EM waves

UNIT –V

ELECTROMAGNETIC WAVES

(3 marks)

CBSE 2016 1. Why are micro waves considered suitable for radar systems used in aircraft navigation? 2. Explain why current flows through an ideal capacitor when it is connected to an a.c. source but not when it is connected to a d.c. source in a steady state. 3. The charging current for a capacitor is 0.25 A. What is the displacement current across its plates? 4. In which situation is there a displacement current but no conduction current? 5. State two properties of electromagnetic waves. How can we show that em waves carry momentum? 6. (i) How are infrared waves produced? Write their one important use. (ii) The thin Ozone layer on top of the stratosphere is crucial for human survival. Why? 7. How are em waves produced by oscillating charges? Draw a sketch of linearly polarized em waves propagating in the Zdirection. Indicate the directions of the oscillating electric and magnetic fields. 8. Write Maxwell’s generalization of Ampere’s Circuital Law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is d∅ I D =∈0 E dt where ΦE is the electric flux produced during charging of the capacitor plates. 9. (i) Identify the part of the electromagnetic spectrum which is: (a) suitable for radar system used in aircraft navigation, (b) produced by bombarding a metal target by high speed electrons. (ii) Why does a galvanometer show a momentary deflection at the time of charging or discharging a capacitor? Write the necessary expression to explain this observation 10. (i) Which segment of electromagnetic waves has highest frequency? How are these waves produced? Give one use of these waves. (ii) Which em waves lie near the high frequency end of visible part of em spectrum? Give its one use. In what way this component of light has harmful effects on humans? CBSE 2015 1. In an a.c. circuit, the instantaneous voltage and current are V = 200 sin 300 t volt and I = 8 cos 300 t ampere respectively. Is the nature of the circuit capacitive or inductive? Give reason. 2. A variable frequency AC source is connected to a capacitor. Will the displacement current change if the frequency of the AC source is decreased? 3. Name the types of e.m. radiations which 1

(i) are used in destroying cancer cells, (ii) cause tanning of the skin and (iii) maintain the earth’s warmth. Write briefly a method of producing any one of these waves. 4. Write the expression for the generalized Ampere’s circuital law. Through a suitable example, explain the significance of timedependent term. 5. Write the expression for the generalized form of Ampere’s circuital law. Discuss its significance and describe briefly how the concept of displacement current is explained through charging/discharging of a capacitor in an electric circuit. 6. How are electromagnetic waves produced? What is the source of the energy carried by a propagating electromagnetic wave? Identify the electromagnetic radiations used in (i) remote switches of household electronic devices; and (ii) as diagnostic tool in medicine. 7. (a) Which one of the following electromagnetic radiations has least frequency: UV radiations, X-rays, Microwaves (b) How do you show that electromagnetic waves carry energy and momentum? (c) Write the expression for the energy density of an electromagnetic wave propagating in free space. 8. Answer the following questions: (i) Why is the thin ozone layer on top of the stratosphere crucial for human survival? Identify to which a part of electromagnetic spectrum does this radiation belong and write one important application of the radiation. (ii)Why are infrared waves referred to as heat waves? How are they produced? What role do they play in maintaining the earth’s warmth through the greenhouse effect? 9. Name the parts of the electromagnetic spectrum which is (a) suitable for radar systems used in aircraft navigation. (b) used to treat muscular strain. (c) used as a diagnostic tool in medicine. Write in brief, how these waves can be produced. CBSE 2014 1. A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the direction of electric and magnetic field vectors? 2. Name the physical quantity that remains same for microwaves of wavelength 1 nm and UV radiations of wavelength 1600 A0 in vacuum. 3. To which part of the electromagnetic spectrum does a wave of frequency 3 x 1013 Hz belong? 4. A capacitor has been charged by a dc source. What are the magnitudes of conduction and displacement currents, when it is fully charged?

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5. Considering the case of a parallel plate capacitor being charged, show how one is required to generalize Ampere’s circuital law to include the term due to displacement current. 6. Arrange the following electromagnetic waves in order of increasing frequency: -rays, microwaves, infrared rays and ultraviolet rays. 7. Write the mathematical expression for the generalized Ampere’s circuital law. 8. How are infrared waves produced? Why are these referred to as 'heat waves' ?Write their one important use. 9. When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used, the current flows continuously. How does one explain this, based on the concept of displacement current? 10. (a) An em wave is travelling in a medium with a velocity v = v i^ . Draw a sketch showing the propagation of the em wave, indicating the direction of the oscillating electric and magnetic fields. (b) How are the magnitudes of the electric and magnetic fields related to the velocity of the em wave? 11. (a) How does oscillating charge produce electromagnetic waves? (b) Sketch a schematic diagram depicting oscillating electric and magnetic fields of an em wave propagating along + z-direction. 12. (a) A capacitor is connected in series to an ammeter across a d.c. source. Why does the ammeter show a momentary deflection during the charging of the capacitor? What would be the deflection when it is fully charged? (b) How is the generalized form of Ampere’s circuital law obtained to include the term due to displacement current? 13. Answer the following questions: (i) Show, by giving a simple example, how em waves carry energy and momentum. (ii) How are microwaves produced? Why is it necessary in microwave ovens to select the frequency of microwaves to match the resonant frequency of water molecules? (iii) Write two important uses of infra-red waves. 14. Answer the following: (a) Name the em waves which are used for the treatment of certain forms of cancer. Write their frequency range. (b) Thin ozone layer on top of stratosphere is crucial for human survival. Why? (c) Why is the amount of the momentum transferred by the em waves incident on the surface so small? 15. Answer the following: (a) Name the em waves which are suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves. (b) If the earth did not have atmosphere, would its average surface temperature be higher or lower than what it is now? Explain. (c) An em wave exerts pressure on the surface on which it is incident. Justify. CBSE 2013:(Delhi Sets)

