Electric Charges and Fields — Class 12 ISC Physics revision notes
A 5-page visual revision sheet for Electric Charges and Fields, written for Class 12 ISC 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
Coulomb, superposition and Gauss, with the integrals done
Charge, and the three properties it has
- quantised: every charge is a whole number multiple of e = 1.6 x 10^-19 C, so a charge of half an electron does not exist
- conserved: the total charge of an isolated system never changes, however the charge is redistributed
- additive: charges add algebraically, with sign, which is what makes superposition possible
- charging happens by friction, conduction or induction, and induction is the one that leaves the inducing body unchanged
- like charges repel and unlike attract, and the force acts along the line joining them
Coulomb's law and the field it defines
- force between point charges
F = (1/4πε_0) q_1 q_2 / r^2 - the constant
1/4πε_0 = 9 x 10^9 N m^2 per C^2, with ε_0 = 8.854 x 10^-12 in SI - in a medium
divide by the dielectric constant K, so the force falls by that factor - electric field
E = F/q_0, the force per unit POSITIVE test charge - field of a point charge
E = (1/4πε_0) q / r^2, directed away from a positive charge - superposition
the net force or field is the VECTOR sum of the individual ones, each computed as if alone
Page 1 of 5. The full sheet, all 5 pages, opens free in the app.
What this Electric Charges and Fields sheet covers
- Charge, and the three properties it has
- Coulomb's law and the field it defines
- force between point charges
- the constant
- in a medium
- electric field
- field of a point charge
- superposition
- Force between two point charges
- Field lines, and what they are allowed to do
- The electric dipole
- Dipole results worth having ready
- torque
- potential energy
- work to rotate
- axial field, far away
- equatorial field, far away
- Continuous charge distributions
- Electric flux and Gauss's law
- The three standard Gaussian results
- infinite line of charge
- infinite plane sheet
- two oppositely charged plates
- shell, outside
- shell, inside
- solid sphere, inside
- Choosing a Gaussian surface
- Conductors in an electrostatic field
- Where the charge collects on an irregular conductor
- Dielectrics, in outline
- The field of a charged ring, and where it peaks
- 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.