# 12th Class Physics Chapter 1 Electrostatics Short Question Answers

## 12th Class Physics Chapter 1 Electrostatics Short Question Answers Below

1.The potential is constant throughout a given region of space.Is the electrical field zero or nonzero in this region?

The electric field intensity at a point can also be define as negative potential gradient at that point i.e.E = -?v/?r.In other word this is rate of change of potential with distance while negative sign shows the direction of E along the direction of decreasing potential.Since potential V is constant throughout the region so ?V = 0.

2.Suppose that you follow an electric field line due to a positive charge.Do electric field and the potential increase or decrease?

If we follow an electric field line due to a positive point charge then it means that we are moving away from it.Thus the distance from the charge increases.Due to increase of distance from positive charge both electric intensity and voltage will decrease as it is clear from the following relations.

E α 1/r

E α 1/r

3.How can you identity that which plate of a capacitor is positively charged?

For testing the sign of a charge on a body a device call gold leaf electroscope is used.We bring the disc of a positively charged electroscope close to the plate of a capacitor.If the divergence of the gold leaf increases then the plate is positively charged and if the divergence in the leaf decreases then plate is negatively charged.

4.Describe the force or force on a positive point charge when placed between parallel plates? a) With similar and equal charges. b) With opposite and equal charges.

a) When a positive point charges is placed between parallel plates with similar and equal charges then the net force on the charge will be zero.Thus the value of resultant eletric intensity ‘E’ is zero because the electric intensity ‘E

E = E

Hence the net force on the positive charge is zero.Thus it will remain at rest.

b) When a positive point charge is placed between parallel plates value of resultant electric intensity is

E = E

because E

_{1}’ due to one plate is equal in magnitude but opposite in direction of electric intensity E_{2}due to the other plate i.e.E = E

_{1}– E_{2}= 0Hence the net force on the positive charge is zero.Thus it will remain at rest.

b) When a positive point charge is placed between parallel plates value of resultant electric intensity is

E = E

_{1}+ E_{2}because E

_{1}due to one plate is equal in magnitude but in the same direction of E_{2}due to other plate.Hence net force on the charge will be from positive to negative plate.Thus it will be accelerated towards negative plate.5.Electric lines of forces never cross?

Electric lines of force never cross each other.This is because E

^{→}has only one direction at any given point.If the lines cress E^{→}could have more than one direction which is physically not correct.6.If a point charge q of mass m is released in a non-uniform electric field will it make a rectilinear motion?

A rectilinear motion means a motion along a straight line.If a point charge q of mass m is placed at any point in the field created by positive point charge +q it will experience a repulsive force.Thus the charge will make rectilinear motion along the field line outward due to the force of repulsion.

7.Is E necessarily zero inside a charged rubber balloon if balloon is spherical?Assume that charge is distributed uniformly over the surface?

8.Is it true that Gauss's law states that the total number of lines of forces crossing any closed surface in the outward direction is proportional to the net positive charge enclosed with in surface?

9.Do electrons tend to go to region of high potential or of low potential?

The electrons are negatively charged particles.When they enter the electric field they tend to go from a region of low potential (negative end) to the region of high potential (positive end.). But in the case of conventional current positive charge flows from higher potential to lower potential regions.

10.State Coulomb’s law?

11.Is Coulomb’s electrostatic interaction consistent with Newton’s third law?

Yes Coulomb’s electrostatic interaction is consistent with Newtons third law.

If two alike point charges q

F

Negative sign shows the opposite direction.Hence the two forces are equal but opposite in direction.This is similar to Newton’s third law.

If two alike point charges q

_{1}and q_{2}are placed at a distance r then the charge q_{1}exerts a force F_{21}on the charge q_{2}which acts in direction away from q_{1}.Similarly at the same time the charge q_{2}exerts a force F_{12}on the charge q_{1}which is equal in magnitude to the force F_{21}exerted by q_{1}but acting away from charge q_{2}that isF

_{12}= -F_{21}Negative sign shows the opposite direction.Hence the two forces are equal but opposite in direction.This is similar to Newton’s third law.

12.What is the effect of medium between the charges upon coulomb’s force?

13.How would you show that the electrostatic force on a charged object is independent of its mass?

If we take objects of different masses having the same charge and bring them one by one in the field of charge q.We will find that the electrostatic force acting on each charged object is the same if the distance of each object from charge q is kept constant.It results that the electrostatic force on a charged object is independent of its mass.

14.If the distance between two point charges is halved,what will happen to the force between them?

15.What is an electric field?

The region around an electric charge in which it exerts force on other charges is called an electric field or field of force.Electric field is a vector quantity as it has magnitude only.

16.Is it possible for an electrostatic field lines to intersect?

No it is not possible for electrostatic field lines to intersect.We have read that the tangent to the field line of force at any point gives the direction of field line or electric intensity ‘E’ at that point.If two or more lines intersect at a point then there will be two or more tangents at that point giving two directions of the field lines or intensity at the same time which is physically not correct.

17.Can two equipotential surfaces inter-sect each other?

