Vidaara.orgClass 12 · Physics
CodeVID-P12-04-CH-01
Electromagnetic Waves — Full Chapter Test
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- This is a full-length test covering the whole chapter — every topic is included.
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks. Show all working for Sections B, C and D.
Section A — Multiple Choice Questions
6 × 1 = 6 marks
1.
The displacement current is caused by a changing:
- A.magnetic flux
- B.electric flux
- C.charge density
- D.resistance
2.
The speed of EM waves in vacuum equals:
- A.$\sqrt{\mu_0\epsilon_0}$
- B.$\frac{1}{\sqrt{\mu_0\epsilon_0}}$
- C.$\mu_0\epsilon_0$
- D.$\frac{\epsilon_0}{\mu_0}$
3.
EM waves are:
- A.longitudinal
- B.transverse
- C.neither
- D.torsional
4.
The ratio $\frac{E_0}{B_0}$ is:
- A.$\frac{1}{c}$
- B.$c$
- C.$c^2$
- D.$\mu_0$
5.
The most penetrating EM radiation is:
- A.radio
- B.infrared
- C.X-ray
- D.gamma
6.
Radiation pressure on a perfect absorber for intensity $I$ is:
- A.$\frac{2I}{c}$
- B.$\frac{I}{c}$
- C.$Ic$
- D.$\frac{I}{2c}$
Section B — Short Answer (2 marks)
4 × 2 = 8 marks
7.
Write the expression for displacement current and state what it physically represents.
8.
An EM wave has $E_0=30\ \text{V/m}$. Find $B_0$.
9.
A microwave has frequency $6\ \text{GHz}$. Find its wavelength.
10.
State two uses of ultraviolet radiation.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
11.
State the four Maxwell equations (integral form) and name each.
12.
An EM wave has $E_0=40\ \text{V/m}$. Find its intensity ($\epsilon_0=8.85\times10^{-12}$, $c=3\times10^{8}$).
Section D — Long Answer (5 marks)
2 × 5 = 10 marks
13.
Explain the need for displacement current using a charging capacitor, write the Ampère–Maxwell law, and state how it leads to the prediction of EM waves with speed $c=\frac{1}{\sqrt{\mu_0\epsilon_0}}$.
14.
Describe the electromagnetic spectrum, giving the bands in order of increasing frequency with one source and one use of each, and state the property all EM waves share.
Answer Key
Section A — Multiple Choice Questions
- (B) electric flux
- (B) $\frac{1}{\sqrt{\mu_0\epsilon_0}}$
- (B) transverse
- (B) $c$
- (D) gamma
- (B) $\frac{I}{c}$
Section B — Short Answer (2 marks)
- $I_d=\epsilon_0\frac{d\Phi_E}{dt}$; it is the current associated with a changing electric flux, which produces a magnetic field even where no charge moves.
- $B_0=\frac{E_0}{c}=\frac{30}{3\times10^{8}}=1\times10^{-7}\ \text{T}$.
- $\lambda=\frac{c}{f}=\frac{3\times10^{8}}{6\times10^{9}}=0.05\ \text{m}=5\ \text{cm}$.
- Sterilising surgical instruments and water; detecting forged signatures/security marks (and treating some skin conditions).
Section C — Short Answer (3 marks)
- Gauss (E): $\oint \vec{E}\cdot d\vec{A}=\frac{q}{\epsilon_0}$; Gauss (B): $\oint \vec{B}\cdot d\vec{A}=0$; Faraday: $\oint \vec{E}\cdot d\vec{l}=-\frac{d\Phi_B}{dt}$; Ampère–Maxwell: $\oint \vec{B}\cdot d\vec{l}=\mu_0 I+\mu_0\epsilon_0\frac{d\Phi_E}{dt}$.
- $I=\frac{1}{2}\epsilon_0 E_0^2 c=\frac{1}{2}\times8.85\times10^{-12}\times1600\times3\times10^{8}\approx2.12\ \text{W/m}^2$.
Section D — Long Answer (5 marks)
- For a loop around the charging wire, a flat surface encloses conduction current $I$ but a surface between the plates encloses none, so Ampère's law is inconsistent. Adding $I_d=\epsilon_0\frac{d\Phi_E}{dt}$ gives $\oint \vec{B}\cdot d\vec{l}=\mu_0 I+\mu_0\epsilon_0\frac{d\Phi_E}{dt}$. Thus a changing $E$ makes $B$ and (Faraday) a changing $B$ makes $E$; these self-sustaining fields propagate at $c=\frac{1}{\sqrt{\mu_0\epsilon_0}}\approx3\times10^{8}\ \text{m/s}$, matching the speed of light.
- Radio (antennas; broadcasting), microwave (magnetrons; RADAR/ovens), infrared (hot bodies; night vision), visible (atoms/hot bodies; sight), ultraviolet (hot bodies/UV lamps; sterilisation), X-ray (fast electrons on metal; medical imaging), gamma (radioactive nuclei; cancer therapy). All travel at $c$ in vacuum and obey $c=f\lambda$; they are transverse and carry energy and momentum.
Generated by Vidaara.org · Assignment VID-P12-04-CH-01 · vidaara.org