CHAPTER 17 AND 18 CHARACTERISTICS OF EM WAVES LEARNING OBJECTIVES CHARACTERISTICS OF EM WAVES 11/10/2014

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1 STUDENT LEARNING GOALS PHYSICAL SCIENCE ELECTROMAGNETISM SC.912.P CHAPTER 17 AND 18 Electromagnetic Spectrum, Light, and Sound Goal: Explore the theory of electromagnetism by comparting and contrasting the different parts of the electromagnetic spectrum in terms of wavelength, frequency, and energy, and relate them to phenomena and applications. 4 Investigate and design an experiment relating to the theory of electromagnetism. 3 Explore the theory of electromagnetism by comparting and contrasting the different parts of the electromagnetic spectrum in terms of wavelength, frequency, and energy, and relate them to phenomena and applications. 2 Explain the theory of electromagnetism, wavelength, frequency, and energy. 1 Define the theory of electromagnetism, wavelength, frequency, and energy. LEARNING OBJECTIVES 1.Describe the characteristics of electromagnetic waves in a vacuum and how Michelson measured the speed of light. 2.Calculate the wavelength and frequency of an electromagnetic wave given its speed. 3.Describe the evidence for the dual nature of electromagnetic radiation. 4.Describe how the intensity of light changes with distance from a light source. 5.Rank and classify electromagnetic waves based on their frequencies and wavelengths. 6.Describe the uses for different waves of the electromagnetic spectrum. 7.Describe the Doppler effect CHARACTERISTICS OF EM WAVES Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields An electric field is a region of space that exerts electric forces on charged particles A magnetic field is a region of space that produces magnetic forces Magnetic forces are produced by magnets, changing electric fields, and vibrating charges CHARACTERISTICS OF EM WAVES EM waves can travel through a vacuum as well as through matter EM radiation is the transfer of energy by EM waves traveling through matter or across space Light and all EM waves travel at the same speed but the wavelength (λ) & frequency (f) can differ The speed of light (& all EM waves) is 3.00 x 10 8 m/s CHANGING ELECTRIC AND MAGNETIC FIELDS Electric field is a region where particles can be pushed or pulled. Wherever there is an electric charge there is an electric field associated w/it. A moving electric charge is part of an electric current An electric current is surrounded by a magnetic field A magnetic field is a region in which magnetic forces are present When electric field changes so does the magnetic field. 1

2 CALCULATIONS Speed = wavelength x frequency For EM waves, speed = 3.0 x 10 8 m/s Frequency = speed/wavelength OR f= c/λ Wavelength = speed/frequency OR λ=c/f The units for speed (c) are m/s The unit for wavelength (λ) is m The unit for frequency (f) is Hz (1/seconds) WAVELENGTH PROBLEMS 1.A radio station broadcasts a radio wave with a wavelength of 3.0 meters. What is the frequency of the wave? 2.A global positioning satellite transmits a radio wave with a wavelength of 19 cm. What is the frequency of the radio wave? (Hint- Convert the wavelength to meters before calculating the frequency) WAVELENGTH PROBLEMS SOLUTIONS WAVELENGTH PROBLEMS 1.Speed= c= 3.00 X 10 8 m/s Wavelength= 3.0 m Frequency=? Speed= Wavelength X Frequency or Frequency=Speed/Wavelength Frequency= 3.00 X 10 8 m/s /3.0 m = 1.0 X 10 8 Hz 2. Speed= Wavelength X Frequency; Frequency=Speed/Wavelength 3.00 X 10 8 m/s/0.19 m= 1.6 X 10 9 Hz 3. The radio waves of a particular AM radio station vibrate 680, 000 times per second. What is the wavelength of the wave? 4. Radio waves that vibrate 160,000,000 times per second are used on some train lines for communications. If radio waves that vibrate half as many times per second were used instead, how would the wavelength change? WAVELENGTH PROBLEM SOLUTIONS 3. Wavelength= Speed/Frequency 3.00 X 10 8 m/s/680,000 Hz=440 m 4. At 160 MHz: Wavelength=Speed/Frequency = 3.00 X 10 8 m/s /160,000,000 Hz= 1.9 m; at 80 MHz Wavelength= Speed/Frequency= 3.00 X 10 8 m/s /80,000,000 Hz= 3.8 m; This wavelength would be 1.9 meters longer. EM RADIATION/ LIGHT INTENSITY EM radiation sometimes behaves like a wave and sometimes like a particle Light, therefore, is classified both as an EM wave and as a particle A photon is an EM packet of energy Each photon s energy is proportional to the frequency of the light The intensity of light decreases as photons travel farther from the source 2

