Solution of Chapter 8. Electromagnetic Waves (Physics - Exemplar Book)

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 linearly polarized electromagnetic wave given as is incident normally on a perfectly reflecting infinite wall at z = a. Assuming that the material of the wall is optically inactive, the reflected wave will be given as

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

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

If E and B represent electric and magnetic field vectors of the electromagnetic wave, the direction of propagation of electromagnetic wave is along

The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is

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

An electromagnetic wave travels in vacuum along z direction : . Choose the correct options from the following:

An electromagnetic wave travelling along z-axis is given as : E = E_o cos (kz - ωt.). Choose the correct options from the following;

A plane electromagnetic wave propagating along x direction can have the following pairs of E and B

A charged particle oscillates about its mean equilibrium position with a frequency of 10⁹ Hz. The electromagnetic waves produced:

The source of electromagnetic waves can be a charge

An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?

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

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

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?

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

The magnetic field of a beam emerging from a filter facing a floodlight is given by

B₀ = 12 × 10^–8 sin (1.20 × 10⁷ z – 3.60 × 10¹⁵ t) T.

What is the average intensity of the beam?

Poynting vectors S is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propagation. Mathematically, it is given by Show the nature of S vs t graph.

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.

Show that the magnetic field B at a point in between the plates of a parallel-plate capacitor during charging is (symbols having usual meaning).

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.

Show that average value of radiant flux density ‘S’ over a single period ‘T’ is given by .

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

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

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 geometrical characteristic of LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?

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.

An infinitely long thin wire carrying a uniform linear static charge density λ is placed along the z-axis (Fig. 8.1). The wire is set into motion along its length with a uniform velocity . Calculate the poynting vector .

Sea water at frequency v = 4 × 108 Hz has permittivity ε ≈ 80 ε_o, permeability μ ≈ μo and resistivity ρ = 0.25 Ω–m. Imagine a parallel plate capacitor immersed in sea water and driven by an alternating voltage source. What fraction of the conduction current density is the displacement current density?

A long straight cable of length l is placed symmetrically along z-axis and has radius a(<<l). The cable consists of a thin wire and a co-axial conducting tube. An alternating current 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 Id.

(iii) Compare the conduction current I₀ with the displacement current .

A plane EM wave travelling in vacuum along z direction is given by and

(i) Evaluate over the rectangular loop 1234 shown in Fig 8.2.

(ii) Evaluate over the surface bounded by loop 1234.

(iii) Use equation to prove .

(iv) By using similar process and the equation , prove that

A plane EM wave travelling along z direction is described by and . Show that

(i) The average energy density of the wave is given by .

(ii) The time averaged intensity of the wave is given by .