Five Chapter 7

Applications of Electromagnetic Waves

• Radio and Television
  • Marconi recognized the potential of transmitting information over distances without wires.
  • Marconi first used this for dot-dash Morse code pulses.
  • Eventually this developed into radio and television
• Transmission (How it used to be done)

• The microphone detects and converts sound to a weak audio signal
• AF (audio frequency) amplifier strengthens the signal
• Strengthened signal interferes with an RF (radio frequency) carrier signal from an RF ocsillator (occurs in mixer)
• RF signal is determined by the relative values of inductance and capacitance in the oscillator circuit.
• Rf signal is different for each radio station
  • 500 kHz to 1600 kHz for AM radio
  • 88 MHz to 108 MHz for FM radio
• The modulated signal is amplified and sent to an antenna for transmission as an electromagnetic wave
• Carrier Frequency - the frequency you "Tune on your radio or Television dial
• There are two ways of mixing the sound with the carrier.
  • AM - Amplitude modulation
    • Audio signal is superimposed on the much higher carrier frequency
    • Carrier varies in amplitude according to audio signal being transmitted
    • frequency remains constant

• FM - Frequency modulation
  • Audio signal is combined with a carrier to produce a modulated signal of constant amplitude but varying frequency.
  • Frequency shift is greater for loud audio

• For a television, the signal is much the same only the carrier is mixed with two other signals
  • an audio signal
  • a vidio signal.
• Reception (How it used to be done)

• The antenna detects the electromagnetic waves. As the wave passes over the antenna, the electric field changes with time, and this causes a force to be applied to the electrons in the metal of the antenna. This drives the electrons up and down the antenna.
• In a coil antenna, the magnetic fields of the electromagnetic waves cause the forces on the electrons to produce the current in the coil
• In both cases, an electrical copy of the signal is created in the antenna
• Many signals are picked up by the antenna. A resonant tuning circuit selects a certain carrier frequency or small range of frequencies.
• Selection is accomplished by means of a resonant tuning circuit that contains an inductance (L) and a capacitance (C).
• By adjusting the value of C or L, we tune the circuit so it resonates on the carrier frequency of the desired station. All other frequencies interfere destructively and cancel out.

• The selected RF signal is amplified and sent into a demodulator which separates the AM or FM carrier signal from the audio signal (subtracts the carrier signal out).
• The AF (audio Frequency) signal remaining is amplified and sent to a speaker.
• With a television, two signals are demodulated and one is sent to the speaker and the other to the display.
• The CRTC, and the American equivalent, regulate carrier frequencies and power outputs.
• In addition to radio and television bands, frequency bands have been assigned for cell phones, citizen bands, ship-to-shore radio, aircraft, police, military, amateur radio etc.
• Digital Signals (Signals now used)
  • Digital signals are now used throughout North America, and there are some important differences to the analogue signals of the past.
  • Normal sounds and pictures that we want to transmitt start as analogue signals. To transmitt them as a digital signal, they first have to be converted into digital format. This is done with an analogue-to-digital converter.
  • Analogue signals can have a wide range of continuously changing values. When converted to a digital signal, this wide range of values needs to be converted to a limited number of discrete values. In fact, the numbers used are zero (0) and one (1).
  • All mathematical numbers can be converted to a binary number.

EG. The mathematical number 12 in the binary system of numbers is 1100 where as the

number 9 would be 1001. The numbers from one to thirteen in the binary system are;

1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010, 1011, 1100, 1101. Hopefully you can

see the pattern.

• Therefore, all sounds (frequency, loudness etc.) , and all parts of a picture (location in the picture, color, brightness etc.), can be assigned a number value which can be written as a binary number.
• The advantage of the binary system of numbers is that there are only two numbers 0 and 1.
• Zeros and ones can be easily represented in electronic circuits as a switch. A zero would be an open switch, and a one would be a closed switch
• A single frequence of electromagnetic waves can also be used to represent ones and zeros. A zero would be the electromagnetic wave turned off, and a one means the electromagnetic wave is turned on.
• Therefore, the number 90 can be represented as an electromagnetic signal like this.

• In essence we are dividing the electromagnetic wave into a series of timed chunks or bits.
• Looking at a magnified section of this wave, we see that a bit consists of two parts, the useful symbol duration and the guard interval.
• Useful symbol duration (SD) - the time interval of a bit or chunk which indicates the type of symbol to be interpreted (a one or zero).
• Guard interval (GI) - the time interval of a bit or chunk which is used to separate one symbol from another.

• There is another method of transmitting ones and zeros, and that is to vary the phase of the wave in each bit or chunk as illustrated below. Again we illustrate the number 90.

