JEE Main & Advanced Physics Electrostatics & Capacitance Methods of Charging

A body can be charged by following methods.

(1) By friction : By rubbing two bodies together, both positive and negative charges in equal amounts appear simultaneously due to transfer of electrons from one body to the other.

(i) When a glass rod is rubbed with silk, the rod becomes positively charged while the silk becomes negatively charged. The decrease in the mass of glass rod is equal to the total mass of electrons lost by it.

(ii) Ebonite on rubbing with wool becomes negatively charged making the wool positively charged.

(iii) Clouds also get charged by friction. (iv) A comb moving through dry hair gets electrically charged. It starts attracting small bits of paper.

(v) During landing or take-off, the tyres of an aircraft get electrified therefore special material is used to manufacture them.

(2) By electrostatic induction : If a charged body is brought near an uncharged body, one side of neutral body (closer to charged body) becomes oppositely charged while the other side becomes similarly charged.

Induced charge can be lesser or equal to inducing charge (but never greater) and its maximum value is given by \[Q'=-Q\left[ 1-\frac{1}{K} \right]\] where Q is the inducing charge and K is the dielectric constant of the material of the uncharged body. It is also known as specific inductive capacity (SIC) of the medium, or relative permittivity er of the medium (relative means with respect to free space)

Different dielectric constants

(3) Charging by conduction : Take two conductors, one charged and other uncharged. Bring the conductors in contact with each other. The charge  (whether \[-ve\]or \[+ve\]) under its own repulsion will spread over both the conductors. Thus the conductors will be charged with the same sign. This is called as charging by conduction (through contact).