Illustration 32.1: Creation of Electromagnetic Waves

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This Illustration shows the electric field lines due to a positive charge. Initially the charge is not moving. The slider can be used to vary the speed of the charge. When translation mode is selected, the slider controls the instantaneous velocity. When oscillation mode is selected, the slider sets the maximum speed.

How are the changing fields in an electromagnetic wave created? Electromagnetic waves such as heat, light, and radio waves are created by a charge that is accelerating. The magnitude of the electric field is related to the acceleration of the charge.

Play the animation in translation mode and move the velocity slider. Notice how the electric field lines form a disturbance that moves away from the charge when the velocity changes. Because a changing electric field will give rise to a changing magnetic field, and a changing magnetic field will cause a changing electric field, a traveling electromagnetic wave is created. Move the slider in translation mode and note that abrupt changes in the velocity produce very complex wave patterns.

Play the animation in oscillation mode and notice the sinusoidal appearance of the electromagnetic disturbance.

You may wonder where the energy that goes into the electromagnetic wave comes from. Is energy being created from nothing? The answer is no, of course. A charge will not oscillate on its own; it must be driven by some force. For example, the charge might be part of an AC current, oscillating as the voltage source oscillates. Although some energy is being radiated from the charge, energy is being put into the charge to keep it oscillating.

Illustration authored by Melissa Dancy and Wolfgang Christian.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.

OSP Projects:
Open Source Physics - EJS Modeling
Physlet Physics
Physlet Quantum Physics
STP Book