• In this last diagram, notice how for both ones and zeros the carrier wave is turned on but the last crest for a zero is nearly on the vertical dashed line and for a one it is displaced to the left. This represents a phase difference.
• For digital signals, the analogue sound or picture is converted to binary numbers by an analogue-to-digital converter in the mixing stage in the first diagram under "Transmission." Then the signal is amplified and sent to the transmitter antenna.
• When the signal is received, instead of an RF tuner and amplifier, we would have a digital tuner and amplifier. The detector demodulator would be replaced by a digital to analogue converter.

X-Rays

• Roentgen noted that a fluorescent screen placed several meters from a cathode ray tube glowed in the dark.
• Even when the tube was covered by heavy black paper, nearby photographic plates became exposed.
• He concluded that some radiation must be emitted from the cathode ray tube.
  • He called them X-rays.
• The Characteristics Roentgen found for x-rays are:
  • They penetrated substances opaque to visible light.
  • They were not deflected by electric or magnetic fields.
• Because of the second observation, he concluded they were electromagnetic waves of very short wavelength.
  • Charged particles are deflected by electric and magnetic fields but electromagnetic waves are not.
• X-rays are produced when high-energy electrons decelerate in striking an obstacle.

• By varying the current (in diagram) we vary the number of electrons colliding and therefore the intensity of the x-rays.
• By varying the potential difference between cathode and anode, we vary the kinetic energy of the electrons, energy of the x-rays, and therefore their penetrating ability.

V = E/q => qV = (1/2)mv² = E_k

This E_k of the electrons is converted to energy of the x-ray

E = hf f = frequency of the x-ray

h = Planck’s constant = 6.63 X 10^-34 Js

• X-rays ionize substances they pass through (knock electrons loose).
  • Therefore if an electroscope is charged, it will discharge when the air around it is irradiated with x-rays
  • The air is ionized, and unlike ions in the air are attracted to those on the electroscope to discharge (neutralize) the electroscope.
• Properties of x-rays:
  • They are produced by the collision of high-energy electrons with the anode of a cathode ray tube.
  • Penetrating ability depends upon
    • Object thickness and density
    • Intensity and wave length of the x-ray
  • Cause photochemical reactions and cause some substances to fluoresce.
  • They ionize air molecules and therefore cause charged objects to discharge.
  • Cannot be deflected by magnetic or electric fields.
  • Travel at the speed of light.
  • They are EM waves with a very short wavelength.
• Sometimes x-rays are referred to as
  • Soft rays - x-rays with longer wavelengths.
  • Hard rays – x-rays with shorter wavelengths.
    • Are more penetrating than soft rays
• Radiograph - An X-ray photograph
• Fluoroscope - A fluorescent screen
• To see inside an object, the object is placed between the X-ray tube and the radiograph or fluoroscope.
• X-rays pass better through some materials than others, this results in different amounts of exposure to the film or screen at different places.
• X-rays can also be used to kill human cells (malignant growths). These rays can also kill healthy cells. When X-rays were first discovered, this was not known. Many early investigators died as a result.
• Today, very sensitive photographic materials are used so exposure times are short.
• Lead screens, leaded glass, and lead covered walls protect X-ray technicians.
• Lead blankets and aprons protect patients.
• Exposing sex cells to X-rays can cause genetic changes which result in birth defects.
• X-rays are used in industry to:
  • examine interiors of complex equipment.
  • check for broken parts.
  • check welding.
  • check for cracks in airplane parts.
  • etc.

Gamma Radiation

• Bequerel found in 1896 that the mineral pitchblende, containing radium and polonium would expose a photographic film.
• Pitchblende emitted some kind of radiation spontaneously to do this.
• Analysis of emissions from various materials resulted in the discovery of three types of radiation.
  • Alpha particles (a) - helium nucleus – a type of particle similar to highspeed helium nuclei with poor penetrating ability
  • Beta particles (b) - high speed electrons – a particle similar to high speed electrons with the ability to penetrate several millimeters of aluminum.
  • Gamma rays (g) - EM radiation – similar to very high energy x-rays, which can penetrate several centimeters of lead.
• Magnetic fields were used to distinguish between the three.

• Alpha, and beta particles leaving the lead block from the radioactive sample will be deflected in different directions in the magnetic field because they have opposite charges (beta - negative and alpha - positive). Because gamma rays are electromagnetic waves, they do not have charge and so travel straight through the magnetic field.
• Gamma radiation was first thought to be a neutral particle. Later observations led physicists to conclude it was electromagnetic radiation of very high frequency emitted from radioactive atoms.
• Gamma radiation affects cancer cells more than normal cells. Therefore, gamma radiation is used to treat cancer.
  • To further reduce the effects on healthy cells, the patient’s body is constantly moved to reduce the exposure of healthy cells, while the gamma rays are always directed at the tumor.
  • Damage to healthy cells usually occurs which may result in radiation sickness with symptoms of:
    • Nausea
    • Fatigue
    • Loss of hair
  • If cell damage is in the reproductive organs, then mutations can occur and be transmitted to future offspring.
• Because of the presence and use of radiation, there is concern with respect to the genetic future of the human race.

January 19,2014