NCERT Exemplar solutions for Class 12 Physics, Chapter 1: Electric Charges and Fields are available here. This chapter has a large number of questions due to which we have provided the solutions in several parts. This is Part I, here you will find solutions for MCQs I (or multiple choice questions with single correct answer). Solutions for rest of the questions are available in other parts. These questions are very important for CBSE 12th Physics board exam & competitive examinations like JEE Main, NEET etc.
NCERT Exemplar Solution for 12th Physics, Chapter 1: Electric Charges and Fields (MCQs I) are given below:
Question 1.1: In figure two positive charges q2 and q3 fixed along the y-axis, exert a net electric force in the + x-direction on a charge q1, fixed along the x-axis. If a positive charge Q is added at (x, 0), the force on q1,
(a) shall increase along the positive x-axis
(b) shall decrease along the positive x-axis
(c) shall point along the negative x-axis
(d) shall increase but the direction changes because of the intersection of Q with q2 and q3
Solution 1.1: (a)
Net force on charge q1, by other charges q2 and q3 is along the + x-direction, so nature of force between q1 & q2 and q1 & q3 is attractive. This is possible when charge q1 is negative.
Now, if a positive charge Q is placed at (x, 0), then, the force on q1 shall increase. The direction will be along positive x axis.
Question 1.2: A point positive charge is brought near an isolated conducting sphere (figure). The electric field is best given by
(a) Fig (i)
(b) Fig (ii)
(c) Fig (iii)
(d) Fig (iv)
Solution 1.2: (a)
Electric field originates from positive charge. It is perpendicular to the surface of the conductor. Also, inside the conducting sphere, it will be zero.
All these conditions are satisfied in Fig (i).
Question1.3: The Electric flux through the surface
(a) in Fig.1.3 (iv) is the largest.
(b) in Fig. 1.3 (iii) is the least.
(c) in Fig. 1.3 (ii) is same as Fig. 1.3 (iii) but is smaller than Fig. 1.3 (iv)
(d) is the same for all the figures.
Solution 1.3: (d)
The flux of electric field through any closed surface S is 1/ε0 times the total charge enclosed by S (Irrespective of shape and size of the surface).
All the figures given in the questions have the same charge i.e., +q. So, the electric flux through the surface is the same for all the figures.
Question1.4: Five charges q1, q2, q3, q4, and q5 are fixed at their positions as shown in Fig. 1.4. S is a Gaussian surface.
The Gauss’s law is given by
Which of the following statements is correct?
(a) E on the LHS of the above equation will have a contribution from q1, q5 and q3 while q on the RHS will have a contribution from q2 and q4 only.
(b) E on the LHS of the above equation will have a contribution from all charges while q on the RHS will have a contribution from q2 and q4 only.
(c) E on the LHS of the above equation will have a contribution from all charges while q on the RHS will have a contribution from q1, q3 and q5 only.
(d) Both E on the LHS and q on the RHS will have contributions from q2 and q4 only.
Solution 1.4: (b)
According to Gauss' theorem, the term q on the right side of the equation includes the sum of all charges enclosed by the surface (irrespective of the position of the charges inside the surface).
If the surface is so chosen that there are some charges inside and some outside, the electric field on the left side of equation will be due to all charges (both inside and outside)
Here, E on LHS of the above equation will have a contribution from all charges while q on the RHS will have a contribution from charges q2 and q4 only.
Question1.5: Figure 1.5 shows electric field lines in which an electric dipole p is placed as shown. Which of the following statements is correct?
(a) The dipole will not experience any force.
(b) The dipole will experience a force towards right.
(c) The dipole will experience a force towards left.
(d) The dipole will experience a force upwards.
Solution 1.5: (b)
We know electric field emerges radially outward from positive point charge.
In the figure given above, space between field lines is increasing (or density of electric field line is decreasing). In other words, the electric force is decreasing while moving from left to right.
Thus, the force on charge ‒q is greater than the force on charge + q in turn dipole will experience a force towards left direction.
Question 1.6: A point charge +q, is placed at a distance d from an isolated conducting plane. The field at a point P on the other side of the plane is
(a) directed perpendicular to the plane and away from the plane.
(b) directed perpendicular to the plane but towards the plane.
(c) directed radially away from the point charge.
(d) directed radially towards the point charge.
Solution 1.6: (a)
When you place a positive charge near a conducting plane, then electric field lines from positive charges will enter into the conducting plane (from the side where positive charge is kept) and emerge from opposite side of the plane.
In both cases, the direction of electric field lines will always be perpendicular to the surface of the plane.