# 4.2: EM Waves from an Accelerating Charge

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## Tutorial 4.2: Accelerated Charge Radiation

Electric charges have electric fields. The simulation first shows a moving positive charge and the electric field around it in two dimensions. If the charge is accelerated there will be a disturbance in the field. This is the origin of electromagnetic waves. Note that the energy carried by the disturbance comes from the input energy needed to accelerate the charge.

## Accelerating Charge

### Questions:

Exercise $$\PageIndex{1}$$

Run the simulation. Change the speed, $$v$$, and describe what you see. How does the change in speed (acceleration) affect the initial disturbance of the field (try changing the speed slowly versus rapidly)?

Exercise $$\PageIndex{2}$$

What happens at constant speed? Is there still a disturbance? What happens if you suddenly slow the charge down?

Exercise $$\PageIndex{3}$$

Now click the Animation 2 button. Describe what you see. Explain what you would notice about the field over time if you were measuring it at a point along the $$x$$-axis and compare that with what you would measure at a distant point along the $$y$$-axis.

A charge oscillating in the $$y$$-direction will produce an electromagnetic wave traveling in the $$x$$-direction as seen in Animation 2. For directions other than along the $$x$$-axis the wave has a lower amplitude (smaller variation from equilibrium), dropping to a zero amplitude along the $$y$$-direction. This configuration is called a dipole antenna. (To be technically correct a single wire with an oscillating charge is a monopole antenna. A dipole is created from two wires with opposite polarities, one in the $$x$$-direction the other in the $$-x$$-direction but in the present context we can ignore this subtlety.) Dipole antennas emit the strongest signal in a direction perpendicular to the antenna as Animation 2 shows (remember, the field is the same strength in both directions but the change in the field is zero in $$y$$-direction, largest along the $$x$$-axis). FM radio, AM radio, TV, cell phone, WiFi and short wave radio sending antennas are dipole or approximately dipole antennas.

Exercise $$\PageIndex{4}$$

Why are sending antennas usually oriented vertically?

Note

Short wave antennas are sometimes oriented horizontally so that the signal can bounce off the ionosphere and return to earth a large distance away.

Exercise $$\PageIndex{5}$$

Try different oscillation speeds for Animation 2. If you were measuring the field on the $$x$$-axis, how would the frequency of the wave compare with the frequency of oscillation of the charge in the antenna?

Exercise $$\PageIndex{6}$$

Now try Animation 3. How would the amplitude of the wave at points along the $$x$$-axis compare with the amplitude of the wave along the $$y$$-axis for this case?

This page titled 4.2: EM Waves from an Accelerating Charge is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Kyle Forinash and Wolfgang Christian via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.