not to be republished NCERT ELECTROMAGNETIC WAVES Chapter Eight MCQ I

Transcription

1 Chapte Eight ELECTROMAGNETIC WAVES MCQ I 8 One equies ev of enegy to dissoiate a abon monoxide moleule into abon and oxygen atoms The minimum fequeny of the appopiate eletomagneti adiation to ahieve the dissoiation lies in (a) visible egion (b) infaed egion () ultaviolet egion (d) miowave egion 8 A linealy polaized eletomagneti wave given as E = Eoˆos i ( kz ωt) is inident nomally on a pefetly efleting infinite wall at z = a Assuming that the mateial of the wall is optially inative, the efleted wave will be given as (a) E = E ˆi os ( kz ωt) o (b) E = Eoˆos i ( kz + ωt)

2 Exempla Poblems Physis () E = E ˆios( kz + ωt) o = E i kz ω (d) E oˆsin ( t) 83 Light with an enegy flux of W/m falls on a non-efleting sufae at nomal inidene If the sufae has an aea of 3 m the total momentum deliveed (fo omplete absoption) duing 3 minutes is (a) 36 5 kg m/s (b) 36 4 kg m/s () 8 4 kg m/s (d) 8 7 kg m/s 84 The eleti field intensity podued by the adiations oming fom W bulb at a 3 m distane is E The eleti field intensity podued by the adiations oming fom 5 W bulb at the same distane is (a) E (b) E () E (d) E 85 If E and B epesent eleti and magneti field vetos of the eletomagneti wave, the dietion of popagation of eletomagneti wave is along (a) E (b) B () B E (d) E B 86 The atio of ontibutions made by the eleti field and magneti field omponents to the intensity of an EM wave is (a) : (b) : () : (d) : 87 An EM wave adiates outwads fom a dipole antenna, with E as the amplitude of its eleti field veto The eleti field E whih 48

3 Eletomagneti Waves tanspots signifiant enegy fom the soue falls off as (a) 3 (b) () (d) emains onstant MCQ II 88 An eletomogneti wave tavels in vauum along z dietion: E = ( E ˆ ˆ) os ( ) i+ Ej kz ωt Choose the oet options fom the following: (a) The assoiated magneti field is given as B = ( E ˆ ˆ i Ej) os (kz-ωt) (b) The assoiated magneti field is given as B = ( E ˆ E ˆ i j) os (kz-ωt) () The given eletomagneti field is iulaly polaised (d) The given eletomagneti wave is plane polaised 89 An eletomagneti wave tavelling along z-axis is given as: E= E os (kz - ωt) Choose the oet options fom the following; (a) The assoiated magneti field is given as B = ˆ = ( ˆ ) k E ω k E (b) The eletomagneti field an be witten in tems of the assoiated magneti field as E = ( B k ˆ ) () ke ˆ =, kˆ B= (d) kˆ E=, kˆ B = 8 A plane eletomagneti wave popagating along x dietion an have the following pais of E and B (a) E x, B y (b) E y, B z () B x, E y (d) E z, B y 49

4 Exempla Poblems Physis 8 A haged patile osillates about its mean equilibium position with a fequeny of 9 Hz The eletomagneti waves podued: (a) will have fequeny of 9 Hz (b) will have fequeny of 9 Hz () will have a wavelength of 3 m (d) fall in the egion of adiowaves 8 The soue of eletomagneti waves an be a hage (a) moving with a onstant veloity (b) moving in a iula obit () at est (d) falling in an eleti field 83 An EM wave of intensity I falls on a sufae kept in vauum and exets adiation pessue p on it Whih of the following ae tue? VSA (a) Radiation pessue is I/ if the wave is totally absobed (b) Radiation pessue is I/ if the wave is totally efleted () Radiation pessue is I/ if the wave is totally efleted (b) Radiation pessue is in the ange I/ < p < I/ fo eal sufaes 84 Why is the oientation of the potable adio with espet to boadasting station impotant? 85 Why does miowave oven heats up a food item ontaining wate moleules most effiiently? 86 The hage on a paallel plate apaito vaies as q = q os πνt The plates ae vey lage and lose togethe (aea = A, sepaation = d) Negleting the edge effets, find the displaement uent though the apaito? 87 A vaiable fequeny a soue is onneted to a apaito How will the displaement uent hange with deease in fequeny? 88 The magneti field of a beam emeging fom a filte faing a floodlight is given by B = 8 sin ( 7 z 36 5 t) T What is the aveage intensity of the beam? 89 Poynting vetos S is defined as a veto whose magnitude is equal to the wave intensity and whose dietion is along the dietion of wave popogation Mathematially, it is given by S = E B μ Show the natue of S vs t gaph 5

