Saturday, June 27, 2009

1. E L E C T R O S T A T I C S .

1. E L E C T R O S T A T I C S .

1. WHAT IS ELECTROSTATICS?
Electrostatics is the branch of Physics, which deals with static electric charges or charges at rest.
2. WHAT IS FRICTIONAL ELECTRICITY?
The electricity produced by friction is called frictional electricity. If the charges in a body do not move, then, the frictional electricity is also known as Static Electricity.
3. STATE THE APPLICATIONS OF PROPERTIES OF ATTRACTION & REPULSION OF CHARGES?
The property of attraction and repulsion between charged bodies
have many applications such as electrostatic paint spraying, powder
coating, fly−ash collection in chimneys, ink−jet printing and photostat
copying (Xerox) etc.
4. WHAT ARE CONDUCTORS & INSULATORS? GIVE EXAMPLES
Bodies which allow the charges to pass through are called conductors. e.g. metals, human body, Earth etc.
Bodies which do not allow the charges to pass through are called insulators. e.g. glass, mica, ebonite, plastic
5. WRITE NOTE ON QUANTISATION OF ELECTRIC CHARGE
Quantisation of electric charge
The fundamental unit of electric charge (e) is the charge carried by the
electron and its unit is coulomb. e has the magnitude 1.6 × 10−19 C.
In nature, the electric charge of any system is always an integral
multiple of the least amount of charge. It means that the quantity can
take only one of the discrete set of values. The charge, q = ne where
n is an integer.
6. WRITE NOTE ON CONSERVTION OF ELECTRIC CHARGES?
Electric charges can neither be created nor destroyed. According
to the law of conservation of electric charge, the total charge in an
isolated system always remains constant. For example, Uranium (92U238) can decay by emitting an alpha particle (2He4 nucleus) and transforming to
thorium (90Th234). 92U238 −−−−> 90Th234 + 2He4
Total charge before decay = +92e, total charge after decay = 90e + 2e.
Hence, the total charge is conserved.

7. STATE COULOMB’S LAW IN ELECTROSTATICS
The force between two charged bodies was studied by Coulomb in
1785. Coulomb’s law states that the force of attraction or repulsion
between two point charges is directly proportional to the product of thecharges and inversely proportional to the square of the distance between them F= K q1 q2 /r2

8. DEFINE ONE COULOMB.
One Coulomb is defined as the quantity of charge, which when
placed at a distance of 1 metre in air or vacuum from an equal and
similar charge, experiences a repulsive force of 9 × 109 N.

9. DEFINE ELECTRIC FIELD
Electric field due to a charge is the space around the test chargein which it experiences a force. The presence of an electric field
around a charge cannot be detected unless another charge is brought
towards it.
10. DEFINE ELECTRIC FIELD INTENSITY
Electric field at a point is measured in terms of electric field
intensity. Electric field intensity at a point, in an electric field is defined
as the force experienced by a unit positive charge kept at that point.
It is a vector quantity. E= F/q. The unit of electric field intensity
is N C−1.
11. DEFINE ELECTRIC LINES OF FORCE.
The concept of field lines was introduced by Michael Faraday as
an aid in visualizing electric and magnetic fields.
Electric line of force is an imaginary straight or curved path along
which a unit positive charge tends to move in an electric field.
12. STATE THE PROPERTIES OF ELECTRIC LINES OF FORCE.
(i) Lines of force start from positive charge and terminate at negative
charge.
(ii) Lines of force never intersect.
(iii) The tangent to a line of force at any point gives the direction of
the electric field (E) at that point.
(iv) The number of lines per unit area, through a plane at right angles
to the lines, is proportional to the magnitude of E. This means
that, where the lines of force are close together, E is large and
where they are far apart, E is small.
(v) Each unit positive charge gives rise to 1 εo lines of force in free
space. Hence number of lines of force originating from a point
charge q is N =q ε in free space.

