physics - electric charges and fields

Class 12 - Physics

Electric Charges and Fields

Q&A
Question:

Refer to the arrangement of charges in figure and a Gaussian surface of radius R with Q at the centre. Then,
ncert-exemplar-problems-class-12-physics-electric-charges-fields-14

Answer:

ncert-exemplar-problems-class-12-physics-electric-charges-fields-15
The charge 5 Q lies outside the surface, thus it makes no contribution to electric flux through the given surface. Hence options (a) and (c) are correct.



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Questions and Answers


Electric Charges and Fields - Questions
1.

A positive charge Q is uniformly distributed along a circular ring of radius R. A small test charge q is placed at the centre of the ring figure. Then,
ncert-exemplar-problems-class-12-physics-electric-charges-fields-16
(a) if q > 0 and is displaced away from the centre in the plane of the ring, it will be pushed back towards the centre
(b) if q < 0 and is displaced away from the centre in the plane of the ring, it will never return to the centre and will continue moving till it hits the ring
(c) if q < 0, it will perform SHM for small displacement along the axis
(d) q at the centre of the ring is in an unstable equilibrium within the plane of the ring for q > 0


Answer
2.

Sketch the electric field lines for a uniformly charged hollow cylinder shown in figure.
ncert-exemplar-problems-class-12-physics-electric-charges-fields-21


Answer
3.

A hemisphere is uniformly charged positively. The electric field at a point on a diameter away from the centre is directed .
(a) perpendicular to the diameter
ncert-exemplar-problems-class-12-physics-electric-charges-fields-12
(b) parallel to the diameter
(c) at an angle tilted towards the diameter
(d) at an angle tilted away from the diameter


Answer
4.

The electric field at a point is
(a) always continuous
(b) continuous if there is no charge at that point
(c) discontinuous only if there is a negative charge at that point
(d) discontinuous if there is a charge at that point


Answer
5.

Consider a coin of Question . ft is electrically neutral and contains equal amounts of positive and negative charge of magnitude 34.8 kC. Suppose that these equal charges were concentrated in two point charges separated by
(i) 1 cm — (1/2  x diagonal of the one paisa coin)
(ii) 100 m (~ length of a long building)
(iii) 106 m (radius of the earth). Find the force on each such point charge in each of the three cases. What do you conclude from these results?


Answer
6.

Figure shows the electric field lines around three point charges A, B and C.
ncert-exemplar-problems-class-12-physics-electric-charges-fields-34
(i) Which charges are positive?
(ii) Which charge has the largest magnitude? Why?
(iii) In which region or regions of the picture could the electric field be zero? Justify your answer.
(a) Near A (b) Near B
(c) Near C (d) Nowhere


Answer
7.

Consider a sphere of radius R with charge density distributed as p(r) = kr for r <= R
= 0 for r > R.
(a) Find the electric field at all points r.
(b) Suppose the total charge on the sphere is 2e where e is the electron charge. Where can two protons be embedded such that the force on each of them is zero. Assume that the introduction of the proton does not alter the negative charge distribution?


Answer
8.

A point positive charge is brought near an isolated conducting sphere (figure). The electric field is best given by
ncert-exemplar-problems-class-12-physics-electric-charges-fields-4


Answer
9.

If the total charge enclosed by a Surface is zero, does it imply that the electric field everywhere on the surface is zero? Conversely, if the electric field everywhere on a surface is zero, does it imply that net charge inside is zero?


Answer
10.

There is another useful system of units, besides the SI/MKS. A system, called the CGS (Centimeter-Gram-Second) system. In this system, Coulomb's law
is given by F = (Q q/r2)r .

ncert-exemplar-problems-class-12-physics-electric-charges-fields-50


Answer
11.

If Фs  E . dS = 0 over a surface, then
(a) the electric field inside the surface and on it is zero
(b) the electric field inside the surface is necessarily uniform
(c) the number of flux lines entering the surface must be equal to the number of flux lines leaving it
(d) all charges must necessarily be outside the surface


Answer
12.

If there were only one type of charge in the universe, then
ncert-exemplar-problems-class-12-physics-electric-charges-fields-13


Answer
13.

An arbitrary surface encloses a dipole. What is the electric flux through this surface?


Answer
14.

A paisa coin is made up of Al-Mg alloy and weight 0.75 g. It has a square shape and its diagonal measures 17 mm. It is electrically neutral and contains equal amounts of positive and negative charges.


Answer
15.

Two charges q and -3q are placed fixed on x-axis separated by a distance d. Where should a third charge 2q be placed such that it will not experience any force?


Answer
16.

Consider a region inside which there are various types of charges but the total charge is zero. At points outside the region,
(a) the electric field is necessarily zero
(b) the electric field is due to the dipole moment of the charge distribution  only j
(c) the dominant electric field is ∠ 1/r3, for large r, where r is the distance from an origin in this regions r
(d) the work done to move a charged particle along a closed path, away from the region, will be zero


Answer
17.

