An Overview of Ncert Books Class 12 Physics Chapter 8 Free Download
FAQs on Ncert Books Class 12 Physics Chapter 8 Free Download
1. What is the marks weightage for Chapter 8, Electromagnetic Waves, in the CBSE Class 12 Physics exam for the 2025-26 session?
For the CBSE Class 12 Physics board exam 2025-26, the chapter on Electromagnetic Waves, when combined with Alternating Current, holds a significant weightage. While the exact marks can vary slightly, this unit is crucial for scoring well. Chapter 8 itself is relatively short and conceptual, typically contributing around 3-4 marks. Students can easily secure these marks by focusing on the key theories and applications.
2. What are the most important properties of electromagnetic waves to remember for the board exam?
From an examination perspective, the following properties of electromagnetic (EM) waves are frequently tested:
- EM waves are transverse in nature, meaning the electric and magnetic field vectors oscillate perpendicular to the direction of wave propagation.
- They do not require a material medium to travel and can propagate through a vacuum.
- In a vacuum, all EM waves travel at the same speed, the speed of light (c ≈ 3 x 108 m/s).
- The electric field (E) and magnetic field (B) are always in the same phase and are mutually perpendicular.
- The energy of an EM wave is distributed equally between its electric and magnetic fields.
3. Why was the concept of displacement current introduced, and what is its significance?
The concept of displacement current was introduced by Maxwell to address an inconsistency in Ampere's Circuital Law. The law was only valid for steady currents. Its primary significance is that it establishes that a changing electric field can produce a magnetic field, just as a changing magnetic field produces an electric field (Faraday's Law). This symmetry is fundamental to the existence of electromagnetic waves, as it explains how these waves can self-propagate through space.
4. Which applications of the electromagnetic spectrum are most important for the CBSE Class 12 exam?
For the board exam, you should focus on at least one key application for each part of the electromagnetic spectrum. The most frequently asked applications are:
- Microwaves: Used in RADAR systems for navigation and in microwave ovens for cooking.
- Infrared Waves: Used in remote controls, thermal imaging (night vision), and physical therapy.
- Ultraviolet (UV) Rays: Used for water purification (killing germs) and in detecting forged documents.
- X-rays: Widely used in medical diagnostics to image bones and in industry to detect cracks in metal structures.
- Gamma Rays: Used in radiotherapy to destroy cancer cells.
5. What types of questions, including MCQs, can be expected from the Electromagnetic Waves chapter?
Given its conceptual nature and limited length, this chapter typically features in the short-answer section. You can expect:
- 1-Mark Questions (MCQ/VSA): Focused on identifying the EM wave for a given application, questions on the source of EM waves (accelerated charges), or the relative order of wavelengths/frequencies.
- 2 or 3-Mark Questions (SA): These often require you to list the properties of EM waves, explain the need for displacement current, or state two uses each for different types of waves like infrared or UV.
- Assertion-Reasoning: Questions might test your conceptual clarity on topics like the transverse nature of EM waves or the definition of displacement current.
6. Why are electromagnetic waves considered transverse in nature?
Electromagnetic waves are considered transverse because the oscillations of both the electric field (E) and the magnetic field (B) are perpendicular to the direction in which the wave is travelling. If an EM wave propagates along the z-axis, the electric and magnetic fields will oscillate in the x-y plane. This is a fundamental characteristic that distinguishes them from longitudinal waves (like sound), where oscillations occur along the direction of propagation.
7. What is the fundamental source of all electromagnetic waves?
The fundamental source of all electromagnetic waves is an accelerated electric charge. A stationary charge produces only an electric field, and a charge moving with uniform velocity produces both electric and magnetic fields (a steady current). However, only when a charge accelerates or oscillates does it radiate energy in the form of electromagnetic waves, where time-varying electric and magnetic fields sustain each other.
8. On what basis are electromagnetic waves classified into different categories like radio waves and gamma rays?
Electromagnetic waves are classified into different categories primarily based on their frequency (ν) or wavelength (λ). The entire range of these waves, arranged in order of increasing frequency (or decreasing wavelength), is known as the electromagnetic spectrum. Although all these waves travel at the speed of light in a vacuum, their different frequencies cause them to interact with matter differently, leading to their distinct properties and applications.
9. Does displacement current involve a physical flow of charge like conduction current? Clarify this common misconception.
No, this is a common misconception. Displacement current does not involve the physical flow of electrons or any other charge carriers. Unlike conduction current, which is due to the drift of charges in a conductor, displacement current is a theoretical concept. It is produced by a time-varying electric flux, typically found in the space between the plates of a capacitor while it is charging or discharging. It is the effect of this changing electric field that produces a magnetic field.
