November 2012

Electric Charges and Field

We began by defining the electric field at any point in space as the total Coulomb force per unit positive charge at that point.

This shift from discussing forces between existing charges to fields that exist at points in space is a major reorientation

Electric charge is a basic property of all matter.

Unlike charges attract, like charges repel.

Conductor allow flow of electric charges through them while insulators don't

Electric charges have three basic properties

Coulomb's law:

the Electrostatic force of interaction between two point electric charges is directly proportional to the product of the charges,
inversely proportional to the square of the distance between them
act along the straight line joining the two charges.

Superposition Principle:

The electric intensity at a point due to several

charges is the
vector sum of electric intensities produced by each

charge individually in the absence of other charges.

Electric field

the region of space around a charge or a system of

charges within which other charged particles

experiences an electrostatic force.

Electric field line

an imaginary straight or curved path along which a unit positive charge is supposed to move when free to do so in any electric field.

Various properties of Electric field lines:

(a) lines are continuous

(b) two field lines cannot cross each other.

(c) Field lines start from positive charge to negative and

cannot form closed loops.

Electric dipole

a pair of equal and opposite point charges separated by a small distance.
Electric field generated by an electric dipole.
The additional charge in the system destroys the spherical symmetry and thereby complicates the general calculation of the field.
In a uniform electric field, a dipole experiences a torque, and net force becomes zero.

Continuous charge distribution

has cylindrical symmetry about the line of charge; the direction of the field is perpendicular to the line and the magnitude of the field decreases as 1/r.