Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Electric Charges and Fields. The best solution of the problem is to practice as many Physics Class 12 Chapter 1 Important Questions as possible to clear the doubts.
1 Q:- What kind of charge are produced when a glass rod is rubbed with silk?
Ans:- Glass rod acquires positive charge and silk acquires negative charge.
2 Q:- Why does an ebonite rod get negatively charged when rubbed with fur or wool?
Ans:- The ebonite rod get negatively charged due to transfer of electrons from fur or wool into ebonite.
3 Q:- Name the experiment which established quantum nature of electric charge.
Ans:- Millikan's oil drop experiment conclusively established the quantum nature of electric charge.
4 Q:- What is the true cause of electrification when one body is rubbed with another body?
Ans:- The true cause of electrification is actual transfer of electrons from one body to another. In present example, electrons migrate from wool to polyester rod.
5 Q:- When a glass rod is rubbed with silk, equal and opposite charge are developed on them. Is there a transfer of mass between them? If yes, from which to whom?
Ans:- Yes, cause of electrification is transfer of electrons from glass to silk so that glass acquires a positive charge and silk acquires equal amount of negative charge. So, there is transfer of mass from glass rod to silk.
6 Q:- Give one example to demonstrate that electrostatic force are much stronger than gravitational force.
Ans:- A charged comb can lift a small piece of paper against the gravitational pull of the Earth on the paper.
7 Q:- Can a body have a charge 8.0 x 10-20 C? Give reason.
Ans:- A body cannot have a charge of 8.0 x 10-20 C, because it is less than the charge on an electron or proton and according to quantised nature of charge, no charge less than e = 1.6 x 10-19 is possible independently.
8 Q:- Does Coulomb's law of electric force obey Newton's third law of motion?
Ans:- Yes, Coulomb's law of electric force obeys Newton's third law of motion. The force experienced by two charges are equal in magnitude but mutually opposite in directions.
9 Q:- Consider three charged bodies A, B and C. If A and B repel each other but A attracts C, what is the nature of force between B and C?
Ans:- Charges of A and B are of same sign but charges of A and C are of mutually opposite signs. Thus, B and C bodies have charges of opposite signs and hence, attract each other.
10 Q:- Does the Coulombian force, exerted by one charge on another, charge if other charges are brought nearly?
Ans:- No, the Coulombian force exerted by one charge on another is not affected due to presence of other charges in its neighborhood.
11 Q:- What is the relationship between relative permittivity and dielectric constant of a medium?
Ans:- For a given medium, value of relative permittivity ε, is exactly same as is its dielectric constant (K).
12 Q:- Give the SI unit of electric field. Is electric field a scalar or a vector?
Ans:- The SI unit of electric field is newton/coulomb (N/C) but it may also be written as volt/meter (V/m). Electric field is a vector quantity.
13 Q:- Electric field inside a conductor is zero. Why?
Ans:- No electric field line passes through the body of a conductor. So, electric field inside a conductor is zero.
14 Q:- What is the use of the concept of electric field?
Ans:- The concept of electric field enables us to find the value of electrostatic force acting on a charge q at any point in the field by using the relation F = qE.
15 Q:- What is the nature of symmetry of electric field due to a point charge?
Ans:- Electric field due to a point charge exhibits spherical symmetry.
16 Q:- What is the basic fact involved in superposition principle involved in electrostatics?
Ans:- The force between two given charges remains unaffected by the presence of the other charges.
17 Q:- How is the number of electric field lines related to field strength at a point?
Ans:- The number of electric field lines crossing per unit surface area around a given point is directly proportional to the strength of electric field at that point. Higher the field strength, more crowded the electric field lines around that point.
18 Q:- Why do we obtain a neutral point in the space between two like charges?
Ans:- In the space between two like charges, there is a particular point where electric field due to two charges are exactly equal and opposite. Hence, net electric field at that point is zero. Such a point is known as the neutral point.
19 Q:- Define the term 'electric dipole moment of a dipole'. State its SI unit.
Ans:- Two equal and unlike point charges separated by a finite distance constitute an electric dipole. The dipole moment of a dipole is given by the product of magnitude of either charge and distance between the two charges. The SI unit of dipole moment is coulomb meter (C-m).
20 Q:- When is the torque on an electric dipole in a uniform electric field maximum?
Ans:- The torque on dipole is maximum when dipole is held making an angle of 90° from the direction of uniform electric field.
