Free Ncert Solutions for 12th Class Physics EM Waves - Studyadda.com

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12th Physics EM Waves

1)

Figure shows a capacitor made of two circularplates each of radius 12 cm and separated by5.0 cm. The capacitor is being charged by anexternal source (not shown in figure). Thecharging current is constant and equal to0.15 A. (a) Calculate the capacitance and the rate ofchange of potential difference between theplates. (b) Obtain the displacement current acrossthe plates. (c) Is Kirchhoff?s first rule valid at eachplate ofthe capacitor? Explain.
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2)

A parallel plate capacitor (shown in figure)made of circular plates each of radius has a capacitance . Thecapacitor is connected to a 230 V a.c. supplywith a (angular) frequency of 300 rad . (a) What is the r.m.s. value of the conductioncurrent? (b) Is the conduction current equal to thedisplacement current? (c) Determine the amplitude of B at a point3.0 cm from the axis between the plates.
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3)

What physical quantity is the same for X-rays of wavelength red light of wavelength and radio waves of wavelength 500 m?
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4)

A plane electromagnetic wave travels invacuum along z-direction. What can you sayabout the directions of its electric andmagnetic field vectors? If the frequency ofthe wave is 30 MHz, what is its wavelength?
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5)

A radio can tune into any station in the 7.5MHz to 12 MHz band. What is thecorresponding wavelength band?
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6)

A charged particle oscillates about its meanequilibrium position with a frequency of. What is the frequency of theelectromagnetic waves produced by theoscillator?
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7)

The amplitude of the magnetic field part of aharmonic electromagnetic wave in vacuumis . What is the electromagneticwaves produced by the oscillator?
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8)

Suppose that the electric field amplitude ofan electromagnetic wave is and that its frequency is . (a)Determine, and . (b) Findexpressions for E and B.
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9)

The terminology of different parts of theelectromagnetic spectrum is given in the text.Use the formula (for energy of aquantum of radiation : photon) and obtain thephoton energy in units of for differentparts of the electromagnetic spectrum. In whatway are the different scales of photon energiesthat you obtain related to the sources ofelectromagnetic radiation?
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10)

In a plane electromagnetic wave, the electricfield oscillates sinusoidally at a frequency ofand amplitude . (a) What is the wavetehgth of the wave? (b) What is the amplitude of the oscillatingmagnetic field? (c) Show that the average energy density ofthe E field equals the average energydensity of the B field. .
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11)

Suppose that the electric field part of anelectromagnetic wave in vacuum is . (a) What is the direction of propagation? (b) What is the wavelength ? (c) What is the frequency ? (d) What is the amplitude of the magneticfield part of the wave? (e) Write an expression for the magnetic fieldpart of the wave.
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12)

About 5% of the power of a 100 W lightbulb is converted to visible radiation. Whatis the average intensity of visible radiation? (a) at a distance of 1 m from the bulb? (b) at a distance of 10 m? Assume that the radiation is emitted is o tropically and neglect reflection.
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13)

Use the formula to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?
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14)

