CBSE Class 12 Physics: Electromagnetic Waves (Marking Scheme)
(a) Factors affecting the speed of an electromagnetic wave in a medium: [1 Mark]
The speed of an electromagnetic wave in a medium is given by the relation:
$$v = \frac{1}{\sqrt{\mu \epsilon}}$$Where:
- $\mu$ is the magnetic permeability of the medium.
- $\epsilon$ is the electric permittivity of the medium.
Thus, the speed depends on the electrical and magnetic properties of the medium.
(b) Production of an electromagnetic wave: [1 Mark]
An electromagnetic wave is produced by an accelerated charge. According to Maxwell's equations, a time-varying electric field produces a time-varying magnetic field, and vice versa. When a charge oscillates or accelerates, it creates a time-varying electric field, which generates a time-varying magnetic field, resulting in the propagation of electromagnetic waves in space.
(c) Schematic diagram (Propagation along z-axis): [1 Mark]
The electric field vector $\vec{E}$ oscillates along the x-axis, and the magnetic field vector $\vec{B}$ oscillates along the y-axis, while the wave propagates along the z-axis.
[Diagram Description: A 3D coordinate system where the wave travels along the z-axis. The E-field is represented as a sine wave in the x-z plane, and the B-field is represented as a sine wave in the y-z plane, perpendicular to both the E-field and the direction of propagation.]
💡 EL10 Concept Insight: Think of an electromagnetic wave like a dance between electricity and magnetism; when a charge wiggles (accelerates), it creates a ripple in the electric field, which immediately forces the magnetic field to wiggle too, and this "tag-team" keeps pushing itself forward through space forever!