Download NCERT Solutions of Class 12 CBSE Physics Chapter 1, Electric Charges and Fields (Q 1-8)


Electric Charges and Fields

NCERT Solutions of Class XII, CBSE Physics Chapter 1 Textbook Exercise Questions

Question 1.1: What is the force between two small charged spheres having charges of 2 x 10-7 C and 3x10-7 C placed 30 cm apart in air ?
Answer: Here, q1 = 2x10-7 C, q2 = 3x10-7 C, r = 0.30m, F =?
Using the relation,

Question 1.2: 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 ?
Answer: Given that,
q1 = 0.4 µC = 0.4x10-6 C, q2 = -0.8 µC = -0.8x10-6 C,
F= electrostatic force between q1 and q2 = 0.2 N.
(a) r=?
Using the relation,

(b) Force on q2 due to q1 =?
We know that electrostatic forces always, appear in priors and follow Newton’s 3rd law of motion.

Question 1.3: Check that the ratio ke2 / Gmemp is dimensionless. Look up a Table of Physical Constants and determine the value of this ratio. What does this ratio signify ?
Dimensions of e2 are = [C2]
Dimensions of k are = [Nm2 c2] = [ML3 T-2 C-2]
Dimensions of G are = [M-1 L3 T-2]
Dimensions of me = [M]

Question 1.4: (a) Explain the meaning of the statement ‘electric charge of a body is quantized’.
(b) Why can one ignore quantization of electric charge when dealing with macroscopic i.e., large scale charges?
(a) The meaning of the statement ‘electric charge of a body is quantized’ is that the charge on it is always some integral multiple of elementary charge of an electron or a proton (= e in magnitude) i.e., discrete packets called quanta or packets of charge. Mathematically, the charge on a body can be expressed as -
q = ± ne where,
n = an integer, e = magnitude of the charge of an electron or proton = 1.6x10-19 C.
A fraction of the fundamental charge e has never been observed in free state.

(b) In practice, the charge on a charged body at macroscopic level is very large while, the charge on an electron is very small. When electrons are added to or removed from a body, the change taking place in the total charge on the body is so small that the charges seems to be varying in a continuous manner. Thus, quantization of electric charge can be ignored at macroscopic level i.e., when dealing with a large scale charged body.

Question 1.5: When a glass rod is rubbed with a silk cloth, charges appear on both. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the other law of conservation of charge.
Answer: Initially both the glass rod and silk cloth are electrically neutral before rubbing. In other words, net charge on the glass rod and silk cloth is zero. When the glass rod is rubbed with silk cloth, thus, glass rod becomes positively charged and silk cloth negatively charged. The positive charge on the glass rod is exactly equal to the negative charge on the silk cloth, so net charge on the system is again zero. Thus, the appearance of charge on the glass rod and silk cloth is in accordance with the law of conservation of charge as the total charge of the isolated system is constant. Similarly, when ebonite rod is rubbed with fur, they acquire –ve and +ve charges respectively and net charge is zero again.
Thus, we conclude that charge is neither created nor destroyed but it is merely transferred from one body to another which is consistent with the law of conservation of charge.

Question 1.6: Four points 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 ?
Answer: Consider the square ABCD of each side 10 cm and centre O. The charge of 1 µC is placed at O. Now clearly,
OA = OB = OC = OD
AB = BC = 10 cm = 0.1 m

Question 1.7: (a) An electrostatic field line is a continuous curve. That is, a field line cannot have sudden breaks. Why not ?
(b) Explain why two field lines never cross each other at any point ?
(a) The electrostatic line of force is the path tangent at every point of which gives the direction of electric field at that point. The direction of electric field generally changes from point to point. So, the lines of force are generally curved lines. Further, they are continuous curves and cannot have sudden breaks if it is so, then it will indicate the absence of electric field at the break points.

b) The electric lines of force never cross each other because if they do so, then, at the point of their intersection, we can draw two tangents which give two directions of electric field at that point which is not possible.
Question 1.8: 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 mid-point O of the line AB joining the two charges ?
(b) If a negative test charge of magnitude 1.5 x 10-29 C is placed at this point, what is the force experienced by the test charge ?

Here, charge at point A, qA = +3 µC = 3x10-6 C. 
Charge at point B, qB = -3 µC = -3x10-6 C.
r = AB = 20 cm = 0.2 m.
Let O be the mid-point of the line AB, then,
OA = OB = r/2 = 0.2/2 = 0.1 m.

(b) Force on a negative charge of magnitude 1.5 x 10-9 C is given by the formula.

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