The electric and magnetic waves are shown together at one instant in time in Figure 3. 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 3. A part of the electromagnetic wave sent out from the antenna at one instant in time. The electric and magnetic fields E and B 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 3 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. Electromagnetic waves carry energy away from their source, similar to a sound wave carrying energy away from a standing wave on a guitar string.
An antenna for receiving EM signals works in reverse. And like antennas that produce EM waves, receiver antennas are specially designed to resonate at particular frequencies. An incoming electromagnetic wave accelerates electrons in the antenna, setting up a standing wave. If the radio or TV is switched on, electrical components pick up and amplify the signal formed by the accelerating electrons. Sometimes big receiver dishes are used to focus the signal onto an antenna.
When designing circuits, we often assume that energy does not quickly escape AC circuits, and mostly this is true. A broadcast antenna is specially designed to enhance the rate of electromagnetic radiation, and shielding is necessary to keep the radiation close to zero. Some familiar phenomena are based on the production of electromagnetic waves by varying currents.
Your microwave oven, for example, sends electromagnetic waves, called microwaves, from a concealed antenna that has an oscillating current imposed on it. There is a relationship between the E — and B -field strengths in an electromagnetic wave. This can be understood by again considering the antenna just described.
The stronger the E -field created by a separation of charge, the greater the current and, hence, the greater the B -field created. It can be shown that the magnitudes of the fields do have a constant ratio, equal to the speed of light. This is true at all times and at all locations in space. A simple and elegant result. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction.
Matter in the medium is periodically displaced by a sound wave, and thus oscillates. When people make a sound, whether it is through speaking or hitting something, they are compressing the air particles to some significant amount. By doing so, they create transverse waves. When people hear sounds, their ears are sensitive to the pressure differences and interpret the waves as different tones.
Water waves can be commonly observed in daily life, and comprise both transverse and longitudinal wave motion. Water waves, which can be commonly observed in our daily lives, are of specific interest to physicists.
Describing detailed fluid dynamics in water waves is beyond the scope of introductory physics courses. Although we often observe water wave propagating in 2D, in this atom we will limit our discussion to 1D propagation. The uniqueness of water waves is found in the observation that they comprise both transverse and longitudinal wave motion.
As a result, the particles composing the wave move in clockwise circular motion, as seen in. Oscillatory motion is highest at the surface and diminishes exponentially with depth. Waves are generated by wind passing over the surface of the sea.
As long as the waves propagate slower than the wind speed just above the waves, there is an energy transfer from the wind to the waves. Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind making the water to go into the shear stress , contribute to the growth of the waves.
In the case of monochromatic linear plane waves in deep water, particles near the surface move in circular paths, creating a combination of longitudinal back and forth and transverse up and down wave motions. When waves propagate in shallow water where the depth is less than half the wavelength , the particle trajectories are compressed into ellipses. As the wave amplitude height increases, the particle paths no longer form closed orbits; rather, after the passage of each crest, particles are displaced slightly from their previous positions, a phenomenon known as Stokes drift.
Plane wave : We see a wave propagating in the direction of the phase velocity. The wave can be thought to be made up of planes orthogonal to the direction of the phase velocity. Since water waves transport energy, attempts to generate power from them have been made by utilizing the physical motion of such waves. Although larger waves are more powerful, wave power is also determined by wave speed, wavelength, and water density. Deep water corresponds with a water depth larger than half the wavelength, as is a common case in the sea and ocean.
In deep water, longer-period waves propagate faster and transport their energy faster. The deep-water group velocity is half the phase velocity. In shallow water for wavelengths larger than about twenty times the water depth as often found near the coast , the group velocity is equal to the phase velocity.
These methods have proven viable in some cases but do not provide a fully sustainable form of renewable energy to date. Water waves : The motion water waves causes particles to follow clockwise circular motion. This is a result of the wave having both transverse and longitudinal properties. Waves are defined by its frequency, wavelength, and amplitude among others. They also have two kinds of velocity: phase and group velocity. Waves have certain characteristic properties which are observable at first notice.
The first property to note is the amplitude. The amplitude is half of the distance measured from crest to trough. We also observe the wavelength, which is the spatial period of the wave e. The frequency of a wave is the number of cycles per unit time — one can think of it as the number of crests which pass a fixed point per unit time.
Mathematically, we make the observation that,. Frequencies of different sine waves. Conversely we say that the purple wave has a high frequency. Note that time increases along the horizontal. In fact,. This is the velocity at which the phase of any one frequency component of the wave travels.
For such a component, any given phase of the wave for example, the crest will appear to travel at the phase velocity. Fig 2 : This shows a wave with the group velocity and phase velocity going in different directions.
The group velocity is positive and the phase velocity is negative. Energy transportion is essential to waves. It is a common misconception that waves move mass. Waves carry energy along an axis defined to be the direction of propagation. One easy example is to imagine that you are standing in the surf and you are hit by a significantly large wave, and once you are hit you are displaced unless you hold firmly to your ground!
In this sense the wave has done work it applied a force over a distance. Since work is done over time, the energy carried by a wave can be used to generate power. Water Wave : Waves that are more massive or have a greater velocity transport more energy. Similarly we find that electromagnetic waves carry energy. Electromagnetic radiation EMR carries energy—sometimes called radiant energy—through space continuously away from the source this is not true of the near-field part of the EM field.
Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. EMR also carries both momentum and angular momentum. Why are EM plane waves transverse?
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