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Chapter 24 | Electromagnetic Waves 1087
electric part, since they are both produced by the same movement and separation of charges in the antenna.
The electric and magnetic waves are shown together at one instant in time in Figure 24.7. The electric and magnetic fields produced by a long straight wire antenna are exactly in phase. Note that they are perpendicular to one another and to the direction of propagation, making this a transverse wave.
Figure 24.7 A part of the electromagnetic wave sent out from the antenna at one instant in time. The electric and magnetic fields ( and ) are in phase, and they are perpendicular to one another and the direction of propagation. For clarity, the waves are shown only along one direction, but they
propagate out in other directions too.
Electromagnetic waves generally propagate out from a source in all directions, sometimes forming a complex radiation pattern. A linear antenna like this one will not radiate parallel to its length, for example. The wave is shown in one direction from the antenna in Figure 24.7 to illustrate its basic characteristics.
Instead of the AC generator, the antenna can also be driven by an AC circuit. In fact, charges radiate whenever they are accelerated. But while a current in a circuit needs a complete path, an antenna has a varying charge distribution forming a standing wave, driven by the AC. The dimensions of the antenna are critical for determining the frequency of the radiated electromagnetic waves. This is a resonant phenomenon and when we tune radios or TV, we vary electrical properties to achieve appropriate resonant conditions in the antenna.
Making Connections: Self-Propagating Wave
Note that an electromagnetic wave, as shown in Figure 24.7, is the result of a changing electric field causing a changing magnetic field, which causes a changing electric field, and so on. Therefore, unlike other waves, an electromagnetic wave is self-propagating, even in a vacuum (empty space). It does not need a medium to travel through. This is unlike mechanical waves, which do need a medium. The classic standing wave on a string, for example, does not exist without the string. Similarly, sound waves travel by molecules colliding with their neighbors. If there is no matter, sound waves cannot travel.
Applying the Science Practices: Wave Properties and Graphs
Exercise 24.1
From the illustration of the electric field given in Figure 24.8(a) of an electromagnetic wave at some instant in time, please state what the amplitude and wavelength of the given waveform are. Then write down the equation for this particular wave.
Solution
The amplitude is 60 V/m, while the wavelength in this case is 1.5 m. The equation is .
Exercise 24.2
Now, consider another electromagnetic wave for which the electric field at a particular location is given over time by
. What are the amplitude, frequency, and period? Finally, draw and label an appropriate graph for this electric field.
Solution
The amplitude is 30 V/m, while the frequency is 2.0 MHz and hence the period is 5.0 × 10−7 s. The graph should be similar to that in Figure 24.8(b).