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As the CBSE Class 12 board exams get closer, it’s important for students to understand the new exam pattern. Starting in the 2024-25 school year, CBSE will include 50% more competency-based questions. These questions will be both multiple choice and written, focusing on how to use what students have learned in real-life situations.
For Class 12 Physics, Unit - 7 on Electromagnetic Waves, CBSE has provided competency-based questions that help improve understanding and application skills. It highlights key competency-based questions and provides answers to help students succeed. These questions are designed to help students strengthen their understanding and apply concepts effectively for their exams.
By practicing these competency-based questions, students will get familiar with the types of questions they are likely to face in the exam. The free PDF includes a variety of questions, such as multiple-choice (MCQs) and free-response questions, with answers provided to guide students in their preparation.
CBSE Class 12 Physics Chapter 7 Electromagnetic Waves Important Competency-Based Questions
Q.1 Study the following statements carefully.
A. Electric and magnetic fields have zero average value in a plane em wave.
B. For an em wave, the ratio k/ω is independent of wavelength.
C. In an em wave, the E and B fields vary with the same frequency and are in opposite phase.
D. Since E = cB, the energy associated with the electric field is much greater than that associated with the magnetic field.
Identify the correct option.
A. only A and B are correct
B. only C and D are correct
C. only A and C are correct
D. only B and D are correct
Answer. A. only A and B are correct
Q.2 5 Which of the following statement/s are incorrect?
A. The displacement current flows in a dielectric of the capacitor when the potential difference across its plates is decreasing with time.
D. Instantaneous energy flow rate is a constant for an electromagnetic wave.
E. Light of uniform intensity shines perpendicularly on a totally absorbing surface.
On decreasing the area of the surface, the intensity remains the same.
A. Only statements A & B
B. Only statements C & D
C. Only statements D & E
D. Only statements A, C & D
Answer. B. Only statements C & D
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Q.3 The diagrams below show the electric and magnetic field components of an electromagnetic wave at a certain time and location.
Which of these electromagnetic waves are travelling towards you?
A. only the em wave in Fig I
B. only the em wave in Fig I and II
C. only the em wave in Fig II and III
D. only the em wave in Fig II, III and IV
Answer. D. only the em wave in Fig II, III and IV
Q.4 An electromagnetic wave of frequency 1 GHz travels through an empty space along the z‐direction. At a specific point in space, the electric field E attains a maximum value of 50 V/m. If the electric wave is polarized along x‐axis, then,
(a) explain and identify the plane in which the magnetic field B will lie. (b) express the electric and magnetic fields as a function ofz and t.
Answer. (a) Since E wave is polarized along x‐direction and the em wave propagates along z‐direction, the magnetic field vector has to be perpendicular to both E wave and the direction of propagation of the wave. So, B vector is aligned along y axis and lies in y‐z plane.
[1 mark for correct explanation and the direction]
Q.5 In a spaceship orbiting around the Earth, two astronauts stationed 2 m apart are speaking to each other. The conversation is transmitted to Earth via electromagnetic waves.
Given that the sound waves travel through the air between the two astronauts in exactly the same time as the em waves take to reach the Earth ground station.
Calculate the distance of the spaceship from the ground station. Take speed of sound in the air between the two astronauts as 340 m/s.
Answer. For the travel of sound waves between the two astronauts: 2 / 340
= t ….(1) For the travel of em waves from the spaceship to the Earth station: d/(3 x 108) = t ….(2)
[0.5 mark for each of the equations for sound and em waves] Equating (1) and (2) and solving for d,
d ≈ 1765 km
[1 mark for correct final result]
Q.6 An unfortunate nuclear explosion leaves behind the residual gamma radiations in the vicinity of the explosion site with an average energy density of 4 × 10−14 J/m3.
(a) What is the rms value of the electric field of the radiation?
(b) Compare the electric field strength with the magnetic field strength in this residual radiation.
Answer. a. As energy density u = ε0E2
= 0.067 N/C
[1 mark for the correct finalresult]
b. In any em radiation, the ratio E/B = c, is always constant [1 mark for the correct application of the ratio between E to B in any em wave]
Q.7 A dish antenna with a circular aperture of a radius 20 cm, receives digital TV signals from a satellite. The average intensity of the em waves that carry a particular TV program is 5 x 10‐14 W/m2.
Determine the following:
(a) electromagnetic energy delivered to the dish during the telecast of 30 minutes of a programme.
(b) average energy density of the em wave.
Answer. (a) Average intensity I = average power P /area A Average power P = I. A = I. πr2 Average energy delivered during the telecast = I. πr2 . t
[0.5 mark for correct formula of energy in terms of intensity, area and time]
E = 5 x 10‐14 x π x (0.2)2 x 1800
11.3 x 10‐12 J
[0.5 mark for correct value of energy]
(b) Energy density u = I/c = 5 x 10‐14 / 3 x 108 = 1.66 x 10‐22 J/m3 [1 mark for correct result of energy density]
Q. 8 A laser emits a sinusoidal em wave of wavelength 10 μm along the x‐axis. The E field of the wave is parallel to the –ve z‐axis with a maximum value of 1 MV/m.
Express the wave equation for E and B for this wave with all appropriate values and directions.
Answer. The standard wave equations: E‐z = Eo sin(kx ‐ ωt) … direction of E field will be along –z‐axis By = Bo sin(kx ‐ ωt) .. direction of B field will be along y‐axis
Where Bo = Eo/c = 106 /3 x 108 = 3.3 x 10‐3 T k = 2π/λ = 2 π / 10 x 10‐6 = 2π x 105 /m
ω = ck = 3 x 108 x 2π x 105 = 6π x 1013 rad/s
[0.5 mark for correct representation of E and correct values of Bo, k and ω] Equations:
E‐z = ‐ k (106 V/m) sin(2π x 105 . x ‐ 6π x 1013 .t)
By = j (3.3 x 10‐3 T) sin(2π x 105 . x ‐ 6π x 1013 .t) [0.5 mark for each of E and B equations]
Q.9 A satellite at a height of 100 km from the Earth’s surface detects a radio signal emitted by a radio station on the ground. If the average power of the signal received is 100 kW, find the amplitudes E0 and B0 of the incoming signal.
Answer. Surface area of the hemisphere on the ground through which the signal is emitted by the radio station
A = 2πR2 = 2π (100 x 1000)2 = 2π x 1010 m
[0.5 mark for correct calculation of surface area]
Intensity of the signal received by the satellite = I = Average power/ Area = 100 x 1000 / 2π x 1010 = 10‐5/2π W/m2
[1 mark for correct calculation of Intensity of signal]
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