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Electric Charge
The word 'electric' is derived from the Greek word 'elektron' meaning amber. The existence of charges were known when charged particles were produced by rubbing (due to friction) of suitable materials. These facts are demonstrated by simple experiments.
Electricity and Matter
It is important to know the atomic picture of matter. The basic unit of all matter is an 'atom'. Each atom consists of a small core called nucleus which accounts to most of its mass consisting of positively charged protons and neutral neutrons and surrounded by lighter negatively charged particles called electrons.
Charging by Induction
A charged plastic rod is brought close to the sphere. Free electrons in the sphere move away due to repulsion and piles up at the other end of the sphere.
Coulomb's Law
It states that "the electrostatic force between two electric charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them ".
Basic Properties of Electric Charges
If a system consists of two point charges q1 and q2, then the total charge of the system is got by adding q1 and q2. Thus, the charge add up like real numbers (scalars). When we add charges, one should take care of its sign.
Multiple Charges
Coulomb's law describes only the interaction of two point charges. Experiments show that when two charges exert forces simultaneously on a third charge, the total force acting on that charge is the vector sum of the forces that the two charges would exert individually. This important property is known as the superposition principle. This principle holds good for any number of charges.
Electric Field
The electric field or electric field strength is the electrostatic force acting on a small positive test charge placed at that point.
Electric Dipole
It is a pair of point charges with equal magnitude and opposite in sign separated by a distance.
Dipole in a Uniform External Field
An electric field is said to be uniform if the electric field strength at every point in the field is the same.
Electric Field Lines
They are nothing but a way of pictorially mapping the electric field around a configuration of charges. It is the curve drawn in such a way that the tangent to it at each point is in the direction of the net field at the point. An arrow on the lines of force is a must to indicate the direction of the electric field.
Electric Flux
Electric field can be quantitatively described by using the concept of electric flux.
Gauss Theorem
We have already learnt to find the electric field intensity due to a charged conductor using Coulomb's law. Gauss' theorem can also be used to calculate the electric field intensity provided there is a symmetry in the charge distribution.
Continuous Charge Distribution
We have so far dealt with discrete charges but a system of charges can be considered as a continuous distribution if the group of charges are located very close together. To find the electric field due to a continuous charge distribution we have to define the following terms.
Application of Gauss Theorem
Gauss' theorem can be used to calculate the electric intensity due to an infinitely long straight charged wire.
Summary
Conductor allow flow of electric charges through them while insulators don't.
Conclusion
Principle of electrostatics and electromagnetism play a very vital role in all modern scientific developments.
Numerical 01
Two identical balls, each of mass 0.1 x 10-3 kg, carry identical charges and are suspended by two threads of equal length. At equilibrium, they position themselves as shown in the figure below. Calculate the charge on either ball.
Numerical 02
Calculate the resultant force on the 10 microcoulomb of charge.
Numerical 03
Find the force on the centre charge.
Numerical 04
he charges shown in the figure are stationary. Find the force on 4mC charge due to the other two.
Numerical 05
n a hydrogen atom, the distance between electron and proton is 5.3 x 10-11 m. Calculate the electrical force of attraction between them.
