Electrostatics :
Definition:
It is branch of science which deals with the study of charges at rest. It is also known as static electricity or frictional electricity.
Electric Charge:
Electric charge is an Intrinsic property of the elementary particles like electrons, protons etc.of which all the objects are made up of.
Quantization of charge:
The quantization of charge means that charge can exist only as integral multiple of electronic charge i.e. q=+-ne,where n is an integer.
• It was first suggested by by faraday.
Charge conservation:
In an isolated system,the total net charge of the system always remains constant.
Charge is always associated with mass :
Charge can not exist without mass though mass can be exist without charge.
Additivity of charge:
Total charge of a system is the algebraic sum of all individual charges in the system.
Charge is invariant:
Charge is independent of the frame of reference.
Coulomb's Law:
Coulomb's law states that the magnitude of the electric force between two charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them and acts along the line joining between two charges.
Fundamental law of electrostatics:
It states that, "Like charges repel and unlike charges attract each other
Methods of charging:
There are three methods of charging a body by (i) Friction (ii) Induction (iii) Conduction.".
Continuous Charge Distribution:
It has three types,
(i) Linear charge distribution: Charge is distributed along a line (One dimensional distribution).
(ii) Superficial charge distribution: Charge is distributed over a surface (two dimensional distribution).
(iii) Cubical charge distribution : Charge is distributed over a volume (three dimensional distribution.)
Electric Field:
Electric field due to a given charge is defined as the modified space around the charge in which electrostatic force of attraction/repulsion due to the charge may be experienced by any other rest charge. The S.I. unit of electric field is NC-1.
Electric Lines of Force:
It is locus of the test charge within an electric field.
Properties of electric lines of force:
(i) It starts from positive charge and ends at negative charge.
(ii) If there is a single charge, they may start or end at infinity.
(iii) For single charge, electric lines of force are straight and for two charges they are continuous smooth curves without any break.
(iv) Electric lines of force never intersect eachother.
(v) Electric lines of force do not form closedoloops. So they are discontinuous.
(vi) Electric lines of force starts or ends normally on the surface of a conductor.
(vii) Electric lines of force never pass through a conductor. So they do not exist inside a conductor. Because electric field inside a conductor is zero.
(viii) Electric lines of force are closer when field is stronger and spread out when field is weaker.
(ix) For uniform electric field electric lines of force are equidistant, parallel straight lines.
(x) It exerts lateral pressure or tendency to expand laterally. Hence, like charges repel.
(xi) Electric lines of force have a tendency to contract lengthwise. Hence, unlike charges attract.
(xii) The tangent to a lines of force at any point gives the direction of the electric field at that point.
Electric Dipole:
A pair of equal and opposite point charges separated by a very small distance constitutes an electric dipole.
Electric Dipole Moment:
The electric dipole moment of an electric dipole is given by the product of one of the two charges constituting the dipole and the vector distance between them. It is a vector quantity and its direction is from negative charge to the positive charge.
Torque on an electric dipole:
Electric Dipole moment may be defined as the torque acting on an electric dipole, placed perpendicular to a uniform electric field of unit strength.
Mathematically, t = PE sin Thita
It E = 1 unit, 0 = 90°, then, t = P
Electric Potential :
Electric potential at a point in an electric field is defined (or measured) as the quantity of work done in moving a unit positive test charge from infinity to that point against the electrostatic force of repulsion. It is a scalar quantity and its S.I. unit is volt (V). Potential Difference: Electric potential difference between two points in an electric field is defined (or measured) as the quantity of work done in moving a unit positive test charge from one point to other.
Equipotential Surface:
Equipotential surface is a surface obtained by joining all the points in an electric field having same electric potential.
Properties of equipotential surfaces:
(i) Two equipotential surfaces or lines never intersect with each other.
(ii) Work done on moving a charge on an equipotential surface is zero.
(iii) Electric field is always normal to the equipotential surface at every point
(iv) Equipotential surfaces are closer together and farther apart in the regions of strong field and weak field respectively.
Potential Gradient:
The rate of change of potential with distance at a point is called potential gradient.
Principle of Capacitor :
The capacitance of a conductor gets increased greatly, when an earth conductor is placed near it.
Uses of capacitor :
(i) In radio circuits for tuning.
(ii) To produce electric field of desired patterns.
(iii) In the tank circuit of oscillators.
(iv) For producing rotating magnetic fields in induction motors.
(v) In power supplies for smoothening the rectified current.
(vi) They store not only charge, but also energy in the electric field between their plates.
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