations at the following URL show how magnetic field lines change as two or more magnets move in relation to each other. http://www.coolmagnetman.com/magmotion.htm

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 and includes forces of attraction and repulsion. A magnet can exert force over a distance because the magnet is surrounded by a magnetic field.

Vocabulary

  • magnet : Object that attracts ferromagnetic materials such as iron.
  • magnetic field : Area around a magnet where it exerts magnetic force.
  • magnetic force : Force of attraction or repulsion exerted by a magnet.
  • magnetic pole : North or south end of a magnet where the magnet exerts the most force.

Practice

At the following URL, take the animated quiz to check your understanding of magnetic field interactions. http://elgg.norfolk.e2bn.org/jsmith112/files/68/151/Law+of+magnetism.swf

Review

  1. What is a magnet?
  2. Describe the magnetic poles of a bar magnet.
  3. Explain why a magnet can exert force over a distance.
  4. 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.

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Description

Difficulty Level:

At Grade

Concept Nodes:

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Sep 05, 2014

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