Electromagnetic Waves
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Displacement Current and Maxwell's Laws
Part of: Displacement Current and Wave Nature
Between charging capacitor plates no charge flows, yet magnetism appears; Maxwell added displacement current, epsilon nought times the rate of electric flux, to complete Ampere's law. Together with the two Gauss laws and Faraday's law, these four statements predict self-travelling electromagnetic waves.
Why it matters. Radio, light and X-rays all follow from these four lines. Calling displacement an ordinary charge flow breaks the continuity of current.
Formulas
- Displacement: Id = e0 x d(flux)/dt
- Ampere-Maxwell: circulation of B = mu0 (I + Id)
- Unit of displacement current: ampere
Key words
- Displacement current
- : Epsilon nought times rate of electric flux, completing current across gaps.
- Ampere-Maxwell law
- : Magnetic circulation equals mu nought times conduction plus displacement current.
- Maxwell's equations
- : Four laws covering electric flux, magnetic flux, induction and magnetism of currents.
Try it in two minutes
- Track electric flux between charging plates.
- Multiply its rate by epsilon nought for displacement current.
- Add it to conduction current in Ampere's law.
Try at home. Explain how current stays continuous across a charging capacitor gap.
Nature of Electromagnetic Waves
Part of: Displacement Current and Wave Nature
EM waves carry linked electric and magnetic fields at right angles to each other and to travel, with E over B equal to light speed. Energy splits equally between the two fields and flows along E cross B. Every frequency shares the same vacuum speed.
Why it matters. Antennas, optics and radiation pressure all use this structure. Treating the fields as parallel, or giving each colour its own speed, breaks wave reasoning.
Formulas
- Field ratio: E0 / B0 = c
- Wave speed: c = 3 x 10^8 m/s in vacuum
- Intensity: I = e0 c E0^2 / 2
Key words
- Transverse wave
- : Wave whose oscillations are perpendicular to travel direction.
- Poynting direction
- : Energy flow direction given by E cross B.
- Intensity
- : Average power per unit area carried by the wave.
Try it in two minutes
- Point E one way and B at right angles to it.
- Find travel along E cross B.
- Divide E amplitude by c for B amplitude.
Try at home. Check that doubling E amplitude doubles B amplitude too.
Spectrum: Radio Waves to Visible Light
Part of: Spectrum: Radio to Visible
From long to short: radio waves carry broadcasts, microwaves serve radar and ovens, infrared carries heat and remote signals, and visible light spans the rainbow our eyes see. Each band comes from accelerating charges, with higher frequency meaning shorter wavelength and more energy per photon.
Why it matters. Choosing antennas, cookers, sensors and lights needs the right band. Mixing up the order scrambles every wavelength and application answer.
Formulas
- Wavelength: lambda = c / f
- Band order: radio, micro, IR, visible, UV, X-ray, gamma
Key words
- Radio waves
- : Longest EM waves, used for broadcasting and communication.
- Microwaves
- : Short radio waves used in radar, ovens and phones.
- Infrared
- : Heat radiation beyond red, used in remotes and night vision.
- Visible light
- : Narrow band our eyes detect, from red to violet.
Try it in two minutes
- List bands from longest to shortest wavelength.
- Divide 3 x 10^8 by station frequency for its wavelength.
- Match each gadget to its band.
Try at home. Compute the wavelength of your favourite FM station near 100 MHz.
Spectrum: UV Rays to Gamma Rays
Part of: Spectrum: UV to Gamma
Beyond violet come ultraviolet from the Sun and lamps, causing tanning and sterilisation but blocked partly by ozone; X-rays from fast electrons striking metal, imaging bones; and gamma rays from nuclei, the most penetrating, used in cancer therapy and needing lead shielding.
Why it matters. Sun safety, medical imaging and radiation protection all depend on this order. Confusing sources or shielding needs gives dangerous answers.
Formulas
- Energy order: gamma > X-ray > UV > visible
- Green light frequency: about 5.5 x 10^14 Hz
Key words
- Ultraviolet
- : Band beyond violet causing tanning, sterilisation and sunburn.
- X-rays
- : Short waves from electron impacts, used to image bones.
- Gamma rays
- : Shortest waves from nuclei, highly penetrating and medically used.
- Ozone layer
- : Atmospheric layer absorbing much of the Sun's ultraviolet.
Try it in two minutes
- Rank UV, X and gamma by wavelength.
- Name one source for each band.
- Name one use and one hazard for each.
Try at home. Explain why dentists step out or shield during X-ray scans.
What your child will practise
- Displacement Current and Maxwell's Laws: Candy Quick Fire (Quick Fire)
- Displacement Current and Maxwell's Laws: Space Puzzle (Clue Puzzle)
- Nature of Electromagnetic Waves: Ocean Puzzle (Clue Puzzle)
- Spectrum: Radio Waves to Visible Light: Jungle Quest (Flip and Match)
- Spectrum: UV Rays to Gamma Rays: Ocean Quick Fire (Quick Fire)
- Quiz: Displacement Current and Maxwell's Laws (8 questions)
- Quiz: Nature of Electromagnetic Waves (8 questions)
- Quiz: Spectrum: Radio Waves to Visible Light (8 questions)
- Quiz: Spectrum: UV Rays to Gamma Rays (8 questions)