CHAPTER -8 ELECTROMAGNETIC WAVES

You are given a 2μF parallel plate capacitor. How would you
establish an instantaneous displacement current of 1mA in the
space between its plates?

Even though an electric field E exerts a force qE on a charged
particle yet the electric field of an EM wave does not contribute to
the radiation pressure (but transfers energy). Explain.

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?

Show that the radiation pressure exerted by an EM wave of
intensity I on a surface kept in vacuum is I/c.

You are given a 2μF parallel plate capacitor. How would you
establish an instantaneous displacement current of 1mA in the
space between its plates?

Electromagnetic waves with wavelength
(i) λ1 is used in satellite communication.
(ii) λ2 is used to kill germs in water purifies.
(iii) λ3 is used to detect leakage of oil in underground pipelines.
(iv) λ4 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.

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.

A variable frequency a.c source is connected to a capacitor. How
will the displacement current change with decrease in frequency?

The charge on a parallel plate capacitor varies as q = q0 cos 2πνt.
The plates are very large and close together (area = A,separation = d). Neglecting the edge effects, find the displacement
current through the capacitor?

Why does microwave oven heats up a food item containing water
molecules most efficiently?

Why is the orientation of the portable radio with respect to
broadcasting station important?

Many people like to watch CID programme on a TV channel. In this programme, a murder mystery is shown. This murder mystery is solved by CID team. Each member of the team works with full dedication. They collect information and evidences from all possible sources and then tend to lead the correct conclusion. Sometimes they also use ultraviolet rays in the forensic laboratory. Some people get surprised to know the advantages of ultraviolet rays because they only aware of the fact that ultraviolet rays coming from the sun produce harmful effects.

a. What values were displayed by the members of CID team?

b. What is the use of ultraviolet rays in forensic laboratory? 

Two friends were passing through the market. They saw two welders using the welding machine. One welder was using the goggles face mask with window in order to protect his face. The other one was welding with naked eyes. They went to the welder who was not using face mask and explained him the advantages of using goggles and face masks. Next day, the welder bought asset of goggles and began to do his work fearlessly.

a. What values were displaced by two friends?

b. Why do welders wear glass goggles or face masks with glass windows while carrying out welding? 

Gopal visits his friend Naresh. In his house Naresh was playing with his kid sister and in spite of the broad day light, Gopal notices the tube light burning and advises him to save electricity; He also claims that the heat inside the room increases which would lead to global warming.

a) What are the values associated with the decision saving electricity?

b) Which electromagnetic wave is responsible for increase in the average temperature of the earth? Give other applications of the electromagnetic wave. 

Two persons were playing and one of them got injured as he fell from the top of a tree. He was taken to the Hospital where, he was admitted for a fracture after X-ray. The boy’s mother, who on hearing the above incident rushed to the hospital, was worried and restless; the other boy consoled her and the doctor who, overheard them, informed her about the latest developments in the Medicine field and assured a speedy recovery of her son.

a) How would you rate the qualities of the boy who consoled his friend’s mother?

b) How X-ray is produced?

c) Mention the fields where X-ray is used. 

Two persons were playing and one of them got injured as he fell from the top of a tree. He was taken to the Hospital where, he was admitted for a fracture after X-ray. The boy’s mother, who on hearing the above incident rushed to the hospital, was worried and restless; the other boy consoled her and the doctor who, overheard them, informed her about the latest developments in the Medicine field and assured a speedy recovery of her son.

a) How would you rate the qualities of the boy who consoled his friend’s mother?

b) How X-ray is produced?

c) Mention the fields where X-ray is used. 

A woman and her daughter of class XII in KV were in the kitchen, preparing a feast for visitors using the new microwave oven purchased last evening. Suddenly, the daughter noticed sparks inside the oven and unplugs the connection after switching it off. She found that inside the microwave oven a metallic container had been kept to cook vegetable. She informs her mother that no metallic object must be used while cooking in microwave oven and explains the reasons for the same.

a) What attitude of the daughter inspires you?

b) Give another use of a microwave oven

c) The frequency of microwave is 3 x 10¹¹ Hz. Calculate its wavelength. 

The electric field of plane an wave is given Ex=0 , Ey=0, Ez=0.5cos{2π x 10⁸(t-4c)}

(i) What is dirⁿ of propagation of wave?

(ii) Compute the components of magnetic fields.     

Write the order of frequency range and one use each of the following e.m. radiation (i) Micro waves (ii) U.V rays (iii) Gamma rays ?     

Write the order of frequency range and one use each of the following e.m. radiation (i) Micro waves (ii) U.V rays (iii) Gamma rays ?     

Name the following constituent radiation of electromagnetic radiation which is suitable for

(a) Radar system used in aircraft navigation
(b)  Treatment of cancer tumors

Write two application of each of them.     

Name the following constituent radiation of electromagnetic radiation which is suitable for

(a) Radar system used in aircraft navigation
(b)  Treatment of cancer tumors

Write two application of each of them.     

Identify the following electromagnetic radiations as given below Write one application of each ?    

(a) 1nm     (b)10^-12m       (c)10^-8m      

Give two uses each of (i) radio waves (ii) Microwaves.      

