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

“An electric line of force is an imaginary smooth curve drawn in an electric field along which a free, isolated positive charge moves. The tangent drawn at any point on the electric line of force gives the direction of the force acting on a positive charge placed at that point.”

OR

An imaginary curve drawn in such a way that the tangent at any given point on this curve gives the direction of the electric field is called an electric line of force.

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 · [\[\frac {ab}{(b − a)}\]]

• Electric field lines originate from positive charges and terminate on negative charges (or at infinity).
• The tangent to a field line at any point gives the direction of the electric field; in a uniform field, the lines are parallel and straight.
• No two electric field lines intersect, as this would imply more than one direction of the electric field at a point.
• Electric field lines do not pass through a conductor, showing that the electric field inside a conductor is zero.
• The density of field lines indicates field strength—closer lines represent a stronger field, while wider spacing represents a weaker field; the lines are continuous and imaginary, though the field is real.