## Thursday, 5 January 2012

### Electric Charges and Fields (Revision Notes)

Electric Charges and Fields
Q.1   Is Coulomb’s law a universal law?
No, Coulomb’s law is not universal law as Coulombic force depends upon the medium between charges. Moreover this law is applicable only for the point charges at rest.
Q.2   A negatively charged rod attracts suspended object. Can we conclude that the object is positively charged?
No, because negatively charged rod can attract neutral objects also by inducing positive charge on them.
Q.3    Vehicles carrying inflammable material usually have chains attached to them, which remain in contact with earth during motion. Why?
Vehicles when moves through dry air will have their body charged due to friction. The accumulation of charge can cause sparking. Thus to ground this charge, metallic chains are used which transfers the charge on the body to vehicle to the ground.
Q.4   Can a body have a charge of 0.8e?
No according to quantization principle, a body can have charge, which is integral multiple of the charge on electron.
Q.5    Two bodies are rubbed together, one gets positively and other negatively charged. Will their                                      mass gets affected due to charging?
charging by rubbing occurs due to transfer of electrons from one body to the other. Thus body which is positively charged has lost electrons thus its mass decreases, whereas the body which has gained electrons is negatively charged and its mass increases.
Q.6    Similar charges repel each other. Can they attract each other?
Yes. This can happen when the charge on one body is very large as compared to the charge on the other body, because larger charge will induce opposite charge on the smaller body and thus attracts it.
Q.7 A special type of rubber is used to manufacture aircraft tyres. Why?
when aircraft is moving on the runway its body gets charged due to air friction. This charge can cause sparking. Thus to ground this charge aircraft tyres are made of conducting rubber.
Q.8 Two point charges q1 and q2 are such that q1q2>0. What is the nature of interaction between charges?
As the product of two charges is always positive, it implies that either both are positive or both are negative. Thus force of interaction between them is repulsive.

Q.9 Is electrostatic force a central force in nature?
Yes, electrostatic force is central force as it always acts along the line joining the two charges.
Q.10 A sensitive instrument is to be shielded from strong electrostatic fields in environment. Suggest a possible method?
The instrument can be shielded by enclosing it inside a metallic surface. Since no electrostatic field can be present within the metallic surface therefore the instrument will be shielded from the electrical influence.
Q.11 No two electric lines of force can cross each other. Why?
The tangent to the electric lines of force gives us the direction of electrical field intensity. Therefore, if two lines of force intersect then there will be two directions of electric field intensity at one point which is not possible.
Q.12 Why one can ignore the quantization of charge while dealing with macroscopic charges?
The charge on the electron is very small. If few electrons are added or removed then the change in the magnitude of the charge is very small. Thus the charge can be assumed to be continuous.
Q.13 Why does the printing paper gets charged when it is passed through the press?
Printing paper gets charged because of the friction between paper and machine. It can be avoided by ionizing the region around the press so that charge on the paper gets grounded.
Q.14. A comb run through one’s hair attracts bits of paper. Why? What happens if the hair is wet or if it is a rainy day?
This happens because of the friction between comb and hair. But if the hair is wet then comb will not get charged due to reduced friction and it will not be able to attract paper.
Q.15. A bird perches on high power line and nothing happens to the bird. A man standing on the ground touches the same line gets fatal shock?
When bird perches on single bare high power line, nothing happens as circuit is not complete and current is not flowing through his body. But when man touches the same line current flows from wire to the ground through his body and het gets fatal shock.

Q.16 can ever whole charge of the body be transferred to the other body? If yes, how and if not why?
Yes, the whole charge of body can be transferred to the conducting body when smaller body is enclosed inside larger body and they are connected by wire. This happens as charge always resides on the outer surface of the conductor.
Q.17 Consider three charged bodies P,Q and R. if P and Q repel each other and P attracts R, what is the nature of force between Q and R?
As P and Q repel each other they are similarly charged whereas R is oppositely charged to P and Q. thus  Q and R will attract each other.
Q.18 What is the relevance of large value of K [=81] for water?
Large value of K is used to explain the electrolysis phenomenon. As k for water is large thus force between oppositely charged ions gets reduced when dissolved in water. Thus it becomes easier to separate them.

19.The electrostatic force on a small sphere of charge 0.4 μC due to another small sphere of charge − 0.8 μC in air is 0.2 N. (a) What is the distance between the two spheres? (b) What is the force on the second sphere due to the first?
Hint:-   ,
(a)The distance between the two spheres is 0.12 m.
(b) Both the spheres attract each other with the same force. Therefore, the force on the second sphere due to the first is 0.2 N.
20. Four point charges qA = 2 μC, qB = −5 μC, qC = 2 μC, and qD = −5 μC are located at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 μC placed at the centre of the square?
21. Two point charges qA = 3 μC and qB = −3 μC are located 20 cm apart in vacuum.
(a) What is the electric field at the midpoint O of the line AB joining the two charges?
(b) If a negative test charge of magnitude 1.5 × 10−9 C is placed at this point, what is the force experienced by the test charge?
Ans-the electric field at mid-point O is 5.4 × 106 N C−1 along OB.                                                                                                   force experienced by the test charge is 8.1 × 10−3 N along OA.
22. A system has two charges qA = 2.5 × 10−7 C and qB = −2.5 × 10−7 C located at points A: (0, 0, − 15 cm) and B: (0, 0, + 15 cm), respectively. What are the total charge and electric dipole moment of the system?
(a)Total charge of the system, q = qA + qB = 0
(b) p = qA × d = qB × 2a=7.5 × 10−8 C m
23. An electric dipole with dipole moment 4 × 10−9 C m is aligned at 30° with the direction of a uniform electric field of magnitude 5 × 104 N C−1. Calculate the magnitude of the torque acting on the dipole.
Hint-τ = pE sinθ=10−4 N m.
24. A polythene piece rubbed with wool is found to have a negative charge of 3 × 10−7 C.
(a) Estimate the number of electrons transferred (from which to which?)
(b) Is there a transfer of mass from wool to polythene?
(a)the number of electrons transferred from wool to polythene is 1.87 × 1012.
(b)Mass = 1.706 × 10−18 kg
25. (a) two insulated charged copper spheres A and B have their centers separated by a distance of 50 cm. What is the mutual force of electrostatic repulsion if the charge on each is 6.5 × 10−7 C? The radii of A and B are negligible compared to the distance of separation.
(b) What is the force of repulsion if each sphere is charged double the above amount, and the distance between them is halved?
(A) The force between the two spheres is 1.52 × 10−2 N.                                                                                         (b) New force = 16 × 1.52 × 10−2 = 0.243 N
26. Suppose the spheres A and B in above question have identical sizes. A third sphere of the same size but uncharged is brought in contact with the first, then brought in contact with the second, and finally removed from both. What is the new force of repulsion between A and B?
the force of attraction between the two spheres is 5.703 × 10−3 N.
28.A point charge of 2.0 μC is at the centre of a cubic Gaussian surface 9.0 cm on edge. What is the net electric flux through the surface?
Hint-      =   = 2.26 × 105 N m2 C−1