Given below are some famous numbersassociated with electromagnetic radiationsin different contexts in physics State the partof the electromagnetic spectrum to whicheach belongs. (a) 21 cm (wavelength emitted by atomichydrogen in interstellar space). (b) 1057 MHz (frequency of radiation arisingfrom two close energy levels in hydrogen;known as Lamb shift.) (c) 2.7 K (temperature associated with theisotropic radiation filling all space-thought to be a relic of the ?big-bang? origin of the universe.) (d) (double lines of sodium) (e) 14.4 keV[energy of a particulartransition in nucleus associated witha famous high resolution spectroscopicmethod (Mossbauer spectroscopy).]
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15) One requires 11eV of energy to dissociate a carbon monoxide molecule into carbon and oxygen atoms. The minimum frequency of the appropriate electromagnetic radiation to achieve the dissociation lies in (a) visible region (b) infrared region (c) ultraviolet region (d) microwave region
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16) A linearly polarized electromagnetic wave given as is incident normally on a perfectly reflecting infinite wall at = a. Assuming that the material of the wall is optically inactive, the reflected wave will be given as (a) (b) (c) (d)
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17) Light with an energy flux of 20 W/cm2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 30 cm2, the total momentum delivered (for complete absorption) during 30 minutes is (a) 36 x10-5 Kg m/s (b) 36 x 10-4 Kg m/s ( b) 108 x104 Kg m/s (d) 1.08 x 1O7-Kg m/s
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18) The electric field intensity produced by the radiations coming from 100 W bulb at a 3 m distance is E. The electric field intensity produced by the radiations coming from 50 W bulb at the same distance is (a)E/2 (b)2E (c) (d)
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19) If and represent electric and magnetic field vectors of the electromagnetic wave, the direction of propagation of electromagnetic wave is along (a) (b) (c) (d)
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20) The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is (a) c: 1 (b) c2: 1 (c) 1 : 1 (d)
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21) An EM wave radiates outwards from a dipole antenna, with E0 as the amplitude of its electric field vector. The electric field E0 which transports significant energy from the source falls off as (a) (b) (c) (d) remains constant
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22) An electromognetic wave travels in vacuum along z direction: correct options from the following : (a) The associated magnetic field is given as (b) The associated magnetic field is given as (c) The given electromagnetic field is circularly polarised (d) The given electromagnetic wave is plane polarized
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23) An electromagnetic wave travelling along z-axis is given as : Choose the correct options from the following; (a) The associated magnetic field is given as (b) The electromagnetic field can be written in terms of the associated magnetic field as (c) (d)
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24) A plane electromagnetic wave propagating along x direction can have the following pairs of and (a) Ex, B y (b)E y, B z (c) B x, E y (d)E z, B y
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25) A charged particle oscillates about its mean equilibrium position with a frequency of 109 Hz. The electromagnetic waves produced : (a) will have frequency of 109 Hz ( b) will have frequency of 2 x 109 Hz (c) will have a wavelength of 0-3 m (d) fall in the region of radio waves
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26) The sources of electromagnetic waves can be a charge (a) moving with a constant velocity (b) moving in a circular orbit (c) at rest (d) falling in an electric field
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27) An EM wave of intensity falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the followings are true? (a) Radiation pressure is c if the wave is totally absorbed (b) Radiation pressure is /c if the wave is totally reflected (c) Radiation pressure is 2 /c if the wave is totally reflected (d) Radiation pressure is in the range/c < p < 2/c for real surface
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28) Why is the orientation of the portable radio with respect to broadcasting station important ?
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29) Why does microwave oven heats up a food item containing water molecules most efficiently ?
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30) The charge on a parallel plate capacitor varies as .The plates are very large and close together (area = A, separation = d). Neglecting the edge effects, find the displacement current through the capacitor?
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31) A variable frequency a.c. source is connected to a capacitor. How will the displacement current change with decrease in frequency ?
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32) The magnetic field of a beam emerging from a filter facing a floodlight is given by B = 12 x 10-8 sin (1.20 x 107 z ? 3.60 x 1015t) T. What is the average intensity of the beam ?
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33) Poynting vector is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propogation. Mathematically, it is given by Show the nature of S vs t graph.
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34) Professor C.V Raman surprised his students by suspending freely a tiny light ball in a transparent vacuum chamber by shining a laser beam on it. Which property of EM waves was he exhibiting? Give one more example of this property.
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35) Show that the magnetic field B at a point in between the plates of a parallel-plate capacitor during charging is (symbols having usual meanings).
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36) Electromagnetic waves with wavelength (i) is used in satellite communication. (ii) is used to kill germs in water purifier. (iii)is used to detect leakage of oil in underground pipelines. (iv) is used to improve visibility in runways during fog and mist conditions. (a) Identify and name the part of electromagnetic spectrum to which these radiations belong. (b) Arrange these wavelengths in ascending order of their magnitude. (c) Write one more application of each.
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37) Show that average value of radiant flux density 'S' over a single period ?T' is given by .
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38) You are given a 2parallel plate capacitor. How would you establish an instantaneous displacement current of 1 mA in the space between its plates?
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39) Show that the radiation pressure exerted by an EM wave of intensity I on a surface type kept in vaccum is I/C.
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40) What happens to the intensity of light from a bulb if the distance from the bulb is doubled ? As a laser beam travels across the length of a room, its intensity essentially remains constant. What geomatrical characteristic of LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb ?
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41) Even though an electric field exerts a force on a charged particle yet the electric field of an EM wave does not contribute to the radiation pressure (but transfers energy). Explain.
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42) An infinitely long thin wire carrying a uniform linear static charge density is placed along the z-axis (Fig. 5(EP).3). The wire is set into motion along its length with a uniform velocity. Calculate the pointing vector .
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43) Sea water at frequency v = 4 x 108 Hz has permittivity permeability and resistivity Imagine a parallel plate capacitor immersed m sea water and driven by an alternating voltage source V (t) = V0 sin (2 vt). What fraction of the conduction current density is the displacement current density ?
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44) A long straight cable of length is placed symmetrically along z-axis and has radius a («). The cable consists of a thin wire and a co-axial conducting tube. An alternating current (t) = sin flows down the central thin wire and returns along the co-axial conducting tube. The induced electric field at a distance s from the wire inside the cable is (i) Calculate the displacement current density inside the cable. (ii) Integrate the displacement current density across the cross-section of the cable to find the total displacement current . (iii) Compare the conduction current with the displacement current
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45) A plane EM wave travelling in vacuum along z direction is given by and (i) Evaluate dl over the rectangular loop 1234 shown in Fig. 5(EP).4. (ii) Evaluate over the surface bounded by loop 1234. (iii) Use equationto porve (iv) By using similar process and the equation , prove that
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46) A plane EM wave travelling along z direction is described by (i) The average energy density of the wawve is given by (ii) The time averaged intensity of the wave is given by
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