A surface is called equipotential surface if the potential at every point on the surface is the same for example the surface of a sphere having a point charge at its center is an equipotential surface.It is impossible for two equipotential surfaces to intersect each other otherwise there will be two values of potential at the point of intersection (due to two surfaces)which is not possible.

18.If the absolute potential at a point is zero what can you say about the electric intensity there?

If the absolute potential at a point is zero the electric intensity at that point may have some value or it may be zero.If we consider a point midway between two equal and opposite charges the potential at that point will be zero but the electric intensity will have some value. But if we consider a point at infinite distance from a charged body the potential as well as the electric intensity will be zero at that point.

19.What do you under stand by electric field lines?

The path along which a unit positive charge moves in an electric field is called electric field is called electric field line. Electric field lines give a three dimensional picture which shows the strength and direction of an electric field strength (or intensity) of particular charge.Thus they are vector quantities.

20.Describe five properties of electric field lines?

(i) No two lines cross eachother. (ii) Electric field lines start from positive charges and on negative charges. (iii)The tangent to a field line at any point gives the direction of the electric field at that point. (iv)The lines are closer when the field is strong and the lines are farther apart when the field is weak. (v) The lines of force do not exist inside the conductor.

21.When we move in an electric field the potential changes at the rate of 10 v/m.What is the electric intensity?

22.What will be flux through a surface placed at right angle to the electric field?

The electric flux through a surface of vector area A placed in an electric field of intensity E

φ = E

= Eacos Θ

Where E is the magnitude of E

As the surface is placed at right angle to the field.The angle between the normal to the surface and the electric intensity will be Zero i.e. Θ =0

φ

Thus maximum flux will pass through the surface placed at right angle to the electric fields.

^{→}is given byφ = E

^{→}.A^{→}= Eacos Θ

Where E is the magnitude of E

^{→}.A is the magnitude of A^{→}and Θ is the angle between the electric intensity(lines of force)and the normal to the surface (because the direction of a vector area is always along the normal to the surface)As the surface is placed at right angle to the field.The angle between the normal to the surface and the electric intensity will be Zero i.e. Θ =0

φ

_{e}= Eacoso= EAThus maximum flux will pass through the surface placed at right angle to the electric fields.

23.What will be the flux through a closed surface which does not contain any charge?

According to Gauss’s Law the flux through any closed surface is 1/Ԑo time the charge enclosed by the surface i.e. φ= Q/ Ԑo If a closed surface does not contain any charge then Q= 0 and therefore the flux through it is also zero.

24.Gauss’s law has been derived by assuming that the charge enclosed by a closed surface is positive.If q negative what happens to the field lines?

When the charge q enclosed b a closed surface is positive the following formula for Gauss ‘s law has been derived as,

φ = – Q/ Ԑ_{o}

Where φ is the flux through the closed surface and Ԑ_{o} is the permitivity of free space.

If charge q is taken as negative then the formula will become as

φ = – q/ Ԑ_{o}

In this case the field lines of forces will enter the closed a surface along the normal’s to the surface while in the case of positive q the field lines were coming out of the closed surface.

.

25.Why does spherical symmetry of a sphere shell demand the E due to charge q ar the center of shere be constant over the surface?

26.What is the difference between electrical potential energy and the electrical potential difference?

(i) Electrical potential energy is defined as the energy stored in the charge q by virtue of its position in an electric field.If in an electric field the electrical potential at a point is V then the electrical P.E.of charge will be given by

Electric P.E. = U = qV.

(ii) Electric potential difference between two points is defined as the work done in moving a unit positive charge from one point to the other keeping the charge in electrostatic equilibrium.

Mathematically it is written as

?V = -Ed or ?V = E.d

where ?V is the potential difference between two points separated by a distance ‘d’ and ‘E’is the electric intensity.

Electric P.E. = U = qV.

(ii) Electric potential difference between two points is defined as the work done in moving a unit positive charge from one point to the other keeping the charge in electrostatic equilibrium.

Mathematically it is written as

?V = -Ed or ?V = E.d

where ?V is the potential difference between two points separated by a distance ‘d’ and ‘E’is the electric intensity.

27.Is electron volt (eV) a unit of potential difference or energy?

Electron volt eV is not a unit of potential difference.This is the unit of energy used for nuclear energy in atomic physics.

Its Definition:-It is defined as the energy gained or lost by an electron

when it is moved between two points with a potential difference of one volt.

1 eV = 1.6 ×10

=1.6 ×10

Its Definition:-It is defined as the energy gained or lost by an electron

when it is moved between two points with a potential difference of one volt.

1 eV = 1.6 ×10

^{-19}C×1 volt=1.6 ×10

^{-19}j28.Convert joule in electron volt?

29.Describe the charging of a capacitor by battery assuming the positive charge as transferred?

When the plates of a capacitor are connected to the terminals of battery then plate A connected to the negative terminal of the battery will lose positive charge and then become negative.This positive charge will then pass through the battery and will be deposited on the plate B.This process will continue till the potential difference between the plates becomes equal to the e,m.f of the battery.Now the capacitor is fully charged.