3 EMR - WAVE OR PARTICLE? Acts like a wave sometimes ie: Polarizing Filter EMR - WAVE OR PARTICLE? Acts like a particle, a photon, sometimes ie: Photoelectric Cell EMS WAVES Long wavelength : Low Frequency & Low Energy Short wavelength : High Frequency & High Energy RADIO & MICROWAVES Longest wavelengths & lowest frequency of the EMS Include Am, FM and Television frequencies AM Amplitude modulation: same frequency waves just a change in the amplitude to get different sounds etc. FM Frequency Modulation: slight changes in frequency MICROWAVE & INFRARED EMR Microwave: used in microwave ovens & cellular phones Infrared: Fast Food Heat Lamps, use as a night time surveillance tool. VISIBLE LIGHT White light is a mixture of the entire visible light spectrum 3

4 RANK AND CLASSIFY The prism separates the wavelengths present in sunlight which is visible light From longest to shortest: ROY G B(I)V The electromagnetic spectrum includes visible plus invisible radiation Increasing frequency from left to right (longest to shortest): radio waves, infrared rays, visible light, UV ray, X rays, and gamma rays UV, X-RAY & GAMMA RAYS UV from the Sun helps the body produce vitamin D, too much exposure can cause skin cancer Xrays: used extensively in medicine to see into the body Gamma Rays: used in medicine to treat cancer or destructive radiation from nuclear explosions. USES FOR EM WAVES Radio waves are used in radio, television, microwaves and radar The shortest radio waves are microwaves Radar is an acronym: radio detection and ranging Radar often uses the Doppler effect to determine how fast something is moving Infrared rays are used as a source of heat & to discover areas of heat difference USES FOR EM WAVES Thermograms use infrared to sensors to show differences in temperature of objects Visible light is used to see, stay safe and communicate UV rays are used in health, medicine and agriculture X rays are used in medicine, industry and transportation to make pictures of the inside of solid objects Gamma rays are used medically to kill cancer cells, make brain pictures and in certain industrial situations such as checking pipelines for cracks or other damage SOUND Sounds are longitudinal waves that require a medium to travel caused by the vibrations of an object. Speed of Sound on average: Air is 767 mph (343 m/s) about 1 mile every 5 sec Water is 3,315 mph (1,482 m/s) Steel is 13,330 mph ( 5,960m/s) The speed of sound depends on the elasticity, density and temperature of the medium. SPEED OF SOUND Speed of Sound: depends on the elasticity, density and temperature Elasticity the ability of an object to bounce back to its original shape. Sound travels faster in more elastic objects. Typically gasses are the least elastic, liquids are next and solids are the most elastic. Density generally speaking, in material of the same state of matter (solid, liquid or gas) the denser the medium the slower the sound travels. Sound travels slower in lead than it does in steel. Temperature generally speaking the higher the temperature the faster the speed of sound. 4

5 BREAKING THE SOUND BARRIER Chuck Yeager first man to fly faster than the speed of sound Andy Green first man to drive a land vehicle faster than the speed of sound. PROPERTIES OF SOUND Intensity the amount of energy the wave carries per second per meter squared intensity = Watts / m 2 Loudness sound level is measured in decibels (db) October 14, 1947 in X1 Glamorous Glennis October 15, 1997 in SuperSonic Car Thrust SSC 763 MPH FREQUENCY & PITCH Frequency the number of vibrations per second Human Hearing between 20 Hz 20,000 Hz Below 20 Hz is called infrasound Above 20,000 Hz is called ultrasound Pitch - dependent of frequency high frequency yields high pitch sounds Low frequency yields low pitch Resonance when the frequency of sound matches the natural frequency of an object DOPPLER EFFECT Sound moves equally in all directions from a source. Circular (or spherical) pattern If the source is moving the origin of successive circles moves. WAVELENGTH SHIFT The normal period is T and the observer moves at speed u. The wavelength ahead of the source is shorter. l = l - ut FREQUENCY SHIFT This can be converted into a frequency shift. The period is related to the wavelength. T l / v l v / f l l u( l / v) l(1 u / v) v / f ( v / f )(1 u / v) f f 1 u / v The wavelength behind the source is longer. l = l + ut Insert + for to get the shift behind the source. 5

6 AMBULANCE MOVING OBSERVER An ambulance is approaching at 130 km/h with a siren at 1.2 khz. What pitch do you hear? Change speed to m/s (130 km/h)/3.6 = 36 m/s Find the Doppler shift. f f 1 u / v If the observer is moving toward the sound the effect is similar, but the wave crests have a different relative spacing. u/v = (36 m/s)/(343 m/s) u/v = f = f/(1-1.05) = 1340 Hz f f ( 1 u / v) A speed gun uses a double Doppler shift. There is a shift for both the pulse out and reflected signal. SHOCK WAVE A moving source can exceed the speed of sound. The sound waves constructively interfere on a front at an angle to the motion. This is called a shock wave. 6