CBSE Class 12 Physics

## Electric charge Matter carries electric charge of two kinds, called positive and negative. Charges of the same kind repel and charges of different kinds attract. A body is charged when it has an excess of one kind, and it is neutral when the positive and negative charges in it balance exactly. Rubbing two different insulators together moves electrons from one to the other, so one ends up negative and the other positive by the same amount; nothing is created, only transferred. The SI unit of charge is the coulomb (C). The smallest free charge found in nature is the magnitude of the charge on an electron, the elementary charge $e$. $$ e = 1.6\times10^{-19}\ \mathrm{C} $$ ### Quantisation of charge The charge on any body is a whole-number multiple of $e$. $$ q = ne \qquad n = 0, \pm1, \pm2, \dots $$ Because $e$ is so small, a charge of even a microcoulomb contains about $6\times10^{12}$ elementary charges, so charge on everyday bodies looks continuous and the steps are invisible. Quantisation matters for atoms and for very small charges. ### Conservation of charge The total charge of an isolated system does not change. Charges can be moved from one part to another, and pairs of equal and opposite charges can appear or vanish together, but the algebraic sum stays the same. ### Additivity of charge Charge is a scalar and it adds algebraically, with signs. A system holding charges $+5\ \mu\mathrm{C}$, $-3\ \mu\mathrm{C}$ and $+1\ \mu\mathrm{C}$ has a total charge of $+3\…