The dimensions of an atom are of the order of an Angstrom. Thus, there must be large electric fields between the protons and electrons. Why then is the electrostatic field inside a conductor zero?


Answer
18.

Two fixed, identical conducting plates (a and P), each of surface area .S' are charged to -Q and q, respectively, where Q > q >0. A third identical plate (j), free to move is located on the other side of the plate with charge q at a distance d (figure). The third plate is released and collides with the plate p. Assume the collision is elastic and the time of collision is sufficient to redistribute charge amongst p and y
(a) Find the electric field acting on the plate y before collision.
(b) Find the charges on P and yafter the collision.
(c) Find the velocity of the plate yafter the collision and at a distance d from the plate /?.

ncert-exemplar-problems-class-12-physics-electric-charges-fields-46


Answer
19.

Five charges q1, q2, q3, q4  and q5  are fixed at their positions as shown in
figure, S is a Gaussian surface. The Gauss' law is given by
ncert-exemplar-problems-class-12-physics-electric-charges-fields-8
Which of the following statements is correct?
(a) E on the LHS of the above equation will have a contribution from q1  ,q5 and q1, q5  and q3  while q on the RHS will have a contribution from q1  and q4  only
ncert-exemplar-problems-class-12-physics-electric-charges-fields-9
(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


Answer
20.

Figure shows electric field lines in which an electric dipole P is placed as shown. Which of the following statements is correct?
ncert-exemplar-problems-class-12-physics-electric-charges-fields-10
(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


Answer
21.

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
ncert-exemplar-problems-class-12-physics-electric-charges-fields-1
(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


Answer
22.

The electric flux through the surface
ncert-exemplar-problems-class-12-physics-electric-charges-fields-6
(a) in Fig. (iv) is the largest
(b) in Fig. (iii) is the least
(c) in Fig. (ii) is same as Fig. (iii) but is smaller than Fig. (iv)
(d) is the same for all the figures


Answer
23.

Refer to the arrangement of charges in figure and a Gaussian surface of radius R with Q at the centre. Then,
ncert-exemplar-problems-class-12-physics-electric-charges-fields-14


Answer
24.

A metallic spherical shell has an inner radius R1 and outer radius R2. A charge Q is placed at the centre of the spherical cavity. What will be the surface charge density on .
(i) the inner surface (ii) the outer surface?


Answer
25.

What will be the total flux through the faces of the cube as given in the figure with side of length a if a charge q is placed at
ncert-exemplar-problems-class-12-physics-electric-charges-fields-24
(a) A a comer of the cube
(b) B mid-point of an edge of the cube (c) C centre of a face of the cube
(d) D mid-point of B and C


Answer
26.

Five charges, q each are placed at the comers of a regular pentagon of side a.
ncert-exemplar-problems-class-12-physics-electric-charges-fields-36
(a) (i) What will be the electric field at O,
the centre of the pentagon?
(ii) What will be the electric field at O if the charge from one of the comers (say A) is removed?
(iii) What will be the electric field at O if the charge q at A is replaced by – q?
(b) How would your answer to (a) be affected if pentagon is replaced by n-sided regular polygon with charge q at each of its comers?


Answer
28.

Total charge -Q is uniformly spread along the length of a ring of radius R. A small test charge +q of mass m is kept at the centre of the ring and is given a gentle push along the axis of the ring.
(a) Show that the particle executes a simple harmonic oscillation.
(b) Obtain its time period.


Answer
29.

The electric flux through the surface
ncert-exemplar-problems-class-12-physics-electric-charges-fields-6
(a) in Fig. (iv) is the largest
(b) in Fig. (iii) is the least
(c) in Fig. (ii) is same as Fig. (iii) but is smaller than Fig. (iv)
(d) is the same for all the figures


Answer
30.

Figure represents a crystal unit of cesium chloride, CsCl. The cesium atohis, represented by open circles are situated at the comers of a cube of side 0.40 nm, whereas a Cl atom is situated at
the centre of the Cube.
ncert-exemplar-problems-class-12-physics-electric-charges-fields-31
The Cs atoms are deficient in one electron while the Cl atom carries an excess electron.
(i) What is the net electric field on the Cl atom due to eight Cs atoms?
(ii) Suppose that the Cs atom at the comer A is missing. What is the net force now on the Cl atom due to seven remaining Cs atoms?


Answer
31.

In 1959 Lyttleton and Bondi suggested that the expansion of the universe could be explained if matter carried a net charge. Suppose that the universe is made up of hydrogen atoms with a number density N, which is maintained a constant. Let the charge on the proton be ep = -(1 + y)e where e is the electronic charge.  
(a) Find the critical value of y such that expansion may start.
(b) Show that the velocity of expansion is proportional to the distance from the centre.


Answer
32.

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


Answer
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