21 Q:- At what points is the field due to an electric dipole parallel to the line joining the two charges?
Ans:- At all points on the axial as well as equatorial line of an electric dipole, its electric field is parallel to the line joining the two charges.
22 Q:- Does an electric dipole always experience a torque when placed in a uniform electric field?
Ans:- No, an electric dipole does not experience a torque when placed either parallel or antiparallel to a uniform electric field.
23 Q:- What happens when an electric dipole is placed at an angle in a nonuniform electric field?
Ans:- An electric dipole placed at an angle θ from the direction of a nonuniform electric field, in general, experience a net translational force as well as a net torque.
24 Q:- Under what condition, does a charged sphere or spherical shell behave as a point charge for the purpose of determining electric field due to it?
Ans:- When the electric field is to be found at a point outside the charged sphere or a charged sphere shell, we can presume whole charge of a sphere or spherical shell to be concentrated at its center.
25 Q:- What is the direction of electric field at a point near a plane, thin infinite charged sheet?
Ans:- If the given sheet is positively charged, then electric field E is directed normally outward. On the other hand, the electric field is directed normally inward if the sheet is negatively charged.
26 Q:- How does a free electron, initially at rest, move in an electric field?
Ans:- As electron is negatively charged, so a free electron, initially at rest, will undergo an accelerated motion along a straight line in a direction opposite to that of electric field.
27 Q:- Two point charges of +5 μC each are 1 m apart. At what point on the line joining the charges will the net electric field be zero, and why?
Ans:- The net electric field is zero at the mid-point of line joining two charges, because electric fields due to two charges have equal magnitudes but are directed in mutually opposite directions.
28 Q:- Does an electric charge experience a force due to the field produced by itself?
Ans:- No, an electric charge does not experience a force due to the electric field produced by itself.
29 Q:- What is the nature of symmetry of electric field due to an electric dipole?
Ans:- The electric field due to an electric dipole exhibit cylindrical symmetry with dipole axis as the axis of cylinder.
30 Q:- If Coulomb's law involved r1/3 dependence would Gauss' law will still hold good?
Ans:- No, in that case, Gauss' law will not hold good. In fact, in such an eventually, electric flux of a closed surface, besides charge enclosed, would also depend on the dimensions of Gaussian surface. Hence, Gauss' law cannot be applied to a surface.
31 Q:- 10 electric dipoles, each of dipole moment p, are located symmetrically inside a closed Gaussian surface. Calculate the electric flux of the surface.
Ans:- Electric flux of the given surface is zero because net charge enclosed within the Gaussian surface due to the presence of dipole is zero.
32 Q:- A conductor with a cavity is given finite value of electric charge. What is the value of electric field at any point within the cavity? Give reason.
Ans:- The electric field at any point within the cavity is zero because field due to the charge outside is zero.
33 Q:- A positively charged glass rod attacks a suspend pith ball. Can we conclude that pith ball is charged?
Ans:- No, we cannot say with certainty that the pith ball is charged one. A positively charged glass rod attracts pith ball either if pith ball is negatively charged or it is unchanged one.
34 Q:- Is there some method of charging a conductor without rubbing it with another object and without bringing it into contact with a charged body?
Ans:- Yes, the conductor can be charged by method of induction by bringing a charged object near the conductor.
35 Q:- Can a charged body attract another uncharged body? Explain.
Ans:- Yes, a charged body can attract another uncharged body. It is because, when the charged body is placed in front of an uncharged body, the induced changes of opposite kind are produced on the uncharged body. Due to this, the charged body attracts the uncharged body.
36 Q:- Can two balls having same kind of charge on them attract each other? Explain.
Ans:- Yes, two balls having same kind of charge can attract each other, if one ball possesses charge quite large as compared to that on the other ball. The reason is that when two charged balls are placed near each other, they induce opposite kinds of charges on the faces of each other. Since on the ball having a small amount of charge, quite a large amount of induced charge is produced, two balls get attracted towards each other due to the force between the large inducting charge of one ball and the large induced charge (opposite in nature) on the other ball.
37 Q:- A comb run through one's dry hair attracts small bits of paper. Why? What happens, if the hair is wet or if it is a rainy day ?
Ans:- When a comb is run through die here it gets charged. As such, it starts attracting small bits of paper, If the hair are wet or it is a rainy day, then in absence of friction, the comb will not get charged, when run through hair. ln the absence of charge on the comb, the bits of paper will not be attracted.
