Electromagnetic Waves

inductor. This means that energy can temporarily be stored in an inductor, similar to that of a capacitor.

ELECTROMAGNETIC WAVES

Maxwell’s equations put together the facts related to magnetism and electricity into a series of statements and equations that are outlined here:

• Electric field lines start on positive charges and end on negative charges with an electric field defined as a force per unit charge, with the strength of the force related to the permittivity of free space.

• Magnetic field lines are continuous and have no beginning or end. The strength of the magnetic field is related to the magnetic constant called the permeability of free space.

• Changing magnetic fields will induce an electromotive force, which creates an electric field. This relates to Lenz’s law, which means that the emf opposes the change in magnetic field.

• Magnetic fields are generated by moving charges or by changing electric fields so that electric current changes will create magnetism.

What these theories put together is the idea that electric fields and magnetic forces are not separate but are different manifestations of the same thing—which is electromagnetic force. These changing fields will propagate from their source like waves. These are now defined since the time of Maxwell to be EM waves or electromagnetic waves. These waves are capable of exerting a force over great distances from their source. How fast do these waves move? When calculated, it turns out that they move at 3 x 108 meters per second, which is the speed of light.

Whenever a current varies, there will be variable electric and magnetic fields that move out from the source like waves. The electric field surrounding the wire is produced by the charge distribution on the wire. The magnetic field B propagates outward as well. Both of these propagate as an electromagnetic wave, such as when a broadcast antenna sends out a signal in a radio or TV station. These waves are similar to other waves we’ve

talked about, with wavelengths identified by lambda and frequencies identified by f, which are inversely proportional to one another.

These electromagnetic waves are transverse waves that have a peak and a trough. The electric wave put out by an antenna will be in phase with the magnetic wave but these will be perpendicular to one another. Charges radiate whenever they are accelerated. Antennae have a varying charge distribution that form standing waves with a certain resonant frequency determined by the dimensions of the antennae. This is why one tunes a radio to get a certain frequency, which is the resonant conditions for a specific antenna.

An antenna is necessary for receiving signals as well. Receiving antennas are designed to resonate at specific frequencies. When the radio is turned on, the electrical components will pick up the signal, converting the signal to audio formats. The same thing is seen with TV signals and the electromagnetic waves that are generated from the station.

The stronger the E-field or electrical field, the greater the current and the greater the Bfield created. The current is directly proportional to voltage, so the voltage is proportional to the field strength E. The ratio of the E field strength and the B field strength is a constant, equal to the speed of light. The E field is much greater than the B field generated because of this ratio being so large and constant.

There are different categories of electromagnetic waves, such as radio waves, infrared waves, and ultraviolet waves. Each wave has a frequency and a wavelength associated with it, and each wave travels at the speed of light. Because they all propagate at the speed of light, the speed of light is equal to the frequency multiplied by the wavelength. The greater the frequency, the smaller the wavelength.

The longest waves are radio waves, with AM wavelengths being longer than FM waves. TV waves are similar to FM waves in wavelength. Microwaves have a shorter wavelength than many radio waves and infrared waves are shorter than that in wavelength. Light waves have a very small bandwidth between infrared waves and ultraviolet waves. Higher than these are x-rays and finally gamma waves, which have

the highest frequency and lowest wavelength. Figure 136 shows the electromagnetic spectrum:

Figure 136.

In general, these things can be made true of electromagnetic waves. High-frequency waves are more energetic and can penetrate more than low-frequency waves. Highfrequency waves carry more information per unit time than low-frequency waves. The shorter the wavelength, the smaller the detail it can resolve.

Radio waves can be produced by the current in wires and circuits. These are used to create microwaves, AM radio waves, FM radio waves, cell phones, and TV pictures. Power lines will generate extra-long wavelengths of electromagnetism as long as many kilometers in wavelength. While power lines do give off waves, there is no evidence that these waves cause disease.

ELF waves or extremely low frequency waves are used as means of communication in submerged submarines. These can penetrate salt water farther than waves of shorter wavelengths. It takes these long wavelengths so the waves aren’t disturbed as much by salt water.