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1. A capacitor has been charged by a d.c. source. What are the magnitudes of conduction and displacement currents, when it is fully charged? 2. (a) An em wave is travelling in a medium with a velocity ⃗v =v i^ . Draw a sketch showing the propagation of the em wave, indicating the direction of the oscillating electric and magnetic fields. (b) How are the magnitudes of the electric and magnetic fields related to the velocity of the em wave? CBSE 2012:(Delhi Sets) 1. Name the physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Å in vacuum. 2. When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used, the current flows continuously. How does one explain this, based on the concept of displacement current? CBSE 2011:(Delhi Sets) 1. A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the direction of electric and magnetic field vectors? 2. How are infrared waves produced? Why are these referred to as ‘heat waves’? Write their one important use. 3. (SET II) A plane electromagnetic wave travels in vacuum along x-direction. What can you say about the direction of electric and magnetic field vectors? 4. (SET III) A plane electromagnetic wave travels in vacuum along y-direction. What can you say about the direction of electric and magnetic field vectors? CBSE 2010: 1. Name the part of electromagnetic spectrum whose wavelength lies in the range of 10 – 10 m. give its one use. (set II) which of the following has the shortest wavelength: Microwave, Ultraviolet rays, X-rays (set III) Arrange the following in descending order of wavelength: X–rays, Radio waves, blue light, infrared light. 2. Arrange the following electromagnetic radiations in ascending order of their frequencies: (i) Microwave (ii) Radio wave (iii) X-rays (iv) Gamma rays Write two uses of any one of these. 3. Draw a sketch of a plane electromagnetic wave propagating along the z-direction. Depict clearly the directions of electric and magnetic field varying sinusoidally with z. CBSE 2009: 1. Name the EM waves used for studying crystal structure of solids. What is its frequency range? [1 2. (a) Optical and radio telescopes are built on the ground while X-ray astronomy is possible only from the satellites orbiting the earth. Why? (b) The small ozone layer on top of the stratosphere is crucial for human survival. Why? CBSE 2008: 1. Identify the following electromagnetic radiations as per the wavelengths given below. Write one application of each.

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(a) 10 -3 nm

(i) (ii) (iii)

(b)10 -3 m

(c) 1nm

CBSE 2007: 1. Name the following constituent radiations of electromagnetic spectrum which (i) produces intense heating effect (ii) is absorbed by ozone layer in atmosphere (iii) is used for studying crystal structure. Write one more application for each of these radiations. CBSE 2006: 1. Write the order of frequency range and one use of each of the following electromagnetic radiations: Microwaves Ultra-violet rays Gamma rays CBSE 2005: 1. Name the constituent radiation of electromagnetic spectrum which (a) is used in satellite communication (b) is used for studying crystal structure. (c) is similar to the radiations emitted during decay of radioactive nuclei. (d) has its wavelength range between 390nm and 770nm? (e) is absorbed from sunlight by ozone layer. (f) produces intense heating effect. CBSE 2004: 1. What is the name given to that part of electromagnetic spectrum which is used in ‘Radar’? 2. Find the wavelength of electromagnetic waves of frequency 5 x 10 19 Hz in free space. Give its two applications. 3. ‘Microwaves are used in Radar.’ Why? 4. Find the wavelength of electromagnetic waves of frequency 4 x 109 Hz in free space. Give its two applications.

Questions from CBSE sample papers: 1. Name the part of Electimagnetic spectrum which is suitable for (i) Physical therapy (ii) TV communication. 2. Which characteristic of the following electromagnetic waves (i) increases (ii) remains same as we move along  radiation, ultraviolet rays, microwaves and radio waves? 3. Which part of electromagnetic spectrum has largest penetrating power? 4. (a) Write the formula for the velocity of light in a material medium of relative permittivity r and relative magnetic permeability r. (b) The following table gives the wavelength range of some constitution of the electromagnetic spectrum. S.No. Wavelength Range 1. 1mm to 700nm 2. 0.1m to 1mm 3. 400 nm to 1nm 4. < 10–3 nm Select the wavelength range, and name the (associated)

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electromagnetic waves, that are used in (i) Radar systems for Aircraft navigation (ii) Earth satellites to observe growth of crops. 5. Explain how one ‘observes an inconsistency’ when Ampere’s 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’? 6. Arrange the following in ascending order of frequency X-rays, green light, red light, microwaves, -radiation. Which characteristic of the above waves is same for all? 7. Identify the type of waves which are produced by the following way and write one application for each: (i) Radioactive decay of the nucleus, (ii) Rapid acceleration and decelerations of electrons in aerials, (iii) Bombarding a metal target by high energy electrons. 8. In an electiomagnetic wave propagating along + x-axis electric field vector is Ey=4x103Cos(3x108t–1.5x) V/m. What is (i) the frequencyof em wave (ii) amplitude of magnetic field.

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