38 Q:- An ebonite rod held in hand can be charged by rubbing with flannel but a copper rod cannot be charged like this. Why?
Ans:- Both the human body and the copper rod conduct electricity. When it is attempted to charge a copper rod by rubbing, the charge flows from the rod to the earth through the hand. However, when ebonite rod is charged by rubbing, the charges so produced stay on the ebonite rod, as it is a bad conductor of electricity.
39 Q:- How the mass of a body is affected on charging?
Ans:- When a body is charged, either electrons get removed (becomes positively charged) or get added (becomes negatively charged) to it. Since electron is a material particle, the mass of a body decreases on getting positively charged and increases on getting negatively charged.
40 Q:- Is Coulomb's law in electrostatics applicable in all situations?
Ans:- No, Coulomb's law in electrostatics does not hold in all situations. It is applicable only in the following situations: 1. The electric charges must be stationary. 2. The electric charges must be points in size. Coulomb's law does not apply to two charged bodies of finite sizes. Say two charged spheres. It is because the distribution of charge does not remain uniform, when the two bodies are brought together.
41 Q:- The test charge used to measure electric field at a point should be vanishingly small. Why?
Ans:- In case, test charge is not vanishingly small, it will produce its own electric field and the measured value of electric field will be different from the actual value of electric field at that point.
42 Q:- Does an electric charge experience a force due to the field it produces itself?
Ans:- No, an electric charge does not experience any force due to the electric field it produces itself.
43 Q:- A proton is placed in a uniform electric field along the positive X-axis. In which direction will it
tend to move?
Ans:- Along the direction of the electric field i.e. along the positive X-axis.
44 Q:- Does an electric dipole always experience a torque, when placed in a uniform electric field?
Ans:- No, it does not experience a torque, when it is placed along the direction of electric field.
45 Q:- When is the torque acting on an electric dipole maximum, when placed in uniform electric field?
Ans:- The torque is maximum, when the electric dipole is placed perpendicular to the direction of electric field.
46 Q:- A charged particle is free to move in an electric field. Will it always move along and electric line of force?
Ans:- The charged particle may or may not move along an electric line of force. If the charged particle was initially at rest, it will move along an electric line of force. In case the charged particle had some initial velocity making certain angle with a line of force, then its resultant path will not be along the line of force.
47 Q:- Electric field intensity within a conductor is always zero. Why?
Ans:- The electric lines of force cannot pass through a conductor As a result there can be no electric field inside a conductor. Hence electric field intensity inside a conductor is zero.
48 Q:- Are the field lines are reality?
Ans:- The electric field lines are purely geometrical constructions, which are used to represent electric field graphically. In other words, the electric field lines them self are imaginary, but the electric field they represent is real.
49 Q:- What is the use of Gaussian surface?
Ans:- In order to find electric field due to a charge distribution by applying Gauss' theorem, one is required to evaluate a surface integral. The surface integral can be evaluated easily by choosing a suitable gaussian surface for the charge distribution.
50 Q:- How does electric field at a point change with distance r from an infinite thin sheet of charge?
Ans:- The magnitude of electric field due to an infinite plane sheet of charge is independent of the distance of the observation point from the sheet of charge.
51 Q:- What is the difference between a sheet of charge and a plane conductor having charge?
Ans:- On a sheet of charge, the same charge shows up on its two sides; whereas in case of a charged plane conductor, the charges showing up on the two surfaces are not the same.
52 Q:- What is the strength of the electric field inside a charged spherical shell?
Ans:- The strength of electric field inside a charged spherical shell is zero.
53 Q:- When kept in an electric field, does a proton move from lower to higher potential or from higher to lower potential region?
Ans:- In electric field, a proton (positive charged particle) will move in the direction of electric field i.e. from the higher to lower potential region.
54 Q:- Electric force between two charge is called central force. Why?
Ans:- Electrostatic force between two charges is central force because it is always directed along the line joining the two charges. Moreover, the magnitude of a central force F depends on r and in the motion under a central force, the angular momentum is always conserved. As electrostatic force between two charges fulfils all these conditions, hence it is a central force.
55 Q:- A small test charge is released at a point in an electrostatic field configuration. Will it travel along the field line passing through that point?
Ans:- A small test charge released at rest at a point in an electrostatic field will travel along the field line at that point. However, it will continue to pass through that very field line only if the field line is a straight line. In case, the electrostatic field line is curved one, the direction of the field line will provide the direction of force acting on a test charge but not the direction of motion of test charge.