Unit 15 Electromagnetic Waves

Transcription

1 Unit 5 Electromagnetic Waves 97

2 SUMMRY 9

3 99

4 MCQ For the answer of the following questions choose the correct alternative from among the given ones. () Who produced the electromagnetic waves first? Marconi () Maxwell (C) J.C. ose (D) Hertz () The dimensional formula of o E o is L T () L T (C) LT (D) (3) plane electromagnetic wave is incident on a material surface. The wave delivers momentum P and energy E P=, E () P, E= (C) P, E (D) P=, E= (4) If V,V x and Vm are the velocity of the v rays, x rays, micro waves respectively in space, then Vr Vx Vm () Vr Vx Vm (C) Vr Vx Vm (D) Vr Vx Vm r x m (5) If, and are the wave lengths of the r-rays, x rays and micro waves respectively in space then r x m () r x m (C) r x m (D) r m x (6) ccording to Maxwell, a changing electric field produces emf () Electric current (C) magnetic field (D) radiation pressure (7) n electromagnetic wave going through vaccum is described by E= E o sin(kx- ω t). Which of the following is independent of the wavelength? ω () k/ ω (C) k ω (D) k () Which of the following have zero average value in a plane electromagnetic wave? Electric energy () Magnetic energy (C) Electric field (D) None of these. (9) If the relative permeability and dielectric constant of a given medium are equal to nd K respectively, then the refractive index r to L T of the medium is equal r K () r Eo (C) o Eo (D) / r k () straonomers have found that electromagnetic waves of wavelength cm are continuously reaching the Earth s surface. Calculate the frequency of this radia- tion. (c= 3 m/s) 4. GHz ().4 khz (C).4 MHz (D).4 GHz

5 () Electric field in an electromagnetic wave is given by E=5 sin ω (t-x/c) N C. Intensity of this wave is - Wm 5 (). (C) 3.3 (D) () The amplitude of the electric field in a parallel beam of light of intensity. Wm is 3.NC () 9.4NC (C) 9.7NC (3) Speed of electromagnetic wave is the same for all wavelengths (C) for all intensities () in all media (D) for all frequencies - (D) None of these. (4) The maximum electric field in a plane electromagnetic wave is 9NC. The wave is going in the x direction and the electric field is in the y direction. The maximum magnetic field in the wave is T 3 () 6 3 (C) 6 7 (D) 7 (5) Electromagnetic waves are produced by a static charge (C) an accelerating charge () a moving charge (D) chargeless particles (6) Maxwells equations are derived from the laws of electricity (C) both electricity and magnetism () magnetism (D) mechanics (7) Which of the following electromagnetic waves has the longest wavelength? Radio waves (C) x rays () Infrared radiations (D) visible rays () Which of the following electromagnetic waves has the highest frequency? radio waves () micro waves (C) r rays (D) x rays (9) Which of the following electromagnetic waves is used in telecommunication? radio waves (C) ultraviolet rays () visible radiations (D) micro waves () The maximum value of E 4 in an electromagnetic waves in air is equal to 6. Vm. The maximum value of is 5 4. T (). T (C). 3 T (D). T () Dimensional formula of intensity of radiation is M L T () M L T (C) 3 M L T (D) 3 M L T

6 () The frequency of an electromagnetic wave in free space is 3MHz. When it passes through a medium of relative permeability ε =4., then its frequency r becomes half () become doubled (C) remain same (D) become times (3) The frequency of electromagnetic wave having wavelength 5mm is Hz. () (C). (D) (4) Unit of energy density of electromagnetic wave is 3 Jm () Jm (C) wm (D) None of these (5) What is the ratio of velocities of light rays of wavelengths 4 and in vaccum? : () : (C) : (D) cannot be determined (6) Which of the following rays are not electromagnetic waves? rays () rays (C) rays (D) heat rays (7) new system of unit is evolved in which the values of and ε are and respectively. Then the speed of light in this system will be.5 ().5 (C).75 (D) () Our eyes respond to wavelength ranging from 4nm to 7nm (C) mm to 7nm () to (D)7nm to nm (9) In microwave oven, we use electromagnetic oscillators which produce electromagnetic waves in the wavelength range mm to m ().7 m to mm (C).m to mm (D). m to.7 m (3) What is the direction of E in an electromagnetic wave? same as that of E () same as that of (C) same as the direction of propagation of electromagnetic wave (D) none of these (3) The wavelength of x rays is of the order of cm () m (C) micron (D)angstrom

