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30 solved problems in waves and astrophysics
- 1. 30 solved problemsin waves and astrophysics Source: www.physicslectures.info By mjaafar
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- #2 why do astronomers observing distant galaxies talk about looking backward in time do the brightest stars that you see in the night sky necessarily give off more light than dimmer stars why did early experimental attempts to measure the speed of light fail? how did olaus roemer measure the speed of light how is the brightness of a light source affected by distance? what atmospheric conditions produce a mirage what atmospheric conditions produce thunderstorms a certain radio wave has a frequency of 2.0 × 106 hz. what is its wavelength? the relation between the frequency, velocity and wavelength of em wave is given by v=fλ where v =c and c is the speed of light λ =c/f λ =3 × 108/2.0× 106 λ =3/2 × 108-6 λ =1500HZ yellow-green light has a wavelength of 560 nm. what is its frequency? From the relation v =fλ where v=c and c is the speed of light f= c/λ f =3 × 108 /560× 10-9 f = what is the wavelength of an infrared wave with a frequency of 4.2 x10^14 hz from the relation v =fλ note all em wave travel at the speed of light therefore v =c λ =c/f λ =3 × 108/4.2 × 1014 if the number on an fm radio dial reads 99.5 mhz, what is the wavelength of this signal? 99.5 mHz is SAME AS 99.5 × 106 Hz v =f λ but v = c λ =c/f =3 × 108/99.5 × 106 if you know the wavelength of any form of electromagnetic radiation, you can determine its frequency because wavelength is inversely proportional to frequency while the velocity for all em waves is constant. therefore λ α1/f λ =c/f f λ =c what factor affects the transmission of an electromagnetic wave through a medium? the density of the medium, this is why light has different speed in solid, liquid and vacuum , this difference in speed is measure of the refractive index of these media which is the property of density of the medium. dense medium tends to have high refractive index than light medium, em waves e.g light will travel fast in media with low index of refraction . therefore density is the factor that affect the transmission of em waves in a medium. a(n) __________ body produces light, and a(n) ________ body reflects light. luminous body and non-luminous body. luminous bodies are object that emits light rays on their own e.g sun, stars, light bulbs et while non-luminous object are object like moon, mirror reflect light falling on them. in a vacuum, electromagnetic radiation of short wavelengths the arrangement of electromagnetic radiation is called spectrum why do radioisotopes undergo radioactive decay to attain stability to say that electric charge is quantized is to say that the charge on an object is a multiple of a whole number, the charge on any substance is the multiple of the magnitude of charge on electron, which is 1.6 × 10-19C. the magnitude of charge on an electron or proton is 1.6 × 10-19C. but the number of charges varies, they in the multiple of the above magnitude. hence they are said to be quantized or simply in the multiple of 1.6 × 10-19C. the electrical force between charges is strongest when the electrical force s strongest when the charges are charges are of higher magnitude and closest to each other, from the coulomb force relation F =kq1q2/r2. where all symbols has their usual meaning, the force is proportional to their masses and but inversely proportional to their separation. meaning the larger their magnitude the greater their product and the closer they are the less the effect of their distance. a main difference between gravitational and electric forces is that electrical forces the electric force is much greater than the electrical force and also the gravitational force is always attractive whereas electrical force can be attractive or repulsive depending on the likeness of the polarity of the charges involved. if the gravitational force is given as fg= Gm1m2/r2 whereas fe =kq1q2/r2 ( where all quantities have their usual meaning) the ratio of the electric force to gravitational force can be calculated as follows assuming the quantity involved are two protons with mass of 1.67×10-27kg and a both having a charge of 1.6× 10-19C. G = 6.673×10-11 N m2 kg-2 and k =9.0 x 109 N. m2 / C2. SUBSTITUTINNG THE ABOVE INTO fe / fg =kq1q22/ Gm1m2 fe / fg ≈1036 times the reason a charged balloon will stick to a wall is that when a balloon is charged by rubbing it on a fur or sweater it, electrons sticks to it, and it becomes negatively charged, when the balloon is placed against a hall, its electron will repel those on the hall and protons will be exposed for attraction, this s why the balloon will sticks to the wall. the charge distribution in some molecules is permanently separated into positive and negative regions such molecules are called answer this question if two negative charges are held close together and then released, the charges will will move away from each other the way a same pole bar magnet will do. it is a simple principle of static electricity, like charges repels while unlike charges attracts. the two negative charges held together are like charges 10000 electrons are removed from a neutral plastic ball. what is the charge of the ball the charge on the ball will be +10000. what is the magnitude of the force on an electron in an electric field of 200 n/c ELECTRIC FIELD INTENSITY IS EQUAL TO force per charge E =f/e 200 =F/1.6 * 10-19 f =200 *1.6 * 10-19 =32 * 10-17N what is the magnitude of the force on an electron in an electric field of 400 n/c ELECTRIC FIELD INTENSITY IS EQUAL TO force per carge E =f/e 400=F/1.6 * 10-19 f =400 *1.6 * 10-19 =64* 10-17N what is the magnitude of the electric force on an electron in a uniform electric field of strength 500n/c ELECTRIC FIELD INTENSITY IS EQUAL TO force per charge E =f/e 500 =F/1.6 * 10-19 f =500 *1.6 * 10-19 =8 * 10-17N what is the magnitude of the electric force on an electron in a uniform electric field of strength 2600 n/c that points due east ELECTRIC FIELD INTENSITY IS EQUAL TO force per charge E =f/e 2600 =F/1.6 * 10-19 f =2600 *1.6 * 10-19 =1.16 * 10-16N what are the magnitude and direction of the electric force on an electron in a uniform electric field of strength 2360 n/c that points due east electric field intensity is equal to force per carge e =f/e 2360 =f/1.6 * 10-19 f =2360 *1.6 * 10-19 =3.116* 10-16 how much voltage is required to make 4 amperes flow through a 12-ohm resistor V=IR V= 4 * 12 =48 ohms what is the current in a 60w bulb connected to a 120v source p =I2R P=IV 60 =120 * I I =0.5 amps "how much current does a 100w lamp draw when connected to 120v" p =I2R P=IV 100 =120 * I I =0.8333 amps "what's the difference between a kilowatt and a kilowatt-hour" kilowatt , meaning 1000 of watts, watt is the unit for measuring electrical power whereas kilowatt-hour, is the quantity of kilowatts of power an appliance can consume in an hour. what is the power rating of a device that draws 0.8 amps when connected to 120 v? p =0.82 * 120 p =I2R power =76.8 watts a resistor carries a current of 2 a when connected to a 10-v battery. what is the resistor's value? V =IR R =V/I R =10/2 R =5 OHMS the number of protons in the nucleus of an electrically neutral atom is equal to the number of ELEECTRONS for an electrically neutral atom , protons and electrons are equal in number and their charges cancel out, but in an ion? the protons may be equal to or greater than the number of electrons. in metallic ions where electrons are given out the atom have greater number of protons whereas in non-metallic ions, were electrons are accepted the number of protons is less than that of electrons. this is w non-mettalic ions are negative and metallic ions are positive. ratio of electric force to gravitational force between two electrons electric force or coulomb force about 1039 time greater than gravitational force how many electrons are in 0.0032 coulombs of charge one electron has 1.6 * 10-19C X electrons = 0.0032 cross multiplying we have x =0.0032/1.6 * 10-19 x =2.0 8 1016 electrons in 0.0032 coulumb of charges