Ch 10Electrostatics — Class 11 Physics, concept-first.

Electrostatics is the branch of physics that studies static (stationary) electric charges -- the forces they exert on each other, and the electric fields and electric potentials that surround them.

Opposite electric charges were already known to the ancient Greeks around 600 BC, who observed that rubbing amber against fur produces equal and opposite charges on the two materials.

Having established what charge IS and how it can be produced (by conduction or induction), this section turns to three fundamental, general PROPERTIES that all electric charge obeys, no matter how it…

Electric charge is ADDITIVE, exactly like mass: the total charge carried by an object is simply the algebraic sum of all the individual charges distributed across its different parts.

Careful experiments -- most famously Millikan's oil-drop experiment -- established that charge cannot take on just any arbitrary value: there exists a smallest possible unit of charge, the ELEMENTARY…

The law of conservation of charge states that in any given physical process, electric charge may be TRANSFERRED from one part of a system to another, but the TOTAL charge of the system can never be cr…

Careful, controlled experiments with charged objects reveal that any two charged bodies brought close together -- without actually touching -- exert a measurable FORCE on each other, and this force ca…

Section 10.3.4 established, qualitatively, that charged bodies exert attractive or repulsive forces on each other depending on their signs.

Statement: The force of attraction or repulsion between two point charges at rest is directly proportional to the product of the magnitudes of the two charges, and inversely proportional to the square…

In stating Coulomb's law above, it was implicitly assumed that the two point charges sit in vacuum (or, to an excellent approximation, in air).

Substituting into Coulomb's law in vacuum gives the numerical constant , so if C and m, the resulting force works out to N.

As shown in Fig. 10.4, let and be two point charges situated at points A and B respectively, with the distance separating them.

The principle of superposition states that when a number of charges act simultaneously, the resultant (net) force on any one particular charge is found by taking the VECTOR SUM of the individual force…

Space around any charge Q gets 'modified' in a physical sense: if a second, test charge is brought into this surrounding region, it experiences a measurable Coulomb force, even though the two charges…

Consider a point charge q placed at a point O inside a medium of dielectric constant K (Fig. 10.7). At a point P, a distance r from O, a small positive test charge experiences a Coulomb force (using t…

Beyond the fundamental point-charge formula, there is a very practical, everyday way of both calculating AND measuring an electric field, using a pair of charged parallel plates.

Michael Faraday (1791-1867) introduced the concept of lines of force as a way to visualise both electric and magnetic fields.

As discussed in the previous section, the intensity (magnitude) of the electric field E can be thought of as the number of lines of force crossing a unit area held perpendicular to those lines, Rearra…

Karl Friedrich Gauss (1777-1855), one of the greatest mathematicians of all time, formulated a law -- now called GAUSS' LAW -- expressing a deep, general relationship between electric charge and the e…

A pair of equal and opposite point charges, and , separated by some finite distance, is called an ELECTRIC DIPOLE (Fig. 10.18).

Consider an electric dipole -- charges at point A and at point B, separated by -- placed in a UNIFORM external electric field , with the dipole's axis making some angle with the direction of the field…

This section finds the electric field intensity due to a dipole (charges at A and at B, separated by , with dipole moment ) at two specific, important points in space -- one on its axial line, and one…

So far, every calculation in this chapter has treated charge as concentrated at one or more discrete POINTS.

When a total charge q is distributed UNIFORMLY along a straight or curved conductor of length l (Fig. 10.22), the LINEAR CHARGE DENSITY is defined as the charge per unit length: Its SI unit is coulomb…

When a total charge q is distributed UNIFORMLY over a surface of area A (Fig. 10.23), the SURFACE CHARGE DENSITY is defined as the charge per unit area: Its SI unit is coulomb per square metre ().

When a total charge q is distributed UNIFORMLY throughout the entire volume V of a body (Fig. 10.24), the VOLUME CHARGE DENSITY is defined as the charge per unit volume: Its SI unit is coulomb per cub…