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Important Questions Electromagnetic Waves

Type A One marks Question Q1. In a plane electromagnetic wave, the electric field oscillates sinusoidally at a frequency of 2.5 x 1010 Hz. What will be the frequency of oscillating magnetic field? Q2. What is the name given to current that comes into play in a region where the electric field is changing with time? Q3. If a portion of emwaves of energy U is propagating with speed c, then what is the linear momentum of associated with such a wave? Q4. What is the direction of emwave in terms of electric field vector

!E and magnetic

field vector !B ?

Q5. Define (a) Displacement current. (b) Electromagnetic wave. (c) Electromagnetic spectrum.

Q6. What is the name given to electromagnetic waves of frequency range 3 x1018 Hz to 5 x1022 Hz? Q7. Name the electromagnetic waves in relation to

(a) In the working of solar water heater and cookers. (b) Navigation of aircrafts (c) In telecommunications (d) For detecting fake currency notes. (e) In currency counting machines (f) In photocells. (g) Detected by Geiger tube. (h) Sterilization for medical equipments (i) To see through haze and fog. (j) Night vision devices. (k) Adjacent to the low frequency end of the electromagnetic spectrum. (l) Adjacent to the low wavelength end of the electromagnetic spectrum.

Q8. Every emwave has certain frequency. Name the parameters of an emwave that oscillates with this frequency. Q9. What is phase difference between electric and magnetic field vectors in an emwave? Q10. Give any two uses of microwaves. Q11. Arrange the following in descending order of wavelengths: Gamma rays, Microwaves and UV Rays Q12. Which component of electromagnetic wave

!E or

!B is responsible for visible

effect? Q13. Write expression for speed of emwaves in a medium of electrical permittivity and magnetic permeability . Q14. Which of the following has longest penetration power: UV, X-rays and microwaves? Q15. Which of the following has least frequency IR, visible and radiowaves? Q16. Which physical quantity is same for microwaves of wavelength 1mm and UV radiation of 1600 in vacuum?

Q17. Name the physical quantity with units same as that of formula 0dEdt

.

Q18. Name the device used for producing microwaves.

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Q19. Identify the part of em spectrum to which the following wavelengths belong: (a) 10 1 m (b) 10 12 m (c) 10 2 m

Mention any one application of these waves. Q20. Which of the following, if any, can act as a source of emwaves?

(a) A charge moving with constant velocity. (b) A charge moving in a circular orbit. (c) A charge at rest.

Q21. The charging current for a capacitor is 0.2 A. What is the displacement current? Q22. Give the ratio of velocities of light waves of wavelengths 4000 and 8000 in vacuum. Q23. Which physical quantity, if any has the same value for waves belonging to the different parts of the electromagnetic spectrum? Q24. An emwave has amplitude of electric field E0 and amplitude of magnetic field is B0. At some instant of time, the value of electric field is 3E0/4. What will be the value of magnetic field at that instant? (Wave is travelling in vacuum). Q25. State two applications of

(a) Infrared radiations (b) Ultraviolet radiation (c) X-rays (d) Gamma rays

Q25. Name the emwave that has frequencies greater than those of UV light but less than those of gamma rays? Q26. A variable ac source is connected across a capacitor. Will the displacement current increase or decrease with increase in frequency? Q27. What is the source of displacement current? Q28. What is the SI unit of displacement current? Q29. Is displacement current a current in true sense? Q30. What modification was made by Maxwell in the Amperes circuital law? Explain why? Q31. What is the importance of the concept of displacement current? Q32. What is the difference between conduction current and displacement current? Q33. The velocity of propagation in vacuum and the frequency of (i) X-rays and (ii) radio waves are denoted as (vx, nx) and (vr, nr) respectively. How do the values of

(a) vx and vr and (b) nx and nr compare with each other?

Q34. Name the characteristics of electromagnetic waves that (i) increases (ii) remains constant in the electromagnetic spectrum as one moves from radio wave region towards ultra violet region.

Type B Two marks Question Q1. Induced electric field due to changing magnetic flux are more readily observed in nature than induced magnetic field due to changing electric field. Why? Q2. A variable frequency ac source is connected to a capacitor. Will the displacement current increase or decrease with increase in frequency of ac source? Q3. Give one use of each of the following

(a) UV rays (b) Gamma rays.

