Production of Electromagnetic Waves

We can get a good understanding of electromagnetic waves (EM) by considering how they are produced. Whenever a current varies, associated electric and magnetic fields vary, moving out from the source like waves. Perhaps the easiest situation to visualize is a varying current in a long straight wire, produced by an AC generator at its center, as illustrated in [link].

A long straight gray wire with an A C generator at its center, functioning as a broadcast antenna for electromagnetic waves, is shown. The wave distributions at four different times are shown in four different parts. Part a of the diagram shows a long straight gray wire with an A C generator at its center. The time is marked t equals zero. The bottom part of the antenna is positive and the upper end of the antenna is negative. An electric field E acting upward is shown by an upward arrow. Part b of the diagram shows a long straight gray wire with an A C generator at its center. The time is marked t equals capital T divided by four. The antenna has no polarity marked and a wave is shown to emerge from the A C source. An electric field E acting upward as shown by an upward arrow. The electric field E propagates away from the antenna at the speed of light, forming part of the electromagnetic wave from the A C source. A quarter portion of the wave is shown above the horizontal axis. Part c of the diagram shows a long straight gray wire with an A C generator at its center. The time is marked t equals capital T divided by two. The bottom part of the antenna is negative and the upper end of the antenna is positive and a wave is shown to emerge from the A C source. The electric field E propagates away from the antenna at the speed of light, forming part of the electromagnetic wave from the A C source. A quarter portion of the wave is shown below the horizontal axis and a quarter portion of the wave is above the horizontal axis. Part d of the diagram shows a long straight gray wire with an AC generator at its center. The time is marked t equals capital T. The bottom part of the antenna is positive and the upper end of the antenna is negative. A wave is shown to emerge from the A C source. The electric field E propagates away from the antenna at the speed of light, forming part of the electromagnetic wave from the A C source. A quarter portion of the wave is shown above the horizontal axis followed by a half wave below the horizontal axis and then again a quarter of a wave above the horizontal axis.

The electric field (E size 12{E} {}

) shown surrounding the wire is produced by the charge distribution on the wire. Both the E size 12{E} {}

and the charge distribution vary as the current changes. The changing field propagates outward at the speed of light.

There is an associated magnetic field (B size 12{B} {}

) which propagates outward as well (see [link]). The electric and magnetic fields are closely related and propagate as an electromagnetic wave. This is what happens in broadcast antennae such as those in radio and TV stations.

Closer examination of the one complete cycle shown in [link] reveals the periodic nature of the generator-driven charges oscillating up and down in the antenna and the electric field produced. At time t=0 size 12{t=0} {}

, there is the maximum separation of charge, with negative charges at the top and positive charges at the bottom, producing the maximum magnitude of the electric field (or E size 12{E} {}

-field) in the upward direction. One-fourth of a cycle later, there is no charge separation and the field next to the antenna is zero, while the maximum E size 12{E} {}

-field has moved away at speed c size 12{c} {}

.

As the process continues, the charge separation reverses and the field reaches its maximum downward value, returns to zero, and rises to its maximum upward value at the end of one complete cycle. The outgoing wave has an amplitude proportional to the maximum separation of charge. Its wavelengthλ size 12{ left (λ right )} {}

is proportional to the period of the oscillation and, hence, is smaller for short periods or high frequencies. (As usual, wavelength and frequencyf size 12{ left (f right )} {}

are inversely proportional.)

Electric and Magnetic Waves: Moving Together

Following Ampere’s law, current in the antenna produces a magnetic field, as shown in [link]. The relationship between E size 12{E} {}

and B size 12{B} {}

is shown at one instant in [link] (a). As the current varies, the magnetic field varies in magnitude and direction.

Part a of the diagram shows a long straight gray wire with an A C generator at its center, functioning as a broadcast antenna. The antenna has a current I flowing vertically upward. The bottom end of the antenna is negative and the upper end of the antenna is positive. An electric field is shown to act vertically downward. The magnetic field lines B produced in the antenna are circular in direction around the wire. Part b of the diagram shows a long straight gray wire with an A C generator at its center, functioning as a broadcast antenna. The electric field E and magnetic field B near the wire are shown perpendicular to each other. Part c of the diagram shows a long straight gray wire with an A C generator at its center, functioning as a broadcast antenna. The current is shown to flow in the antenna. The magnetic field varies with the current and propagates away from the antenna as a sine wave in the horizontal plane. The vibrations in the wave are marked as small arrows along the wave.