AM radio waves are called this because they are “amplitude modulation” waves that have a particular frequency emanating from the radio station. The wave received varies in amplitude but not in frequency. These are tuned in by antennas at the detection site. FM radio waves are called “frequency modulation” waves, which is another way of carrying information. There is a carrier wave sent out by the radio station that is modulated in frequency by the audio signal producing a wave of constant amplitude but

varying frequency. FM waves are less subject to noise from stray sources because the amplitude stays the same.

Television waves require a higher bandwidth to carry visual and audio information. The entire FM radio band is within the range of the TV bandwidth. TV channels are divided into VHF (very high frequency) and UHF (ultra-high frequency) waves. Cable TV and satellite TV transmission is ultra-high frequency, which can carry more information in the high-definition or HD format. The higher the frequency of these types of waves, the more direct the wave transmission and reception must be because they can’t travel around structures.

Microwaves are the highest-frequency EM waves that can be produced by an electric circuit. They are called microwaves because of their comparison to the frequency of radio and TV waves. Microwaves can be generated by the thermal agitation of atoms and molecules at any temperature above absolute zero. These will transmit information from communication sources but require a clear line of sight between the transmitter and receiver. Radar is an example of the use of microwaves.

Microwaves are used to generate an alternating electric field with waves that get absorbed by food in a microwave oven. Polar molecules like water will absorb microwaves, resulting in dielectric heating or an increase in water’s temperature. Because it is water and similar molecules that absorb the radiation, the plate does not heat up. Rotating turntables will spread out the hot spots. Microwave diathermy uses microwaves to heat body areas affected by strains and sprains.

Infrared regions will overlap a little with microwave regions. Infrared radiation is produced by the thermal motion and the vibration and rotation of atoms and molecules. The frequency is just below the red visual light region, which is why it’s called infrared radiation. Human bodies will give off infrared radiation so that night-vision scopes can detect it and convert it to visible light. Infrared emissions are proportional to the fourth power of the absolute temperature of an object. About half the radiation arriving on earth from the sun is infrared radiation, with a lesser amount being visible light. A small amount is ultraviolet waves.

Visible light is a very narrow band of waves on the spectrum. It is produced by vibrations and rotations of atoms and molecules as well as by electronic transitions within atoms and molecules. Colored light has different wavelengths between 400 and 750 nanometers. Red light has the lowest frequency and the longest wavelength, while violet has the highest frequency and shortest wavelength.

UV light or ultraviolet light, which has a higher frequency than violet light is produced by molecular motions and electron transitions within atoms and molecules. These wavelengths are invisible to the human eye. The sun has UVA radiation, UVB radiation, and UVC radiation (which have increasing frequencies). Most UVB and UVC waves are absorbed by the ozone in the atmosphere, making 99 percent of this radiation of the UVA type.

UVB radiation is what causes skin cancer. All UV radiation will damage collagen fibers, resulting in wrinkle formation. Sunburn is caused by large exposures, while skin cancer is caused by repeated exposures. Tanning is a defense mechanism in which the body makes pigments to prevent future exposure to skin from the sun. UVB exposure is linked to cataracts, especially in the equatorial regions of the body.

X-rays are created by high-voltage discharges. An electron can be accelerated in a vacuum tube by a high voltage that strikes a metal plate, producing x-rays, thereby ionizing an atom. X-rays can penetrate the skin (unlike ultraviolet rays), causing damage to atoms in cells and making x-rays both causative of and treatment for cancer. X-rays penetrate differently depending on tissue density, which makes them helpful in medical applications. These types of rays can also precisely identify the shapes of molecules, using a technique called x-ray diffraction.

Gamma rays are extremely high-frequency electromagnetic waves, which is the electromagnetic radiation emitted by a nucleus, which is why it’s called nuclear radiation, used in weaponry and nuclear reactors. These are nearly identical at some frequencies to x-rays, differing only in the source. At higher frequencies, these can be very damaging to human tissue. It is the type of radiation used in nuclear medicine. Gamma radiation is used to protect food from spoilage by killing the microorganisms.

No one knows if this damages the food. X-rays and gamma rays are used to scan luggage.