Electromagnetic waves can bring energy into a system by virtue of their electric and magnetic fields. These fields can exert forces and move charges in the system and, thus, do work on them. If the frequency of the electromagnetic wave is the same as the natural frequencies of the system such as microwaves at the resonant frequency of water molecules , the transfer of energy is much more efficient. The behavior of electromagnetic radiation clearly exhibits wave characteristics.
But we shall find in later modules that at high frequencies, electromagnetic radiation also exhibits particle characteristics. These particle characteristics will be used to explain more of the properties of the electromagnetic spectrum and to introduce the formal study of modern physics. Another startling discovery of modern physics is that particles, such as electrons and protons, exhibit wave characteristics.
This simultaneous sharing of wave and particle properties for all submicroscopic entities is one of the great symmetries in nature. Figure 1. Energy carried by a wave is proportional to its amplitude squared.
With electromagnetic waves, larger E -fields and B -fields exert larger forces and can do more work. But there is energy in an electromagnetic wave, whether it is absorbed or not. Once created, the fields carry energy away from a source.
If absorbed, the field strengths are diminished and anything left travels on. Clearly, the larger the strength of the electric and magnetic fields, the more work they can do and the greater the energy the electromagnetic wave carries. This is true for waves on guitar strings, for water waves, and for sound waves, where amplitude is proportional to pressure.
In electromagnetic waves, the amplitude is the maximum field strength of the electric and magnetic fields. See Figure 1. The intensity of light moving at speed in vacuum is then found to be. Algebraic manipulation produces the relationship. One more expression for in terms of both electric and magnetic field strengths is useful.
Substituting the fact that the previous expression becomes. We can use whichever of the three preceding equations is most convenient, because the three equations are really just different versions of the same result: The energy in a wave is related to amplitude squared. Furthermore, because these equations are based on the assumption that the electromagnetic waves are sinusoidal, the peak intensity is twice the average intensity; that is,.
The beam from a small laboratory laser typically has an intensity of about Assuming that the beam is composed of plane waves, calculate the amplitudes of the electric and magnetic fields in the beam. Use the equation expressing intensity in terms of electric field to calculate the electric field from the intensity. The amplitude of the magnetic field can be obtained from Equation A light bulb emits of power as visible light.
What are the average electric and magnetic fields from the light at a distance of? The intensity falls off as the distance squared if the radiation is dispersed uniformly in all directions.
A radio transmitter on Earth sends its signal to a satellite away Figure The area over which the power in a particular direction is dispersed increases as distance squared, as illustrated in the figure. Change the power output by a factor of and change the area by the same factor to keep the same. Then use the proportion of area in the diagram to distance squared to find the distance that produces the calculated change in area. Using the proportionality of the areas to the squares of the distances, and solving, we obtain from the diagram.
The range of a radio signal is the maximum distance between the transmitter and receiver that allows for normal operation. In the absence of complications such as reflections from obstacles, the intensity follows an inverse square law, and doubling the range would require multiplying the power by four. Skip to content By the end of this section, you will be able to: Express the time-averaged energy density of electromagnetic waves in terms of their electric and magnetic field amplitudes Calculate the Poynting vector and the energy intensity of electromagnetic waves Explain how the energy of an electromagnetic wave depends on its amplitude, whereas the energy of a photon is proportional to its frequency.
With electromagnetic waves, doubling the fields and fields quadruples the energy density and the energy flux. In the figure above, the electric field in red is vertically polarized. Think of a throwing a Frisbee at a picket fence. In one orientation it will pass through, in another it will be rejected. This is similar to how sunglasses are able to eliminate glare by absorbing the polarized portion of the light. The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy.
This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two. Radio and microwaves are usually described in terms of frequency Hertz , infrared and visible light in terms of wavelength meters , and x-rays and gamma rays in terms of energy electron volts. This is a scientific convention that allows the convenient use of units that have numbers that are neither too large nor too small.
The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz Hz , after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz. Electromagnetic waves have crests and troughs similar to those of ocean waves.
The distance between crests is the wavelength. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet! An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts eV. An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens.
Consider a jump rope with its ends being pulled up and down. More energy is needed to make the rope have more waves. Top of Page Next: Wave Behaviors. Anatomy of an Electromagnetic Wave. Retrieved [insert date - e. Science Mission Directorate. National Aeronautics and Space Administration.
Anatomy of an Electromagnetic Wave Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another.
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