5 Eletomagneti Waves 8 Pofesso CV Raman supised his students by suspending feely a tiny light ball in a tanspaent vauum hambe by shining a lase beam on it Whih popety of EM waves was he exhibiting? Give one moe example of this popety SA 8 Show that the magneti field B at a point in between the plates of a paallel-plate apaito duing haging is ε μ de (symbols dt having usual meaning) 8 Eletomagneti waves with wavelength (i) λ is used in satellite ommuniation (ii) λ is used to kill gems in wate puifies (iii) λ 3 is used to detet leakage of oil in undegound pipelines (iv) λ 4 is used to impove visibility in unways duing fog and mist onditions (a) Identify and name the pat of eletomagneti spetum to whih these adiations belong (b) Aange these wavelengths in asending ode of thei magnitude () Wite one moe appliation of eah 83 Show that aveage value of adiant flux density S ove a single peiod T is given by S = E μ 84 You ae given a μf paallel plate apaito How would you establish an instantaneous displaement uent of ma in the spae between its plates? 85 Show that the adiation pessue exeted by an EM wave of intensity I on a sufae kept in vauum is I/ 86 What happens to the intensity of light fom a bulb if the distane fom the bulb is doubled? As a lase beam tavels aoss the length of a oom, its intensity essentially emains onstant 5

6 Exempla Poblems Physis What geomatial haateisti of LASER beam is esponsible fo the onstant intensity whih is missing in the ase of light fom the bulb? x V Z Fig 8 a 87 Even though an eleti field E exets a foe qe on a haged patile yet the eleti field of an EM wave does not ontibute to the adiation pessue (but tansfes enegy) Explain LA 88 An infinitely long thin wie aying a unifom linea stati hage density λ is plaed along the z-axis (Fig 8) The wie is set into y motion along its length with a unifom veloity v =v k Calulate the poynting veto S = ( E B ) μ o 89 Sea wate at fequeny ν = 4 8 Hz has pemittivity ε 8 ε o, pemeability μ μ o and esistivity ρ = 5 Ω m Imagine a paallel plate apaito immesed in sea wate and diven by an altenating voltage soue V(t) = V o sin (π νt) What fation of the ondution uent density is the displaement uent density? 83 A long staight able of length l is plaed symmetially along z-axis and has adius a(<<l) The able onsists of a thin wie and a o-axial onduting tube An altenating uent I(t) = I o sin (πνt) flows down the ental thin wie and etuns along the o-axial onduting tube The indued eleti field at a distane s fom s the wie inside the able is E( st, ) =μ ˆ oioνos( πνt) In k a (i) Calulate the displaement uent density inside the able (ii) Integate the displaement uent density aoss the osssetion of the able to find the total displaement uent I d (iii) Compae the ondution uent I with the dispalement uent I d o 83 A plane EM wave tavelling in vauum along z dietion is given by E = E sin( kz ωt)ˆi and B = B sin( kz ωt) ˆj (i) Evaluate Edl ove the etangula loop 34 shown in Fig 8 (ii) Evaluate Bds ove the sufae bounded by loop 34 ˆ z 5

7 Eletomagneti Waves d B (iii) Use equation Edl φ = to pove dt E B = (iv) By using simila poess and the equation dφ dt E Bdl = μi + ε, pove that = μ ε 83 A plane EM wave tavelling along z dietion is desibed by E = E sin( kz ω t)ˆi and B = B sin( kz ω t)ˆj Show that (i) (ii) The aveage enegy density of the wave is given by u B μ av = εe The time aveaged intensity of the wave is given by I = ε E av y x 4 3 h Fig 8 z 53