Electromagnetic Waves: Class 12 Physics Chapter 8 Notes in Hindi
Ab tak humne dekha ki badalta hua magnetic field electric field paida karta hai (Faraday's Law). Chapter 8 mein hum iska ulta padhenge: Badalta hua Electric Field bhi Magnetic Field paida karta hai. Isi se banti hain Electromagnetic Waves.
1. Displacement Current (Visthapan Dhara)
Maxwell ne dekha ki jab capacitor charge hota hai, toh plates ke beech koi wire nahi hota, fir bhi wahan magnetic field hota hai. Iska matlab wahan ek current hai jise Displacement Current ($I_d$) kehte hain.
Formula:
$$I_d = \epsilon_0 \frac{d\phi_E}{dt}$$Yeh current tab paida hota hai jab Electric Flux change hota hai.
2. Maxwell’s Equations
Maxwell ne puri electromagnetism ko 4 equations mein sameta:
Gauss’s Law (Electrostatics): $\oint E \cdot dA = q/\epsilon_0$
Gauss’s Law (Magnetism): $\oint B \cdot dA = 0$
Faraday’s Law: $\oint E \cdot dl = -d\phi_B/dt$
Ampere-Maxwell Law: $\oint B \cdot dl = \mu_0(I_c + I_d)$
3. Electromagnetic Waves Kya Hain?
Jab electric aur magnetic fields ek dusre ke perpendicular (90°) oscillate karte hain, toh wo space mein aage badhte hain. Inhi waves ko EM Waves kehte hain.
Speed: Vacuum mein inki speed light ki speed ke barabar hoti hai ($c \approx 3 \times 10^8 m/s$).
Nature: Ye Transverse waves hoti hain (yani inki oscillation direction wave ki direction ke perpendicular hoti hai).
Inhe chalne ke liye kisi medium (hawa ya pani) ki zaroorat nahi hoti.
4. Electromagnetic Spectrum (Sabse Important)
EM waves ki alag-alag wavelengths aur frequencies hoti hain. Inka order yaad rakhna exam ke liye zaroori hai:
| Wave Type | Use (Upyog) |
| Radio Waves | TV aur Radio communication mein. |
| Microwaves | Radar aur Microwave oven mein. |
| Infrared | Remote control aur night vision mein. |
| Visible Light | Isse hum duniya dekh paate hain. |
| Ultraviolet (UV) | Water purification aur surgery mein. |
| X-Rays | Haddiyon ke fracture dekhne mein. |
| Gamma Rays | Cancer ke ilaaj (Radiotherapy) mein. |
5. Properties of EM Waves
Ye waves charge-less hoti hain (electric/magnetic field se deflect nahi hoti).
Inki energy electric aur magnetic field mein barabar banti hoti hai.
Relation:
$$c = \frac{E_0}{B_0}$$(Yahan $E_0$ electric field aur $B_0$ magnetic field ki peak value hai).
6. Top Important Questions (Chapter 8)
Blog post ke end mein ye questions SEO aur students dono ke liye best hain:
Short Questions:
Ozone layer ka kya kaam hai? Ye sun se aane waali harmful UV rays ko rokti hai.
Greenhouse effect mein kaunsi wave use hoti hai? Infrared waves.
Wavelength ka order bataiye: Radio waves (sabse badi) > Gamma rays (sabse choti).
Long & Conceptual Questions:
Identify the wave: "Kaunsi wave ka use radar mein hota hai?" (Ans: Microwaves).
Show that EM waves are transverse in nature.
Write Maxwell’s four equations and explain their meaning.
Calculate the speed of EM waves:
$$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$$
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