7 (3) plane electromagnetic wave of frequency 5MHz travels in free space along the x direction. t a particular point in space and time E = 6.3 ĵ Vm then at this point is. iˆ T (). kˆ T (C).9 9 kˆ T (D).5 7 kˆ T (33) The magnetic field in a plane electromagnetic wave is given y = 7 3 x. The expression for electric field is sin ( t)t 3 Ex 6 sin(.5 x.5 t) Vm 3 () Ez 6 sin(.5 x.5 t) Vm 3 (C) Ez 6 sin(.5 x.5 t) Vm 5 3 (D) Ez 6 sin(.5 x.5 t) Vm 3-3 (34) Light with an energy flux of w m or Wm falls on a non-reflecting surface at normal to surface. If the surface has an area of m. The average force exerted on the surface during 3 minutes is N () 3.6 N (C) 6. N (D) 3.6 N (35) Energy density of an electromagnetic wave of intensity -. Wm is Jm () Jm (C) 3.5 Jm (36) The waves used in communication are generally called (D) none of the above rays () rays (C) microwaves (D) radiowaves (37) For an electromagnetic wave, the phase difference between vectors E and (far away from the source) () (C) (D) 3 (3) In an electromagnetic wave, if the amplitude of magnetic field is 3 T, the amplitude of the associated electric field will be 9 Vm () 3 Vm (C) 3 Vm (39) The electric and magnetic field of an electromagnetic wave are in phase and perpendicular to each other () in phase and parallel to each other (C) in opposite phase and perpendicular to each other (D) in opposite phase and parallel to each other (D) Vm 3

8 (4) The direction of electric and magnetic vector for an electromagnetic wave travelling along X-axis are shown in fig. and then is correct, is wrong () is correct, is wrong E (C) oth and are correct (D) oth and are wrong (4) The dimensions of are E C C M LT () M L T (4) The dimension of k r is (C) M L T (D) M LT M L T () M LT (C) M L T (D) M L T (43) The frequency of light wave of wavelength 5 is Hz 4 6 ().5 (C).5 (D) 6 (44) Unit of C is same as that of current () resistance (C) electric charge (D) velocity (45) The amplitude of the magnetic field part of an electromagnetic wave in vaccum is m = 5 nt. Then the amplitude of the electric part of the wave is.53 V / m ().53 V/m (C).53 V / m (D).53 V / m (46) If the direction of magnetic field at some instant is along +ve Z direction and the electromagnetic wave is propagating along +ve X direction, then the direction of electric field E at that instant is along -ve Y direction (C) along +ve X direction () along +ve Y direction () along -ve X direction (47) Relation between amplitudes of electric and Magnetic field is... E () E c (C) E (D) E c c 6 (4) If a source is transmitting electromagnetic wave of frequency. Hz, then the wavelength of the electromagnetic waves transmitted from the source will be 36.6m () 4.3m (C)4.5m (D)5.9m 4

