Electric Charges and Fields — Class 12 CBSE Physics revision notes
A 6-page visual revision sheet for Electric Charges and Fields, 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
Electric Charges and Fields in one look
one law between two charges, one field mapping the space around them
- Charge: the source, quantised and conserved
- Force: Coulomb's law, on q2 due to q1
- Field: force per unit +test charge
Charge, properly
- rubbing moves electrons: glass rubbed with silk turns +, the silk −, by equal amounts
- additive: net charge is the plain scalar sum, signs included
- conserved: the total charge of an isolated system never changes; charge is only moved
- quantised: q = ne, n an integer, e = 1.6×10^-19 C the smallest free charge
Conductors and induction
- conductors let charge move (metals, earth, body); insulators hold it (glass, plastic)
- induction: a charged rod nearby shifts free electrons, near side opposite, far side the same
- rod near, earth on, earth off, rod away: a charge opposite to the rod's stays
- conduction needs a touch; induction never does
Like charges vs unlike charges
- repel: same sign, q_1q_2 > 0
- attract: opposite sign, q_1q_2 < 0
Page 1 of 6. The full sheet, all 6 pages, opens free in the app.
What this Electric Charges and Fields sheet covers
- Charge
- Force
- Field
- Charge, properly
- Conductors and induction
- Like charges vs unlike charges
- Like charges
- Unlike charges
- Coulomb's law
- force between two point charges
- Coulomb's constant
- vector form, along the line joining them
- the pair, always
- Superposition principle
- net force on q_0 from many charges
- Electric field
- defined at a point
- field of a point charge q, at distance r
- field of a system of charges, superposed
- Field line rules
- Field lines of charged shapes
- Electric dipole
- dipole moment, − to + charge
- field on the axis (r >> a)
- field on the equator (r >> a)
- Dipole in a uniform field
- torque, turning p toward E
- Electric flux
- flux through a small area
- Gauss's law
- any closed surface
- Units and dimensions
- q charge
- E electric field
- φ flux
- ε_0 permittivity
- Three standard results
- infinite line, linear density λ
- infinite sheet, surface density σ
- charged shell, outside (r>R)
- charged shell, inside (r<R)
- uniformly charged sphere, inside (r<R)
- Conductors in an electric field
- Limits of the standard results
- Field of a ring, an arc, a disc
- ring of charge Q, field on the axis
- that field peaks at
- arc, half-angle α, at its centre
- disc of charge, field on the axis
- Symbols
- 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 Electric Charges and Fields 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.