Important Questions Class 12 Physics Chapter 8: Electromagnetic Waves
Important Questions Class 12 Physics Chapter 8: Electromagnetic Waves
Electromagnetic Waves is a short but important chapter. It introduces displacement current and Maxwell's correction to Ampere's law, the nature and properties of electromagnetic waves, the relation between the electric and magnetic fields in a wave, and the electromagnetic spectrum from radio waves to gamma rays with their uses.
This page gives 12 important questions from Electromagnetic Waves with answers by Extramarks experts. You will find numericals on displacement current in a capacitor, energy density and amplitude of fields, and short questions on the different parts of the electromagnetic spectrum. These are quick marks in the exam if the concepts are clear.
Q1.
Determine the displacement current between the capacitor plates as a function of time. If the separation between the plates is doubled while the plate area, dielectric medium, voltage amplitude, and frequency remain unchanged, analyse how the maximum displacement current changes.
Answer:
Hence, doubling the plate separation reduces the maximum displacement current to half of its original value.
Q2.
Determine the direction of the magnetic field and write its corresponding mathematical expression. Verify the relation between the amplitudes of the electric and magnetic fields, and justify the direction of propagation of the wave using the field vectors.
Answer:
Hence,
Also,
Q3.
Compare with each other.
Answer: (i)vx=vR
Q4. A plane electromagnetic wave travelling along the x-direction has a wavelength of 3 mm. The variation in the electric field
and t respectively.
Answer:
Q5.
Answer: Average energy density of E
Q6. Suppose that electric field part of an electromagnetic wave in vacuum is
(a) What is the direction of propagation? (b) What is the wavelength λ? (c) What is the frequency υ ? (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.
Answer:
Q7. Describe the primary sources of electromagnetic waves and discuss the characteristic nature of these waves.
Answer: Electromagnetic waves are generated by accelerated charges, for example oscillating dipoles in antennas or electrons in atoms. Time‐varying currents produce changing electric and magnetic fields that propagate outward. These waves are transverse in nature.
Q8. Differentiate clearly between ultraviolet radiation and infrared radiation based on their frequencies, energy, and applications.
Answer: Frequency and Energy:
making ultraviolet radiation more energetic and capable of causing ionization. Applications: Ultraviolet radiation is used in sterilization, fluorescence, and vitamin D production in human skin. Infrared radiation is primarily used in heating devices, remote controls, and thermal imaging cameras.
making ultraviolet radiation more energetic and capable of causing ionization.
Q9. Define electromagnetic waves. Describe how they are produced, using various examples for explanation.
Answer: Electromagnetic (EM) waves are a form of energy propagation that consists of oscillating electric and magnetic fields that are perpendicular to each other and also perpendicular to the direction of wave propagation. These waves do not require a medium to travel; they can propagate through vacuum. Production of Electromagnetic Waves: Electromagnetic waves are produced by accelerating electric charges. When a charged particle is in non-uniform motion (i.e., it accelerates or decelerates), it creates a time-varying electric field, which in turn produces a time-varying magnetic field. This dynamic interaction sustains and propagates the electromagnetic wave through space. For example: An oscillating charge in a radio antenna produces radio waves. Accelerated electrons in X-ray tubes produce X-rays. Vibrating molecules in hot bodies emit infrared radiation. Thus, any system in which charges are accelerated can act as a source of electromagnetic radiation.
Q10. What are microwaves and mention two practical applications?
Answer:
than infrared waves. Practical applications of microwaves include microwave ovens (used for cooking due to their energy being absorbed by water molecules) and radar systems (used in navigation, aircraft detection, and weather forecasting).
Q11. Mention any three properties of electromagnetic waves.
Answer:
2.They are transverse waves with electric and magnetic fields perpendicular to each other and also perpendicular to the direction of propagation. 3.Electromagnetic waves carry energy and momentum, and their propagation does not require any material medium.
Q12. Define displacement current density. Explain why Maxwell introduced it and how it resolves the inconsistency in
Ampere’s law for a charging capacitor.
Answer: Displacement current density is defined as
This restores charge conservation and makes Ampere’s law universally valid.
Related Study Material on Extramarks
- Important Questions for Class 12 Physics Chapter 1 Electric Charges and Fields
- Important Questions for Class 12 Physics Chapter 2 Electrostatic Potential and Capacitance
- Important Questions for Class 12 Physics Chapter 3 Current Electricity
- Important Questions for Class 12 Physics Chapter 4 Moving Charges and Magnetism
- Important Questions for Class 12 Physics Chapter 5 Magnetism and Matter
- Important Questions for Class 12 Physics Chapter 6 Electromagnetic Induction
- Important Questions for Class 12 Physics Chapter 7 Alternating Current
- Important Questions for Class 12 Physics Chapter 8 Electromagnetic Waves
- Important Questions for Class 12 Physics Chapter 9 Ray Optics and Optical Instruments
- Important Questions for Class 12 Physics Chapter 10 Wave Optics
- Important Questions for Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter
- Important Questions for Class 12 Physics Chapter 12 Atoms
- Important Questions for Class 12 Physics Chapter 13 Nuclei
- Important Questions for Class 12 Physics Chapter 14 Semiconductor Electronics
Q1-Among the following rays of the electromagnetic spectrum, which one has the shortest wavelength?
Opt-
A-Microwaves
b-Ultraviolet rays
c-Infrared rays
d-X-rays
Ans-
X-rays has shortest wavelength in the electromagnetic spectrum.
FAQs (Frequently Asked Questions)
Electromagnetic Waves is important for boards because it gives direct formula and spectrum questions. Students often get questions on displacement current, Maxwell equations, EM wave speed, and applications.
Displacement current is current due to changing electric flux. Its formula is id = ε0 dΦE/dt.
The formula of electromagnetic wave speed is c = 1/√(μ0ε0). In vacuum, the same speed also equals c = νλ.
Infrared waves are called heat waves because water molecules and many other molecules absorb them. This absorption increases thermal motion.
Electromagnetic waves are used in radio broadcasting, radar, microwave ovens, remote controls, water purifiers, X-ray diagnosis, and cancer treatment. Each use depends on wavelength and frequency.