9 (49) The velocity of light in vaccum can be changed by changing frequency ()wavelength (C)amplitude (D) none of these (5) n electromagnetic wave going through vaccum is described by E=Eo sin(kx- ω t) then =o sin(kx-ωt) then E = ωk () E k = ω (C) E ω = k (D)none of these (5) If the wavelength of light is 4 then the number of waves in mm length will be.5 ()5 (C)5 (D)5 (5) The SI unit of displacement current is coulomb ()henry (C) ampere (D)faraday (53) The electromagnetic waves do not transport energy () charge (C)momentum (D)information (54) n electric charge oscillating with a frequency of kilo cycles/s can radiates electromagnetic waves of wavelength km ()km (C) 3km (D)4km (55) The frequency 57MHz of radiation arising from two close energy levels in hydrogen belongs to radio waves ()infrared waves (C)micro waves (D) rays (56) Electromagnetic waves travelling in a medium which has relative permeability.3 and relative permeability.4 speed of electromagnetic waves in this medium will be m / s (). m / s (C). m / s (D) 3.6 m / s (57) plane electromagnetic wave is incident on a material surface. If the wave delivers momentum p and energy E, then p=,e= () p, E (C) p, E = (D) p =, E (5) Maxwell s modified form of mpere s circuital law is. ds (). ds oi de (C). d l dq oi + o (D). d o + dt l i dt (59) The wavelength of x rays is of the order of 3 m () 5 m (C) m (D) m (6) point source of electromagnetic radiation has an average output power of W. The maximum value of electric field at a distance of 4.m from the source is 64.7Vm () 57.Vm (C) 56.7Vm (D) 54.77Vm 5 o

10 (6) plane electromagnetic wave E cos(6 t 4 x) Vm propagate in a medium of refractive index z.5 (). (C).4 (D)4. - (6) plane electromagnetic wave of wave intensity ωm strikes a small mirror of area cm, held perpendicular to the approaching wave. The radiation force on the mirror will be 6. 6 N ().33 N (C).33 N (D) 6.6 N (63) n observer is at m from an isotropic point source of light emitting 4w power. The rm.s value of electric due to the source at the position of the observer is 5.77 Vm () 7.3Vm (C) 57.7 Vm (D).73Vm (64) Electromagnetic waves used in medicine to destroy cancer cells radio waves (C)gamma rays ()infrared rays (D)ultraviolet rays d (65) What is the name associated with the equation E. dt dt Gauss law for electricity (C) ampere s law (66) What oscillates in an electromagnetic wave? () Gauss law for magnetism (D)faraday s law E and () (C) E (D)none of these (67) Which of the following rays are not electromagnetic waves? rays () rays (C) rays (D)heat rays (6) The rms value of the electric field of the light coming from the sun is 7 N/c. The average tatal energy density of the electromagnetic wave is Jm () Jm (C) 3.35 Jm (D) Jm (69) What is the wave length of range of electromagnetic waves? - m to 5 m (C) -5 m to 5 m (7) What is the wavelength range of visible light? to () () -5 m to m (D) m to 5 m 4 to 7 (C) to (D) to 5 6

11 (7) Unit of oc is same as that of current ()resistance (C)electric charge (D) velocity (7) In electromagnetic spectrum, the visible light lie between radiowaves and microwaves () ultraviolet rays and infrared rays (C) ultraviolet rays and x rays (D) infrared rays and microwaves (73) Which of the following statements is not true in case of electromagnetic waves? they are light waves (C) propagates through space ()theay are transverse waves (D) they are longitudinal waves (74) The oscillating electric and magnetic field vectors of an electromagnetic waves far away from source are oriented along Mutually perpendicular direction and differ in phase by 9 () Mutually perpendicalar and in same phase (C) In same direcfi on and in same phase (D) In same divecfi on and differ in phaseby 9 o (75) Which of the following option of electromagnetic waves is in order of increasing frequency? microwaves, ultraviolet rays, x rays () gamma rays, ultraviolet rays, radiowaves (C) radiowaves, visible light, infrared rays (D) gamma rays, visible light, ultraviolet rays (77) The sum delivers 3 Wm - of electromagnetic flux to earth s surface. The total power that is incident on a roof of dimension m x m will be 5 4 w () 4.56 w (C) w (D) 5.6 w (77) olometer is used to detect. infrared rays () ultraviolet rays (C) x rays (D) rays (7) Range of frequency of microwaves is about. 53kHz to 7kHz ()54MHz to 9MHz (C) 3GHz to 3GHz (D) 4 4 Hz to 4 7 Hz (79) SI unit of displacement current is coulomb ()ampere (C)faraday (D) 7