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Q4. Represent EM waves propagating along the x-axis. Represent the equation of em wave. Q5. Show that the average energy density of electric field equals the average energy density of magnetic field in an emwave. Q6. Find the wavelength of electromagnetic waves of frequency 5 x 1019 Hz in free space. Give its two applications. Q7. An ideal capacitor is charged by a dc battery; but no current flows. However, when an ac source is used, the current flows continuously. Explain this on the basis of displacement current. Q8. An ideal capacitor is charged by a connecting to a dc battery along with an ammeter. Will the ammeter show some momentarily deflection during the process of charging. Explain this on the basis of displacement current. Q9. What is displacement current? Give its mathematical form as well.

Q4. Show that 0

Eddt represent current.

Q10. Name the electromagnetic waves used for the following and arrange them in increasing order of their penetration power:

(a) Water purification (b) Remote sensing (c) Treatment of cancer.

Q13. Why are infrared radiations referred to as heat waves also? Name the radiations which are next to these radiations in electromagnetic spectrum having (i) Shorter wavelength. (ii) Longer wavelength. Q15. Experimental observations have shown that X-rays (i) travel in vacuum with a speed of 3 x 108 m/s. (ii) exhibit the phenomenon of diffraction and can the polarized. What conclusion can be drawn about the nature of X-rays from each of these observations? Q16. How is the generalized form of Amperes circuital law obtained to include the term due to displacement current ?

Type C Three marks Question

Q1. Name the EM radiations used (a) In the treatment of cancer (b) For detecting flaws in pipes carrying oil. (c) In sterilizing surgical instruments

Q2. How would you experimentally show that EM waves are transverse in nature? Q3. List any three properties of EM waves. Q4. Electromagnetic waves of wavelength:

(a) 1 are used to treat muscular strain. (b) 2 are used by FM radio station for broadcasting (c) 3 are used to detect fracture in bones (d) 4 are absorbed by ozone layer of the atmosphere.

Identify and name the part of em spectrum to which these radiation belongs. Arrange these wavelengths in ascending order of magnitude. Q5. Name the constituents radiation of electromagnetic spectrum which:

(a) Is used in satellite transmission. (b) Is used to study crystal structure. (c) Is similar to the radiations emitted during decay of radioactive nuclei.

Q6. Name the constituents radiation of electromagnetic spectrum which:

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(a) Has a wavelength in the range of 390nm and 770 nm. (b) Is absorbed from sunlight by ozone layer. (c) Produce intense heating effect.

Q7. Discuss the inconsistency in Amperes Circuital law. What modification was made by Maxwell in this law? Q8. Conduction current and displacement currents are individually discontinuous, but their sum is continuous. Comment. Q9. Explain how one observes an inconsistency when Amperes circuital law is applied to the process of charging a capacitor? How this contradiction gets removed by introducing the concept of an additional current, known as the displacement current?

Type D Numericals Question Q1. In a plane electromagnetic wave, the amplitude of electric field is 480 V/m. What will be the amplitude of oscillating magnetic field? Q2. Relative permittivity of a medium is 8 and relative permeability of same medium is close to unity. What will be the speed of emwave in such a medium? Q3. Suppose the electric field amplitude of an emwave is 120 N/C and its frequency is 50 MHz.

(a) Determine B0, , k and . (b) Find expression for E and B.

Q4. The amplitude of the magnetic field vector of an electromagnetic wave travelling in vacuum is 2.4 mT. Frequency of the wave is 16 MHz. Find:

(a) Amplitude of electric field vector and (b) Wavelength of the wave.

Q5. An EM wave travelling through a medium has electric field vector: Ey = 4x105 cos 3.14x108 t 1.57x in N /C

Here x is in meter, and t is in second. Then find (a) Amplitude of magnetic field. (b) Wavelength of wave. (c) Name the spectrum to which it belongs.

Q6. A PPC consists of two parallel plates each of radius 10cm and separated by 2mm. The capacitor is being charged by an external battery. The charging current is constant and equal to 0.5A. Calculate (a) the capacitance (b) the rate of change of p.d. across the plates and (c) the displacement current. Q7. The magnetic field in a plane em wave is given by By = 2 x 10 7 sin(0.5 x 103x + 1.5 x 1011t) tesla. What is the value of wavelength and frequency of em waves? Also write the expression for electric field. Q8. A plane electromagnetic wave propagating in the X-direction has a wavelength of 6mm. The electric field in the Y-direction has amplitude of 33V/m. Write suitable equation for the electric and magnetic fields as the function of x and t. Q9. A PPC of area 50cm2 and plate separation 3mm is charged initially to 80C. Due to radio-active source present nearby, the medium between the plates gets slightly conducting and the plate loses charge initially at the rate of 1.5 x 10 8 C/s. What is the magnitude and direction of displacement current? What is the magnetic field between the plates? Q10. A How would you establish a displacement current of 3A in the space between the plates of a capacitor of 3F capacitance?