The magnetic field lines also propagate away from the antenna at the speed of light, forming the other part of the electromagnetic wave, as seen in [link] (b). The magnetic part of the wave has the same period and wavelength as the 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 [link]. 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.

A part of the electromagnetic wave sent out from the antenna at one instant in time is shown. The wave is shown with the variation of two components, E and B, moving with velocity c. E is a sine wave in one plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. E and B are in phase, and they are perpendicular to one another and to the direction of propagation.

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 [link] 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.

Receiving Electromagnetic Waves

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. The signal is then converted to audio and/or video format. Sometimes big receiver dishes are used to focus the signal onto an antenna.

In fact, charges radiate whenever they are accelerated. 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.

Relating E size 12{E} {}

-Field and B size 12{B} {}

-Field Strengths

There is a relationship between the E size 12{E} {}

-field strengths in an electromagnetic wave. This can be understood by again considering the antenna just described. The stronger the E size 12{E} {}

-field created by a separation of charge, the greater the current and, hence, the greater the B size 12{B} {}

-field created.

Since current is directly proportional to voltage (Ohm’s law) and voltage is directly proportional to E size 12{E} {}

-field strength, the two should be directly proportional. It can be shown that the magnitudes of the fields do have a constant ratio, equal to the speed of light. That is,

E B = c size 12{ { {E} over {B} } =c} {}

is the ratio of E size 12{E} {}

-field strength to B size 12{B} {}

-field strength in any electromagnetic wave. This is true at all times and at all locations in space. A simple and elegant result.

Calculating B size 12{B} {} -Field Strength in an Electromagnetic Wave

What is the maximum strength of the B size 12{B} {}

-field in an electromagnetic wave that has a maximum E size 12{E} {}

-field strength of 1000 V/m size 12{"1000" {V} slash {m} } {}

?

Strategy

To find the B size 12{B} {}

-field strength, we rearrange the above equation to solve for B size 12{B} {}

, yielding

B=Ec. size 12{B= { {E} over {c} } } {}

Solution

We are given E size 12{E} {}

, and c size 12{c} {}

is the speed of light. Entering these into the expression for B size 12{B} {}

yields

B=1000 V/m3.00 × 108 m/s= 3.33×10-6 T, size 12{B = { {"1000 V/m"} over {3 "." "00 " times " 10" rSup { size 8{8} } " m/s"} } =" 3" "." "33" times "10" rSup { size 8{ +- 6} } " T"} {}

Where T stands for Tesla, a measure of magnetic field strength.

Discussion

The B size 12{B} {}

-field strength is less than a tenth of the Earth’s admittedly weak magnetic field. This means that a relatively strong electric field of 1000 V/m is accompanied by a relatively weak magnetic field. Note that as this wave spreads out, say with distance from an antenna, its field strengths become progressively weaker.

The result of this example is consistent with the statement made in the module Maxwell’s Equations: Electromagnetic Waves Predicted and Observed that changing electric fields create relatively weak magnetic fields. They can be detected in electromagnetic waves, however, by taking advantage of the phenomenon of resonance, as Hertz did. A system with the same natural frequency as the electromagnetic wave can be made to oscillate. All radio and TV receivers use this principle to pick up and then amplify weak electromagnetic waves, while rejecting all others not at their resonant frequency.

Take-Home Experiment: Antennas

For your TV or radio at home, identify the antenna, and sketch its shape. If you don’t have cable, you might have an outdoor or indoor TV antenna. Estimate its size. If the TV signal is between 60 and 216 MHz for basic channels, then what is the wavelength of those EM waves?

Try tuning the radio and note the small range of frequencies at which a reasonable signal for that station is received. (This is easier with digital readout.) If you have a car with a radio and extendable antenna, note the quality of reception as the length of the antenna is changed.

PhET Explorations: Radio Waves and Electromagnetic Fields

Broadcast radio waves from KPhET. Wiggle the transmitter electron manually or have it oscillate automatically. Display the field as a curve or vectors. The strip chart shows the electron positions at the transmitter and at the receiver.