12 () The frequencies of x rays, rays and ultraviolet rays are respectively p, q and r then p<q, q>r () p>q, q>r (C)p>q, q<r (D)p<q, q<r () t room temperature, if the relative permitivity of water is and the realtive permiability be. then the velocity of light in water is m/s.5 ().5 (C) 3.5 (D) 3 () If the electric field associated with a radiation of frequency MHz is E=sin(kxωt)mV/m then its energy density is 3 Jm ( ε o=.5 C N m ) () (C) 6.5 (D) 4.5 () In an electromagnetic wave in free space, the direction of electric field vector E is along y axis and magnetic field vector is along z axis then which of the following is true E E () E (C) E (D) none of these (4) When a plane electromagnetic wave travels in vaccum, the average electric energy density is given by (Eo is the amplitude of the electric field) 4 oeo () oeo (C) o Eo (D) 4 o Eo (5) In a plane electromagnetic wave, the electric field oscillates sinusoidaly at a frequency of. Hz. if the peak value of electric field is 6 Vm the average energy density (in Jm -3 ) of the magnetic field of the wave will be (given o = 7 4 Tm/) 7 7 () (C) 7 4 (D) 4 and (6) Which of the following pairs of the component of space and time varying E = Ε xˆ i + Eyj ˆ + Ezkˆ = xˆ i + yj ˆ + zk ˆ would generate a plane electromagnetic wave travelling in +ve y direction x i, E zi () Ey i, z i (C) E x i, x i (D) E z i, x i (7) Infrared radiations are detected by spectrometer ()bolometer (C)photocells (D)geiger tubes 7

13 () Electromagnetic wave is produced by oscillating electric and magnetic fields E and. Choose only the incorrect statement from the following E is perpendicular to. () E is perpendicular to the direction of propagation of the wave (C) is perpendicular to the direction of propagation of the wave (D) E is parallel to (9) The potential difference between the plates of a parallel plate capacitor is charg- 6 ing at the rate of Vs. If the capatance is F. The displacement current in the dielectric of the capacitor will be 4 ()3 (C) (D) (9) Which of the following electromagnetic wave has the least frequency? radiowave () infrared radiation (C)microwave (D)x rays (9) Which of the following electromagnetic wave has the least wavelength? rradiowave ()visible wave (C) ultraviolet rays (D) microwaves (9) Which of the following waves are not transverse in nature? light emitted from a sodium lamp () sound waves travelling in air (C) xrays from an x ray machine (D) microwaves used in radar (93) n electromagnetic wave can be deflected by electric field () can be deflected by magnetic field (C) can be deflected bye both electric and magnetic field (D) none of these (94) When an electromagnetic wave encounters a dielectric medium, the transmitted wave has same frquency but different amplitude () same amplitude but different frequency (C) same frequency and amplitude (D) different frequency and amplitude 9

14 (95) ccording to Maxwell, a changing electric field produces emf ()radiation pressure (C) electric current (D)magnetic field (96) was the first to predict the existence of electromagnetic waves Maxwel () Faraday (C)mpere (D) hertz (97) If the earth were not having atmosphere, its temperature. would have been low () would have been high (C)would hav remain constant (D) none of these (9) is responsible for the green house effect infrared rays ()ultraviolet rays (C)x raya (D)radiowaves (99) The dimensional formula of energy density is M L T () M L T M L T M L T (C) 3 (D) 3 KEY NOTES D C 35 5 C 69 D 9 D 36 D C C C 7 4 D C C C 5 C 75 C 5 4 D 59 C C 43 6 D 77 D C 45 C 6 C 79 9 C C 3 C C 4 3 D 4 65 D 5 C 3 49 D C C D D C C D C C D D