This animation visualises an oscillating electric charge and the electric field lines caused by it. The charge is executing simple harmonic motion in the vertical direction, with the maximum amplitude of the oscillation varying from zero to a maximum value and then back to zero.
Initially the animation is paused. You may used the "radio buttons" in the lower right-hand part of the animation to choose whether the speed of light is finite or infinite.
If the speed of the light is finite, then the field line at some distance away from the charge responds to changing positions of the charge only after a time delay: the delay is the time necessary for a signal propagating at the speed of light to travel from the charge to that position on the field line.
If the speed of light is set to be infinite, then every point on all the field lines respond instantly to the changing position of the charge. This, of course, is not realistic: the speed of light is large but not infinite.
There are 3 playback controls:
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Causes the charge to begin oscillating. |
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Stops the charge's oscillation. If the speed of light is finite, the electric field lines continue propagating outwards after this button is clicked. |
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Rewinds the animation and stops. This will allow you to change the speed of light. |
A moment's reflection may convince you that if the charge's position is changing sinusoidally in time, then the moving charge is generating an electric current whose value is also changing sinusoidally. Thus a magnetic field is generated, whose value close to the charge is also changing sinusoidally. However, this changing magnetic field also propagates outwards at the speed of light.
So although the animation only shows the electric field, there is also a magnetic field propagating outwards which looks similar to the shown field lines. Except the magnetic field due to a current is perpendicular to it, so the magnetic field lines will be circular about the axis of oscillation of the charge.
There is one more small matter. Changing magnetic fields cause electric fields, as Faraday showed. Similarly, changing electric fields cause magnetic fields, as Maxwell showed. So all these changing electric fields, as shown in the animation, causes further changes in the magnetic fields, and vice versa.
So, finally, the animation can lead us to the realization that an oscillating electric charge generates a wave of electric and magnetic fields propagating outwards at the speed of light. Note that if the speed of light were infinite no such wave would be generated.
This animation requires Version 6 or better of the Flash player. The player is available free from http://www.macromedia.com.
Although the size of the animation file is fairly small (about 40k) there is considerable computational overhead. For example, under Windoze a 665 MHz Pentium III processor is about the minimum required for the animation to run at full speed of 12 frames per second. In fact, the animated parts of the demonstration consists of only two parts: the graphic of the charge and a 3 pixel black circle; all the motions are generated by about 200 lines of ActionScript code.
This animation was written by David M. Harrison, Dept. of Physics, Univ. of Toronto, mailto:harrison@physics.utoronto.ca in February 2003.
The animation is Copyright © 2003 David M. Harrison.
The animation may be distributed only subject to the terms and conditions set forth in the Open Content License, v1.0 or later (the latest version is presently available at http://opencontent.org/opl.shtml). I will be pleased to send you the swf or the fla files for these animations provided you agree to the terms of this license.
The animation was inspired by a similar one written by Don Ion at the Dept. of Physics, Santa Barbara City College; his "home page" is http://www.cs.sbcc.cc.ca.us/~physics/di-home.html. The original animation can be accessed via this page. However he has also supplied us with a copy of the animation. It does not have the same computational requirements as our version, but is 1.4 Megabytes so it takes longer to load.
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Warning! In the usual case you accessed this page by clicking on the About This Animation button in our version of the animation. You need to kill that animation before accessing his, so the Flash player on your computer is only trying to do one thing at a time. Thus: |
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