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Electromagnetism

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Electromagnetism
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Someone's at the door! She lets the person inside the home know she's at the door by 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. 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 flows through a wire, how might this affect the magnetic field surrounding the wire?

A: With more current, the magnetic field is stronger.

Right Hand Rule

The direction of the magnetic field created when current flows through a wire depends on the direction of the current. A simple rule, called the right hand rule, makes it easy to find the direction of the magnetic field if the direction of the current is known. The rule is illustrated in the Figure below . When the thumb of the right hand is pointing in the same direction as the current, the fingers of the right hand curl around the wire in the direction of the magnetic field. You can see the right hand rule in action at this URL: http://www.youtube.com/watch?v=eK1Ar5WPJj8 .

Right hand rule applied to magnetic fields

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 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.

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.
  • When the thumb of the right hand is pointing in the same direction as the current flowing through a wire, the fingers of the right hand curl around the wire in the direction of the magnetic field.
  • 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.

Vocabulary

  • electromagnetism : Magnetism produced by electric current.

Practice

Watch the video about electromagnetism at the following URL. Then complete the task at the end of the video.

http://www.youtube.com/watch?v=bSge-qDcS4Y

Review

  1. Define electromagnetism.
  2. Apply the right hand rule to determine the direction of the magnetic field around the wire shown below. The arrow inside the wire shows the direction of electric current flowing through the wire. Add more arrows to the sketch to show the direction of the magnetic field.

  1. Why is electromagnetism more useful than regular magnetism?

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