CBSE Class 12 · Physics
Electromagnetic Waves
Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.
This lesson follows the official chapter “Electromagnetic Waves” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.
- CBSE Class 12
- Medium level
- 8 concepts
1Displacement Current
Displacement current: Maxwell’s extra as taught, Id = ε0 dΦE/dt, so Ampere stays honest in a charging capacitor. Displacement is a changing-E stand-in, not a second wire-current. One charging-gap mention is enough.
A still E as a leftover Id is a miss.
Figure. Between the plates there is no conduction path. Maxwell’s displacement current Id equals the wire current Ic that charges the plates.
How it works
- Name a changing electric fluxThe given.
- Write Id=ε0 dΦE/dt as taughtDisplacement current.
- Keep it the gap-story in a charging CThe use.
2Electromagnetic Waves
Electromagnetic waves: changing E makes B, changing B makes E as taught — a travelling pair. c = 1/sqrt(μ0 ε0) as framed. An EM wave needs no wire. Wave is a pair-in-space, not a sound.
A string-wave as this heading is a steal.
Figure. An electromagnetic wave carries E and B at right angles, both at right angles to the travel direction c. B is marked as a box out of the page — the vocabulary has no circle-dot.
How it works
- Name coupled E and B travellingThe wave.
- Keep c as the school speed in vacuumThe size.
- Refuse a wire as the needFree space.
3Electromagnetic Spectrum
Spectrum: radio … gamma as taught, one range with many uses. A spectrum is a frequency-map, not a rainbow-poster only. One named band plus its school use is enough (radio, light, X-ray as framed).
Calling every EM wave “light” is a steal.
Figure. Frequency rises from radio toward X and gamma. Band widths here are equal on purpose: a real spectrum spans decades and has no honest linear scale.
How it works
- Name the band and a use as taughtThe spectrum.
- Keep them all as EM wavesOne family.
- Refuse visible as the whole mapA slice.
4A definition is a test you can run
EM-wave-word is a test: displacement current, a travelling E–B pair, or a band. If you only say “radiation”, you have a heading.
A glow-sticker is not the test.
Figure. Maxwell’s test: a changing electric field is a source of B; a changing magnetic field is a source of E. Either miss and it is not an electromagnetic wave.
How it works
- Name Id, the pair, or a bandThe object.
- Give the school sentenceThe test.
- Then the word has contentThe definition ran.
5Name the given before the unknown
The given is a charging C or a named band. The unknown is Id or the family-word. Copy “radio” before you treat it as gamma.
Using a sound-speed as c is a silent swap.
Figure. Name c and the given wavelength before solving for frequency. The unknown is f, not a second copy of c.
How it works
- Copy the case or the bandThe given.
- Name Id, c, or the band-useThe unknown.
- Then writeGiven first.
6One worked case is enough at this class
One worked case is enough: a charging-capacitor gap as Id, or “light is one EM band”. Do not stack ten bands.
A poster of the whole rainbow is not more science.
Figure. One case: a 3 cm wave. c = 3×10^8 m/s and λ = 0.03 m force f = 1×10^10 Hz. Speed is shared; frequency is not.
How it works
- Take one gap or one bandThe case.
- Name Id or the familyThe teach.
- Stop — one caseThis class.
7A miss: swapping the school name for the picture
A miss: swapping the school name for a picture. “Displacement current” is ε0 dΦE/dt — a glowing gap is a setting.
A pretty ripple without E–B talk is a mute picture.
Figure. Infrared is not a second kind of ray. It is the same electromagnetic wave, just longer than red. The school name “heat ray” is a nickname, not a new field.
How it works
- Keep Id or EM-wave as the nameThe science.
- Use a picture only as a settingHonest.
- Refuse a swap of name for glowThe miss named.
8Check by the opposite action or the opposite test
Check by the opposite: a steady E in a gap should not give Id; a sound-wave should not sit in this family. The opposite look kills the claim.
A still capacitor as “EM wave source” with no change is a miss.
Figure. If the detector stays silent, the wave did not arrive — or E and B were not the pair this chapter named. The opposite test is the missing signal, not a new formula.
How it works
- Ask whether E or B is changingThe need.
- Ask whether it is a sound-lookThe opposite family.
- If the opposite sits, redo the wordThe check.
Displacement current needs
- A changing electric flux as taught
- A steady E only
- A sound wave
The gap-story.
Notes
- Mapped to the official NCERT chapter “Electromagnetic Waves”. Original teaching only — no textbook sentences.
- Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.
Formulas
- Id=ε0 dΦE/dt
- c=1/sqrt(μ0 ε0) as taught
Recap
Hold these pegs from the official chapter “Electromagnetic Waves”. The wording is ExamMaster’s teaching, not a textbook recap.
- Displacement Current
- Displacement current: Maxwell’s extra as taught, Id = ε0 dΦE/dt, so Ampere stays honest in a charging capacitor.
- Electromagnetic Waves
- Electromagnetic waves: changing E makes B, changing B makes E as taught — a travelling pair.
- Electromagnetic Spectrum
- Spectrum: radio … gamma as taught, one range with many uses.
- A definition is a test you can run
- EM-wave-word is a test: displacement current, a travelling E–B pair, or a band.
- Name the given before the unknown
- The given is a charging C or a named band.
- One worked case is enough at this class
- One worked case is enough: a charging-capacitor gap as Id, or “light is one EM band”.
Practise Electromagnetic Waves
Reading is free and needs no account. Practice, mocks and progress live in the app.
- A 4-question practice set that ends the chapter
- 1 quick check with worked explanations
- Timed mocks scored with the real marking scheme
- Readiness tracked per topic, kept on your device