Identify the following e.m. radiation as per wave length given below. Write one application of each.     
(i)10^-3 nm  (ii)10^-3 m (iii)1nm

The magnitude of magnetic field in a plane e.m. wave is given as B_X = 0, B_Y = 2 x 10^-7 sin(0.5 x 10³ x + 1.5 x 10¹¹t)T.    

(a) Determine the wavelength and frequency of wave.
(b) Write an expression for the electric field.

The magnitude of magnetic field in a plane e.m. wave is given as B_X = 0, B_Y = 2 x 10^-7 sin(0.5 x 10³ x + 1.5 x 10¹¹t)T.    

(a) Determine the wavelength and frequency of wave.
(b) Write an expression for the electric field.

Arrange the following electromagnetic radiation in ascending order of their frequencies.

(a)Microwaves   (b)Radiowaves     (c)X-rays    (d) Gamma rays

  Write two uses any one of this.     (2 marks)

Draw a sketch of a plane e.m. wave propagating along x axis. Depict clearly the directions of electric and magnetic field varying sinusoidally.     

Electromagnetic radiations with wavelength

(i) λ₁ are used to kill germs in water purifiers
(ii) λ₂ are used in T.V communication system
(iii) λ₂  plays an important role in maintaining the earth’s warmth.

Name the part of e.m. spectrum to which these radiation belong. Arrange these radiation in decreasing order of wavelength

From the following, identify the e.m. waves having (i) maximum (ii) Minimum frequency.      (2 marks)

(a) Radiowaves  (b)Gammarays  (c)Visiblelight (d)Microwaves

 (e)U.V.rays (f)I.R.rays

Why are infrared radiations referred to as heat waves also? Name the radiations which are next to these radiations in spectrum having (i) shorter wavelength and (ii) longer wavelength.      

Find the wave length of e.m. waves of frequency 5 x 1019 Hz. Give its two applications.     

The oscillating electric field of an electromagnetic wave is given by E_Y=30 sin(2 x 10¹¹t  + 300πx) Vm^-1. Find the dirⁿ of propagation of wave and write down the expression for magnetic field ?      

The oscillating magnetic field in a plane electromagnetic wave is given by B_Y=8 x 10^-6 sin(20 x 10¹¹t + 300πx)T. Calculate the wavelength of e.m. wave. Write down the expression for oscillating electric field.     

Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along z-direction.     

Write the expression for the velocity of e.m. waves in terms of permittivity and permeability.     

Identify the electromagnetic radiations as given : frequency = 10²⁰ HZ.     

Name the electromagnetic radiation used for viewing objects through haze and fog.     

Name the EM waves used for treatment of cancer tumors.     

Name the EM waves used for studying crystal structure of solids.      

Give a reason to show that  microwaves are better carrier of signal for long range transmission.      

Which part of the electromagnetic spectrum has the largest penetrating power?     

Name the electromagnetic radiation to which waves of wavelength in the range 10^-2 m belong. Give one use of this part of Em spectrum.     

Write the following radiations in ascending order in respect of their frequencies X-ray, microwaves, radio waves.     

What is the ratio of speed of infrared rays and ultra violet rays in vacuum?     

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 radio telescopes 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?

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 Ã… - 5896 Ã… [double lines of sodium]

(e) 14.4 keV [energy of a particular transition in ⁵⁷Fe nucleus associated with a famous high resolution spectroscopic method

(Mössbauer spectroscopy)].

Use the formula λm T= 0.29 cm K to obtain the characteristic temperature ranges for different parts of the electromagnetic spectrum. What do the numbers that you obtain tell you?

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 1 m from the bulb?

(b) at a distance of 10 m?

Assume that the radiation is emitted isotropically and neglect reflection.

Suppose that the electric field part of an electromagnetic wave in vacuum is E = {(3.1 N/C) cos [(1.8 rad/m)y + (5.4 × 10⁶ rad/s)t]} .

(a) What is the direction of propagation?

(b) What is the wavelength λ?

(c) What is the frequency ÃŽÂ½?

(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.

In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of 2.0 x 10¹⁰ Hz and amplitude 48 V m^-1.

(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. [c = 3 x 10⁸ m s^-1.]

The terminology of different parts of the electromagnetic spectrum is given in the text. Use the formula E =hν (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?

Suppose that the electric field amplitude of an electromagnetic wave is E0 = 120 N/C and that its frequency is ÃŽÂ½ = 50.0 MHz. (a) Determine, B0, Ã‰, k, and λ. (b) Find expressions for E and B.

The amplitude of the magnetic field part of a harmonic electromagnetic wave in vacuum is B0 = 510 nT. What is the amplitude of the electric field part of the wave?

A charged particle oscillates about its mean equilibrium position with a frequency of 10⁹ Hz. What is the frequency of the electromagnetic waves produced by the oscillator?

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

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?

What physical quantity is the same for X-rays of wavelength 10^-10 m, red light of wavelength 6800 Ã… and radiowaves of wavelength 500 m?

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.

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 0.15 A.

(a) Calculate the capacitance and the rate of charge 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.