Chapter 8 : Electromagnetic Waves

Class 12 Physics Chapter 8 Electromagnetic Waves Exarcise Questions and Answers : 

Example 8.1 A parallel plate capacitor with circular plates of radius 1 m has a capacitance of 1 nF. At  = 0, it is connected for charging in series with a resistor R = 1 M Ω across a 2V battery (Fig. 8.3). Calculate the magnetic field at a point P, halfway between the centre and the periphery of the plates, after  = s. (The charge on the capacitor at time  is  , where the time constant  is equal to .)

Example 8.2 A plane electromagnetic wave of frequency 25 MHz travels in free space along the -direction. At a particular point in space and time, . What is B at this point?

Example 8.3 The magnetic field in a plane electromagnetic wave is given by  .

(a) What is the wavelength and frequency of the wave?

(b) Write an expression for the electric field.

Example 8.4 Light with an energy flux of 18 W/ falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 , find the average force exerted on the surface during a 30 minute time span.

Example 8.5 Calculate the electric and magnetic fields produced by the radiation coming from a 100 W bulb at a distance of 3 m. Assume that the efficiency of the bulb is 2.5% and it is a point source.

Class 12 Physics Chapter 8 Electromagnetic Waves Exarcise Questions and Answers : 

8.1 Figure 8.6 shows a capacitor made of two circular plates each of radius 12 cm, and separated by 5.0 cm. The capacitor is being charged by an external source (not shown in the figure). The charging current is constant and equal to 0.15A.

(a) Calculate the capacitance and the rate of change of potential difference between the plates.

(b) Obtain the displacement current across the plates.

(c) Is Kirchhoff’s first rule (junction rule) valid at each plate of the capacitor? Explain.

8.2 A parallel plate capacitor (Fig. 8.7) made of circular plates each of radius R = 6.0 cm has a capacitance C = 100 pF. The capacitor is connected to a 230 V ac supply with a (angular) frequency of 300 rad s-1 .

(a) What is the rms value of the conduction current?

(b) Is the conduction current equal to the displacement current?

(c) Determine the amplitude of B at a point 3.0 cm from the axis between the plates.

8.3 What physical quantity is the same for X-rays of wavelength  , red light of wavelength 6800 Å and radiowaves of wavelength 500 m?

8.4 A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?

8.5 A radio can tune in to any station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band?

8.6 A charged particle oscillates about its mean equilibrium position with a frequency of  Hz. What is the frequency of the electromagnetic waves produced by the oscillator?

8.7 The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is . What is the amplitude of the electric field part of the wave?

8.8 Suppose that the electric field amplitude of an electromagnetic wave is  and that its frequency is  MHz. (a) Determine  ,, , and  . (b) Find expressions for  and B  .

8.9 The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula  (for energy of a quantum of radiation: photon) and obtain the photon energy in units of eV for different parts of the electromagnetic spectrum. In what way are the different scales of photon energies that you obtain related to the sources of electromagnetic radiation?

8.10 In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of   Hz and amplitude 48 V  .

(a) What is the wavelength of the wave?

(b) What is the amplitude of the oscillating magnetic field?

(c) Show that the average energy density of the E field equals the average energy density of the B field. [  m  ]

Class 12 Physics Chapter 8 Electromagnetic Waves Additional Exarcise Questions and Answers : 

8.11 Suppose that the electric field part of an electromagnetic 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 magnetic field part of the wave?

(e) Write an expression for the magnetic field part of the wave.

8.12 About 5% of the power of a 100 W light bulb is converted to visible radiation. What is the average intensity of visible radiation (a) at a distance of 1m from the bulb? (b) at a distance of 10 m? Assume that the radiation is emitted isotropically and neglect reflection.

8.13 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?

8.14 Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the part of the electromagnetic spectrum to which each belongs.

(a) 21 cm (wavelength emitted by atomic hydrogen in interstellar space).

(b) 1057 MHz (frequency of radiation arising from two close energy levels in hydrogen; known as Lamb shift).

(c) 2.7 K [temperature associated with the isotropic radiation filling all space-thought to be a relic of the ‘big-bang’ origin of the universe].

(d) 5890 Å - 5896 Å [double lines of sodium]

(e) 14.4 keV [energy of a particular transition in  nucleus associated with a famous high resolution spectroscopic method (Mössbauer spectroscopy)].

8.15 Answer the following questions:

(a) Long distance radio broadcasts use short-wave bands. Why?

(b) It is necessary to use satellites for long distance TV transmission. Why?

(c) Optical and radiotelescopes are built on the ground but X-ray astronomy is possible only from satellites orbiting the earth. Why?

(d) The small ozone layer on top of the stratosphere is crucial for human survival. Why?

(e) If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now?

(f ) Some scientists have predicted that a global nuclear war on the earth would be followed by a severe ‘nuclear winter’ with a devastating effect on life on earth. What might be the basis of this prediction?