Interference of Waves

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Shanna Todd
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1 Interference of Waves Particles in a medium do not move with the wave. Instead, these particles are. When two waves, traveling in the same medium at the same time, meet, the particles are displaced by both waves. The net result is different than either incident wave during the interaction. (time plays an important role) This combination is referred to the Principle of Superposition. Before animations and real time demos could be used, students examined wave interference through still pictures of the two waves. The amplitude of each wave would be added, keeping its orientation in mind, to determine the new, combined amplitude. Sometimes the new amplitude was greater than the original waves. This type of interference was said to be. If the amplitude was reduced, the interference was said to be. Destructive Interference occurs in a transverse wave when a meets a. Constructive Interference occurs in a transverse wave when a meets a or when a meets a. If the of the waves are then the corresponding destructive interference is said to be Total Destructive Interference. Students are given interference diagrams to check their understanding of this principle. The interference pattern created represents only one moment in time and is usually drawn when the waves are exactly on top of each other. 1

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3 To challenge students, geometric shapes are used instead of real waves. 3

There are two very important interference patterns that have been found in nature and they have been

com/watch?v=pcyv0_qpk 4 The lines represent regions where there is always 2.

edu/drussell/demos/superposition/superposition.html http://www.youtube.com/watch?

frequencies, the wave looks like it is not moving.

4 There are two very important interference patterns that have been found in nature and they have been found to exist in several situations. 1. Two point source interference pattern 4 The lines represent regions where there is always 2. A standing wave gr7kmtorx0&feature=related A standing wave is not actually a still wave but when viewed at high frequencies, the wave looks like it is not moving. It occurs when two waves meet which have an identical and ( We do not have to mention its wavelength because for a given medium the speed is constant. Therefore, since v=f λ, then if f is the same then so to is ) 4

5 Interference and Sound Waves It is possible to create a pure sound wave. A tuning fork is an example of a device which produces a "pure" sound. Most sounds made are combinations of several sound waves which interfere. Because the wave originates from the same place, the pattern created during the production of sound stays in tack as it travels away from the source. This wave pattern (created from interference) is study as "the quality of sound". Sound which is pleasing to our ear has some type of interference pattern which has a mathematical relationship allowing for a discernable pattern with the wave. Sound which is not pleasing to our ear interference in such a way that. The main frequency produced is referred to as the fundamental frequency. The additional waves which add richness to a good sound must be mathematically related to the fundamental to facilitate the development of a pattern in the combined wave. These additional waves are referred to as overtones. Noise in physics are sound waves which lack quality. This can exist for two reasons there is no sustained fundamental note OR the overtones are not mathematically related to the fundamental such that the interference pattern created has no regular flow. Audacity is a computer program which allows you to see the sound waves mapped as a transverse wave. Back to Speakers and Electromagnetism A microphone takes a sound wave and moves a coil in a magnetic field producing an electric current. The electric current varies according to the way it was produce which maintains the "wave" characteristics (frequency and amplitute) control the current produced. This changing current is then sent to a speaker (often it is electrically amplified) where it the changing current is sent into a coil in the presence of a permanent magnet. The changing force created moves the cone reproducing the sound sciences.com/en/v9 loudspeaker principles underlying its functionin a loudspeaker works 5

6 v=fλ v= T f = 1/T v=λ/t v=d/t Formula 1. A metronome beats 60 times over a 40 s time interval. Determine the frequency and period of its motion. 2. A longitudinal wave in a 12.0 m long spring has a frequency of 40.0 Hz and a wavelength of m. Calculate the speed of th wave and the time that it would take to travel the length of the spring. 3. Determine the speed of a plane traveling at Mach 2.5 if the temperature of the air is 22 o C. 6

Chs. 16 and 17 Mechanical Waves

Chs. 16 and 17 Mechanical Waves The nature of waves A wave is a traveling disturbance that carries energy from one place to another, and even though matter may be disturbed as a wave travels through a