30.What are the factors upon which the electric flux depends?

Electric flux depends upon the following factors; i) Magnitude of electric intensity. ii)Area of the surface. iii)Orientation of the area.

31.Define dielectria constant for capacitors.Also write its unit?

32.Describe the factors upon which the capacitance of a capacitor depends?

The capacitance depends upon the following factors. i)Area of the plates. ii)Distances between the plates. iii)Dielectric used between the plates.

33.What is the electric field on a sphere of radius r1 placed inside a concentric sphere of radius r.There is a charge Q on bigger sphere.Give brief reason for your answer?

The electric field on inner sphere is zero.As the charge on the sphere is always on the outer surface of the shear therefore the electric field on a sphere of radius r1 placed inside a concentric sphere of radius r will be zero.

34.How can you increase the capacity of a capacitor?

The capacity of a capacitor can be increased by i)Increasing the area of plates. ii)Reducing the separation between the plates. iii)Inserting a dielectric between the plates.

35.What is an electric dipole?

It consists of two charges equal in magnitude but opposite sign separated by a small distance.

36.Define potential difference between two points.Give its unit and define it?

The potential difference is defined as the work done in moving a unit positive charge from one point to the other point in the electric field.

Units:- Its SI unit is volt.

Definition of Volt:- If a work of 1j is done in moving 1C change from one point to the other then the potential difference between two points will be one volt.In other words it can be written as

1V 1j/1c

Units:- Its SI unit is volt.

Definition of Volt:- If a work of 1j is done in moving 1C change from one point to the other then the potential difference between two points will be one volt.In other words it can be written as

1V 1j/1c

37.What is Farad?Define it?

Farad is the unit of capacitance. Its def:-The capacitance of a capacitor is one farad if a charge of one coulomb given to one of the plates of a parallel plate capacitor produces a potential difference of one volt between them.

38.What is xerography ?

Xerography means method of dry writing .The main part of this machine cylinder coated with layer of selenium which leaves positive static charges.In turn the drum collects toner dust generates negative static charges.These negative charges spread over the drum.The toner from the drum is transferred on to a sheet of paper on which the document is to be copied.Heated pressure rollers then melt the toner into the paper to produce the permanent impression of the document.

39.Describe the working of inkjet printer briefly?

It is a type of printer in which electric charge is used for its operation.An inkjet print-head ejects a steady flow of ink droplets.The charging electrodes are used to charge the droplets that are not needed on the paper.Charged droplets are deflected into a gutter by the deflection plates while uncharged droplets fly straight on the paper.

40.Explain briefly the term’’ potential gradient?

Potential gradient gives a relation between the change of potential ?V due to the motion and an electric field.It is defined as the rate of change of potential with respect to distance and is called as potential gradient.It is mathematically written as E = – ?V/ ?V

Negative sigh indicates that charge is being moved from a point of lower potential to a point of higher potential.

Negative sigh indicates that charge is being moved from a point of lower potential to a point of higher potential.

41.Describe the principle of Millikan’s oil drop method?

The basic principle of Millikan oil drop method is that the gravitational force F

F

Or qE= mg

_{g}acting on the droplet is equal to the electrical force F_{e}i,e.F

_{e}= F_{g}Or qE= mg

42.What is the result of Millikan oil drop method?

The minimum charge on any drop is equal to 1.6×10

^{-19}C.this is assumed to be the integral multiple of charge on any droplet.According to Millikan the charge of minimum quantity 1.6×10^{-19}C is the charge on an electron.43.Write any one use of charging and discharging of a capacitor?

The wipes of a car is one of the practical application of charging and discharging of a capacitor.The wiper timing for on and off is set by a time constant factor.This is done by a resistor capacitor combination circuit.

44.What is time constant of a capacitor?

It is defined as the time required by the capacitor to deposit 0.63 times the equilibrium change qo.As the unit of product RC is that of time so this product is known as time constant.

45.Why does each plate of a capacitor receive same amount of change during charging of a capacitor?

When a parallel plate capacitor is connected across a battery the electrons transferred from positive plate is exactly equal to electrons received b the opposite negative plate.This transfer of electrons continues till the potential between the plates becomes equal to the plates becomes equal to the external potential of the battery.In this way number of electrons received by the other plate.

46.What is meant by electric polarization?

The molecules of dielectric under the action of electric field become dipoles and the dialect is said to be polarized.The increase in the capacity of a capacitor due to presence of some dielectric material is called the polarization effect of dielectric.

47.Can there be a potential difference between two conductors that carry like charges of the same magnitude?

No there can be no potential difference between two conductors carrying like charge of the same magnitude because the two conductors should have unlike charges to make the potential difference between them i.e.positively charged at higher potential and negatively charged at lower potential.

48.How can we shield any apparatus from electric field?

The apparatus can be shielded by covering it with an insulated material.Thus the electric field will have no effect upon it.

49.What is Capacitor?

A device used for storing electric charge is called a capacitor.A simple form of capacitor consists of two conductors or metal plates placed near one another separated by air or vacuum or some in saluting material called dielectric.

50.Define electrical potential?

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