Section Summary

Conceptual Questions

The direction of the electric field shown in each part of [link] is that produced by the charge distribution in the wire. Justify the direction shown in each part, using the Coulomb force law and the definition of E=F/q size 12{E= {F} slash {q} } {}

, where q size 12{q} {}

is a positive test charge.

Is the direction of the magnetic field shown in [link] (a) consistent with the right-hand rule for current (RHR-2) in the direction shown in the figure?

Why is the direction of the current shown in each part of [link] opposite to the electric field produced by the wire’s charge separation?

In which situation shown in [link] will the electromagnetic wave be more successful in inducing a current in the wire? Explain.

Part a of the diagram shows an electromagnetic wave approaching a long straight vertical wire. The wave is shown with the variation of two components E and B. E is a sine wave in vertical plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a horizontal plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves. Part b of the diagram shows an electromagnetic wave approaching a long straight vertical wire. The wave is shown with the variation of two components E and B. E is a sine wave in horizontal plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a vertical plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves.

In which situation shown in [link] will the electromagnetic wave be more successful in inducing a current in the loop? Explain.

Part a of the diagram shows an electromagnetic wave approaching a receiver loop connected to a tuner. The wave is shown with the variation of two components E and B. E is a sine wave in vertical plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a horizontal plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves. Part b of the diagram shows an electromagnetic wave approaching a receiver loop connected to a tuner. The wave is shown with the variation of two components E and B. E is a sine wave in horizontal plane with small arrows showing the vibrations of particles in the plane. B is a sine wave in a vertical plane perpendicular to the E wave. The B wave has arrows to show the vibrations of particles in the plane. The waves are shown intersecting each other at the junction of the planes because E and B are perpendicular to each other. The direction of propagation of wave is shown perpendicular to E and B waves.

Should the straight wire antenna of a radio be vertical or horizontal to best receive radio waves broadcast by a vertical transmitter antenna? How should a loop antenna be aligned to best receive the signals? (Note that the direction of the loop that produces the best reception can be used to determine the location of the source. It is used for that purpose in tracking tagged animals in nature studies, for example.)

Under what conditions might wires in a DC circuit emit electromagnetic waves?

Give an example of interference of electromagnetic waves.

[link] shows the interference pattern of two radio antennas broadcasting the same signal. Explain how this is analogous to the interference pattern for sound produced by two speakers. Could this be used to make a directional antenna system that broadcasts preferentially in certain directions? Explain.

The picture shows an overhead view of a radio broadcast antenna sending signals in the form of waves. Two waves are shown in the diagram with concentric circular wave fonts. The crest and trough are marked as bold and dashed circles respectively. The points where the bold circles of the two different waves meet are marked as points of constructive interference. Arrows point outward from the antenna, joining these points. These arrows show the directions of constructive interference.

Can an antenna be any length? Explain your answer.

Problems & Exercises

What is the maximum electric field strength in an electromagnetic wave that has a maximum magnetic field strength of 5.00×10−4 T size 12{5 "." "00"´"10" rSup { size 8{-4} } " T"} {}

(about 10 times the Earth’s)?

150 kV/m

The maximum magnetic field strength of an electromagnetic field is 5×10−6T size 12{5 times "10" rSup { size 8{ - 6} } T} {}

. Calculate the maximum electric field strength if the wave is traveling in a medium in which the speed of the wave is 0.75c size 12{c} {}

.

Verify the units obtained for magnetic field strength B

in [link] (using the equation B=Ec

) are in fact teslas (T).

Glossary

electric field
a vector quantity (E); the lines of electric force per unit charge, moving radially outward from a positive charge and in toward a negative charge
electric field strength
the magnitude of the electric field, denoted E-field
magnetic field
a vector quantity (B); can be used to determine the magnetic force on a moving charged particle
magnetic field strength
the magnitude of the magnetic field, denoted B-field
transverse wave
a wave, such as an electromagnetic wave, which oscillates perpendicular to the axis along the line of travel
standing wave
a wave that oscillates in place, with nodes where no motion happens
wavelength
the distance from one peak to the next in a wave
amplitude
the height, or magnitude, of an electromagnetic wave
frequency
the number of complete wave cycles (up-down-up) passing a given point within one second (cycles/second)
resonant
a system that displays enhanced oscillation when subjected to a periodic disturbance of the same frequency as its natural frequency
oscillate
to fluctuate back and forth in a steady beat
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