13. DEFINE ELECTRIC DIPOLE.GIVE EXAMPLES.
Two equal and opposite charges separated by a very small distance
constitute an electric dipole. Water, ammonia, carbon−dioxide and
chloroform molecules are some examples of permanent electric dipoles
14. DEFINE DIPOLE MOMENT.WRITE EXPRESSION,UNIT.
The magnitude of the dipole moment is given by the product of the
magnitude of the one of the charges and the distance between
them. Electric dipole moment, p = q2d or 2qd. It is a vector quantity
and acts from –q to +q. The unit of dipole moment is C m.
15. DEFINE TORQUE EXPERIENCED BY ELECTRIC DIPOLE?
The magnitude of torque is,
τ = One of the forces x perpendicular distance between the forces
= F x 2d sin θ = qE x 2d sin θ = pE sin θ (q × 2d = P)
Note : If the dipole is placed in a non−uniform electric field at an
angle θ, in addition to a torque, it also experiences a force.
16. DEFINE ELECTRIC POTENTIAL ENERGY OF AN ELECTRIC DIPOLE?
Electric potential energy of an electric dipole in an
electrostatic field is the work done in rotating the dipole to
the desired position in the field.
17. DESCRIBE THE WORKING OF MICROWAVE OVEN.
It is used to cook the food in a short time. When the oven is
operated, the microwaves are generated, which in turn produce a non−
uniform oscillating electric field. The water molecules in the food which
are the electric dipoles are excited by an oscillating torque. Hence few
bonds in the water molecules are broken, and heat energy is produced.
This is used to cook food.
18. DEFINE POTENTIAL DIFFERENCE.
The potential difference between two points in an electric field is
defined as the amount of work done in moving a unit positive charge
from one point to the other against the electric force.
The unit of potential difference is volt
19. DEFINE THE UNIT OF POTENTIAL DIFFERENCE.
The potential difference between two points is 1 volt if 1 joule of work is done in moving 1 Coulomb of charge from one point to another against the electric force.
20. DEFINE ELECTRIC POTENTIAL.
The electric potential in an electric field at a point is defined as the amount of work done in moving a unit positive charge from infinity to that point against the electric forces.
21. DEFINE ELECTRIC POTENTIAL ENERGY.
The electric potential energy of two point charges is equal to the work done to assemble the charges or workdone in bringing each charge or work done in bringing a charge from infinite distance

22. DEFINE EQUIPOTENTIAL SURFACE,
If all the points of a surface are at the same electric potential,then the surface is called an equipotential surface .If the charge is to be moved between any two points on an equipotential surface through any path, the work done is zero.
23. DEFINE ELECTRIC FLUX
The electric flux is defined as the total number of electric lines of
force, crossing through the given area. Ø = E . ds
24. DEFINE GAUSS’S LAW
Gauss’s law states that the total flux of the electric field E over any closed surface is equal to1/εo times the net charge enclosed by the surface. Ø =q/εo
25. DEFINE ELECTROSTATIC SHEILDING
Electrostatic sheilding is the process of isolating a certain region of space from external field. It is based on the fact that electric field inside a conductor is zero.
26. WHY IS IT SAFER TO SIT INSIDE A BUS THAN STANDING UNDER A TREE?
During a thunder accompanied by lightning, it is safer to sit inside a bus than in open ground or under a tree. The metal body of the bus provides electrostatic shielding, where the electric field is zero.During lightning the electric discharge passes through the body of the bus.
27. DEFINE ELECTROSTATIC INDUCTION
It is possible to obtain charges without any contact with another
charge. They are known as induced charges and the phenomenon of
producing induced charges is known as electrostatic induction. It is
used in electrostatic machines like Van de Graaff generator and
capacitors.
28. DEFINE CAPACITANCE OF A CONDUCTOR.
The capacitance of a conductor is defined as the ratio of the
charge given to the conductor to the potential developed in the
conductor. C = q /V : unit=farad.
The unit of capacitance is farad. A conductor has a capacitance
of one farad, if a charge of 1 coulomb given to it, rises its potential by
1 volt. The practical units of capacitance are μF and pF.
30. WHAT ARE DIELECTRICS?
A dielectric is an insulating material in which all the electrons are
tightly bound to the nucleus of the atom. There are no free electrons
to carry current. Ebonite, mica and oil are few examples of dielectrics.
31. DEFINE A NON-POLAR MOLECULE.GIVE EXAMPLE.
A nonpolar molecule is one in which the centre of gravity of the positive
charges (protons) coincide with the centre of gravity of the negative charges (electrons). Example: O2, N2, H2. The nonpolar molecules do not have a permanent dipole moment.
32. DEFINE POLAR MOLECULE,GIVE EXAMPLE.
A polar molecule is one in which the centre of gravity of the
positive charges is separated from the centre of gravity of the negative
charges by a finite distance. Examples : N2O, H2O, HCl, NH3. They have
a permanent dipole moment.

33. DEFINE ELECTRIC POLARISATION.
The alignment of the dipole moments of the permanent or
induced dipoles in the direction of applied electric field is called
polarisation or electric polarisation.
34. STATE THE APPLICATIONS OF CAPACITORS
(i) They are used in the ignition system of automobile engines
to eliminate sparking.
(ii) They are used to reduce voltage fluctuations in power
supplies and to increase the efficiency of power transmission.
(iii) Capacitors are used to generate electromagnetic oscillations
and in tuning the radio circuits.
35. WHAT IS EFFECTIVE CAPACITANCE OF CAPACITORS CONNECTED IN SERIES AND PARALLEL?
When a number of capacitors are connected in series, the reciprocal of
the effective capacitance is equal to the sum of reciprocal of the
capacitance of the individual capacitors. 1/CS = 1/C1 + 1/C2 + 1/C3.
The effective capacitance of the capacitors connected in parallel is the sum
of the capacitances of the individual capacitors. CP = C1 + C2 + C3
36. WHAT IS CORONA DISCHARGE OR ACTION OF POINTS?
The leakage of electric charges from the sharp points on the
charged conductor is known as action of points or corona discharge.
This principle is made use of in the electrostatic machines for collecting
charges and in lightning arresters.