CBSE Class 12 Physics – Chapter 1: Electric Charges and
Fields
1. Electric Charge
Definition: Property of matter that causes it to experience a force when placed in an
electric field.
Types: Positive (+), Negative (−).
Basic properties:
1. Quantisation: q=neq = neq=ne, where e=1.6×10−19e = 1.6 \times 10^{-
19}e=1.6×10−19 C.
2. Conservation: Total charge in an isolated system remains constant.
3. Additivity: Net charge is the algebraic sum of charges.
Conductors vs. Insulators:
o Conductors allow charge flow (metals, human body).
o Insulators don’t (rubber, plastic).
2. Coulomb’s Law
F=14πϵ0⋅∣q1q2∣r2F = \frac{1}{4\pi\epsilon_0} \cdot \frac{|q_1 q_2|}{r^2}F=4πϵ01⋅r2∣q1q2∣
Vector form:
F 12=14πϵ0⋅q1q2r2r^12\vec{F}_{12} = \frac{1}{4\pi\epsilon_0} \cdot \frac{q_1 q_2}{r^2}
\hat{r}_{12}F12=4πϵ01⋅r2q1q2r^12
Key points:
o Force is along the line joining charges.
o Like charges repel, unlike charges attract.
o ϵ0\epsilon_0ϵ0 (permittivity of free space) = 8.85×10−12 C2/N⋅m28.85 \times
10^{-12} \, C^2/N·m^28.85×10−12C2/N⋅m2.
3. Superposition Principle
Net force on a charge = vector sum of individual forces due to all other charges.
4. Electric Field (E \vec{E}E)
Definition: Force per unit positive test charge.
E =F q0\vec{E} = \frac{\vec{F}}{q_0}E=q0F
