Electromagnetic Waves — Class 12 CBSE Physics revision notes
A 3-page visual revision sheet for Electromagnetic Waves, written for Class 12 CBSE Physics. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.
Page 1 as PDF
Electromagnetic Waves in one look
a changing E makes a B, a changing B makes an E, and the pair carries itself forward
- Changing E: an electric field that varies with time
- Displacement current: Maxwell's term: it makes a B, just as a conduction current does
- Self-sustaining wave: each one's change regenerates the other
Displacement current, said precisely
- Ampere's law, one loop: the current i pierces its flat surface
- stretch that surface into a bag between the plates: no charge crosses it
- yet the electric flux through the gap changes as the plates charge
- Maxwell: a changing E is a current too, the displacement current i_d = ε_0dφ_E/dt
- Ampere-Maxwell law: oint B· dl = μ_0 i_c + μ_0ε_0dφ_E/dt
- constant, AC or rising: i_d = i at every instant, the same B
- not a flow of charge, yet it makes B; steady (dc) state: no i_d
Sources: an accelerating charge
- charge at rest: only a steady E, no wave
- steady speed (a steady current): a steady B added, still no wave
- an accelerated charge radiates: oscillating E makes oscillating B, which makes E again
- the wave has the frequency of the charge's oscillation; its energy comes from the source
- Hertz made and detected them (1887); J.C. Bose, waves of a few mm (1895)
Page 1 of 3. The full sheet, all 3 pages, opens free in the app.
What this Electromagnetic Waves sheet covers
- Changing E
- Displacement current
- Self-sustaining wave
- Displacement current, said precisely
- Sources: an accelerating charge
- What an electromagnetic wave is
- Speed and the relation between E and B
- speed in vacuum
- numerically
- in a material medium
- amplitudes are locked together
- frequency and wavelength
- Named cases of speed and wavelength
- Energy and momentum in the wave
- The electromagnetic spectrum, low to high frequency
- Symbols
- How each band is made and detected
- Traps that cost marks
- Tricks that save time
How CapCompass uses this sheet
These notes are not a standalone PDF. CapCompass plans Physics chapter by chapter against your real exam dates and free hours, and when the plan reaches Electromagnetic Waves the sheet opens inside the study block itself — at the page covering the sub-topic that block is on, with the timer still running. Revision arrives at the moment the plan says revise, instead of having to be gone looking for.