PHYSICS. Speed of Sound. Mr R Gopie

Transcription

1 Speed of Sound Mr R Gopie

2 a) Reciprocal firing Methods of determining the speed of sound in air include: Diag. 20 The time interval, t, between the flash and the sound represents the time taken for sound waves to travel from the source of sound to the receiver of sound (assuming that the light waves from the gunshot, source of the flash seen, arrive instantaneously). The roles of the two experimenters are then exchanged (while their positions are maintained) and the procedure is repeated (to eliminate errors due to wind assistance and to reaction time). An average value of t is determined. The distance between the two experimenters is measured several times and an average value, d is determined. The speed of sound v = d/t Page 2 of 18

3 b) Echo method Diag. 21 The person doing the clapping does so in time with the preceding echoes i.e. he synchronizes the claps with the echoes. The other persons then counts the claps and times the interval over which they occurred, for instance, n claps in t s. Calculations: Time interval between successive claps = t/n. During this time interval sound waves travel to and from the wall (returning as an echo, i.e. a distance of 2d. So speed of sound waves, v = distance 2d/ time to travel this distance, t/n = 2dn/t Page 3 of 18

4 The electromagnetic spectrum Electromagnetic Waves Diag. 1 from light All electromagnetic waves are progressive/travelling waves (through in certain circumstances they can be changed into stationary/ standing waves). They are also transverse wave. They vary in wavelength, frequency and energy, but they all travel with the same speed in air or a vacuum (i.e. the so called speed of light, c = 3.0 x 10 8 ms - 1 ). They all show the properties, reflection, refraction, diffraction and interference. They all obey the wave equation v = f x λ Waves (of all types ) emitted from a point source obey an inverse square law and this law dictates that the intensity of a wave (which is a measure of wave energy per second per square metre at right angles to its direction of propagation) varies inversely, as the square of the distance from the source, i.e. I 1/r², where I is the intensity of the waves at a distance r from the point source of the wave. Since the intensity also depends on the amplitude of the wave (actually the amplitude squared) then the amplitude also decreases with increasing distance from a source of waves. Consider the following : if the intensity of waves when 2m from a point source is 4 Wm - 2, what is the intensity when the distance is a) 1 m b) 4m Page 4 of 18

5 Wave Band Origin Source Detector Special properties and uses. ɣ - rays X- rays U.V. Energy changes in the nuclei of atoms/ions High energy changes in the shells of atoms Decelerated electrons Energy changes in the shells of atoms Certain radioactive nuclides Cosmic rays X- ray tubes (including TVs ) Very hot bodies such as the sun, electric arcs, sparks. Electric discharges, through certain gases, (eg. Hg vapour), i.e. vapour lamps and florescent tubes. Photographic film Geiger- muller tube and counter Photographic film Fluorescent screen Photographic film Photo cells Fluorescent chemicals High energy photons/waves which are very penetrating and very dangerous. they are used to kill cancer growths, to detect flaws in metals, to detect leaks in underground pipes and to sterilize equipment Also very penetrating and dangerous. Used to take X- ray pictures (i.e. radiography) and to treat skin disorders. Also used to study crystal structures (i.e. X- ray crystallography) Absorbed by glass causing many chemical reactions. Damages and kills living cells. (e.g. causes sun burn) U.V. lamps used in medicine for skin treatment (but dangerous to the eyes) produce fluorescents in Page 5 of 18

6 certain washing powders. Fluorescent is also used to detect forgeries. Wave Band Origin Source Detector Special properties and uses. Visible light I.R. radiation Microwaves and Radio waves Energy changes in the shells of atoms Low level energy changes in the shells of atoms Very low- level energy changes in the shells of atoms Hot bodies at or above red- heat, such as flames, lamp, filaments, lasers and the sun All matter over a wide range of temperature (from 0K upwards) e.g. warm and hot bodies, fires, people, and the sun. Radio transmitting circuits and associated The eye Photographic film photocells Special photographic film Semiconductor devices such as L.D.R.s and photodiodes Skin Thermopile (i.e. a set of thermocouples connected in series). Aerials connected to tuned electric circuits in radio Refracted by glass and water. Essential for photosynthesis and plant growth. Used to identify certain elements in flame tests in chemistry. Used in communication systems involving lasers and optical fibres. Causes heating when absorbed (e.g. makes skin feel warm in the sun.) used for heating (e.g. radiators and fires emit I.R radiation) used for photography through haze and fog (since I.R. radiation is not scattered as much as visible light). I.R. photographs taken by satellite provide special information (about crops etc). Are spread round hills and buildings but diffraction. Used Page 6 of 18

7 High frequency oscillatory electric circuits aerial equipment used with radio and TV. microwaves and TV sets for radio, TV, telephone and satellite communications. Used for radar detection of ships, aircrafts and missiles. Used in radio astronomy. Microwaves are used for cooking. Page 7 of 18

8 June 1994 paper 2 #3 TUTORIAL Page 8 of 18

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11 June 1992 paper 2 #5 Page 11 of 18

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14 June 2006 paper 3 #3 Page 14 of 18

15 June 2007 paper 2 #4 Page 15 of 18

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17 June 2007 paper 3 #2 Page 17 of 18

18 June 2001 paper 3 #4 Page 18 of 18