Revision: Electrostatics Physics HSC Science Class 12 Tamil Nadu Board of Secondary Education

The smallest unit of electric charge, denoted by e, is called the elementary charge.

OR

The smallest unit of free electric charge, denoted e, with value e ≈ 1.602 × 10⁻¹⁹ C.

A charged body whose size is negligibly small compared to the distance between the charges under consideration, is called a point charge.

OR

A charged body is treated as a point charge when its physical size is negligible compared to the distance between it and other charges under consideration.

Electric line of force is an imaginary curve drawn in an electric field such that the tangent at any point on it gives the direction of the electric field at that point.

• It represents the path along which a unit positive test charge would tend to move if free to do so.
• Lines are imaginary — they have no physical existence; they are a visualization tool only.

The electric dipole moment is defined as the product of the magnitude of one of the charges and the distance between the two equal and opposite charges.

“The line joining the two charges, pointing from the negative charge to the positive charge. This is known as the ‘direction of dipole axis’.”

OR

The line passing through both charges +q and −q is called the dipole axis (also called the axial line or axis of the dipole).

Electric dipole moment \[\vec p\] is a vector quantity defined as the product of the magnitude of either charge and the separation between them.

Mathematical definition: \[\vec p\] = q × 2a

An electric dipole is a pair of equal and opposite point charges placed at a short distance apart.

OR

A system formed by two equal and opposite point charges placed at a small distance apart is called an electric dipole.

OR

A system of two equal and opposite point charges +q and −q separated by a small fixed distance 2a is called an electric dipole.

• The total charge of an electric dipole is zero
• Zero net charge does not mean zero electric field - the field exists because the charges are spatially separated​
• The midpoint of the line joining −q and +q is called the centre of the dipole

The midpoint of the line joining the two charges is called the centre of the dipole.

The line passing through the centre of the dipole and perpendicular to the dipole axis is called the equatorial line.

OR

The plane passing through the centre of the dipole and perpendicular to the dipole axis is called the equatorial plane; the line along which the equatorial field is evaluated is the equatorial line (perpendicular bisector).

Electrostatic potential at a point is the work done by an external agent in bringing a unit positive test charge slowly from infinity to that point without acceleration.

The potential difference between two points P and R is the work done by an external force in moving a unit positive test charge from one point to the other.

The ratio of the charge Q given to one of the conductors of a capacitor to the potential difference V between the conductors is called its capacitance, given by C = Q/V.

The ability of a conductor to store charge is called the capacity of conductor.

The maximum electric field that a dielectric medium can withstand without breakdown (of its insulating property) is called its dielectric strength.

A system consisting of two conductors having equal and opposite charges separated by an insulator or dielectric is called a capacitor.

If the work done in bringing charge q from infinity to point P is W, then

V_P ​= \[\frac {W​}{q}\]

If the potential energies at points P and R are U_P and U_R​, then

\[V_P-V_R=\frac{U_P-U_R}{q}\]

C = Q/V

C = \[\frac {2πkε₀ l}{2.303 log(b/a)}\]

C = 4πkε₀ · [\[\frac {ab}{(b − a)}\]]

• Quantisation of charge: Electric charge exists in discrete packets, and the charge on any body is given by
  q = ±ne
  where n is an integer and e = 1.6 × 10⁻¹⁹ C is the elementary charge.
• No fractional charge: Charge cannot exist as a fraction of e (like 0.5e or 2.3e); hence, electric charge is atomic in nature.
• Conservation of charge: The total electric charge of an isolated system remains constant; charge can neither be created nor destroyed, only transferred.
• Experimental support: Processes such as rubbing, pair production and annihilation, and radioactive decay always conserve the net charge of the system.
• Invariance of charge: The value of electric charge does not change with velocity, unlike mass, which varies with speed.