, ELECTRIC CHARGES AND FIELDS
2015
Q1. What is the electric flux through a cube of side 1 cm which encloses an
electric dipole? (1M)
Ans.zero
Q2. (a) An electric dipole of dipole moment p consists of point charges +q and
-q separated by a distance of 2a apart. Deduce the expression for the electric
field E due to the dipole at a distance x from the centre of the dipole on its
axial line in terms of the dipole moment p. Hence show that in the limit x >> a,
(b) Given the electric field in the region x is 2E , find the net electric flux through
the cube and the charge enclosed by it. (5M)
Ans. (a) (b)
Q3. (a) Explain, using suitable diagrams, the di erence in the behaviour of an (i)
conductor and (ii) dielectric in the presence of an external electric field. Define
the terms polarisation of a dielectric and write its relation with susceptibility.
(b) A thin metallic spherical shell of radius R carries a charge Q on its surface. A
point charge Q/2 is placed at its centre C and another charge +2Q is placed
outside the shell at a distance x from the centre as shown in the figure.
Find (i) the force on the charge at the centre of the shell and at point A and (ii)
the electric flux through the shell. (5M)
,Ans. (b) (i) (ii)
2016
Q1. A point charge +Q is placed at point O as shown in the figure. Is the
potential di erence VA – VB positive, negative or zero? (1M)
Ans. positive
Q2. How does the electric flux due to a point charge enclosed by a
spherical Gaussian surface get a ected when its radius is increased? (1M)
Ans. does not get a ected
Q3. A charge is distributed uniformly over a ring of radius ‘a. Obtain an
expression for the electric intensity E at a point on the axis of the ring.
Hence show that for points at large distances from the ring, it behaves
like a point charge. (3M)
Ans.
2017
Q1. (a) Derive an expression for the electric field E due to a dipole of
length ‘2a’ at a point distant r from the centre of the dipole on the axial
line. (b) Draw a graph of E versus r for r >> a. (c) If this dipole were kept in
a uniform external electric field Eo, diagrammatically represent the
position of the dipole in stable and unstable equilibrium and write the
expressions for the torque acting on the dipole in both cases. (5M)
Ans. c) i) 0 ii) 0
Q2. (a) Use Gauss’s theorem to find the electric field due to a uniformly
charged infinitely large plane thin sheet with surface charge density σ. (b)
An infinitely large thin plane sheet has a uniform surface charge density
+σ. Obtain the expression for the amount of work done in bringing a
point charge q from infinity to a point, distant r, in front of the charged
plane sheet. (5M)
, Ans. a) b)
2018
Q1. Define electric flux and write its SI unit. The electric field components
in the figure shown are Ex = ax, Ey = 0, Ez = 0 where a = 100 N/Cm. Calculate
the charge within the cube, assuming a = 0.1m. (3M)
Ans. SI unit is Nm^2c^(− 1), q = 8.85 × 10^− 13 C
2019
Q1. Draw the pattern of electric field lines, when a point charge – Q is kept
near an uncharged conducting plate. (1M)
Q2. (a) Draw the equipotential surfaces corresponding to a uniform
electric field in the z-direction. (b) Derive an expression for the electric
potential at any point along the axial line of an electric dipole. (3M)
Q3. (a) Derive an expression for the electric field at any point on the
equatorial line of an electric dipole. (b) Two identical point charges, q
each, are kept 2m apart in the air. A third point charge Q of unknown
magnitude and sign is placed on the line joining the charges such that
the system remains in equilibrium. Find the position and nature of Q. (5M)
Ans. b) r = 1 m & Q=q/4
2020
Q1. If the net electric flux through a closed surface is zero, then we can
infer
(A) no net charge is enclosed by the surface.
(B) uniform electric field exists within the surface.
(C) electric potential varies from point to point inside the surface. (D)
charge is present inside the surface. (1M)
2015
Q1. What is the electric flux through a cube of side 1 cm which encloses an
electric dipole? (1M)
Ans.zero
Q2. (a) An electric dipole of dipole moment p consists of point charges +q and
-q separated by a distance of 2a apart. Deduce the expression for the electric
field E due to the dipole at a distance x from the centre of the dipole on its
axial line in terms of the dipole moment p. Hence show that in the limit x >> a,
(b) Given the electric field in the region x is 2E , find the net electric flux through
the cube and the charge enclosed by it. (5M)
Ans. (a) (b)
Q3. (a) Explain, using suitable diagrams, the di erence in the behaviour of an (i)
conductor and (ii) dielectric in the presence of an external electric field. Define
the terms polarisation of a dielectric and write its relation with susceptibility.
(b) A thin metallic spherical shell of radius R carries a charge Q on its surface. A
point charge Q/2 is placed at its centre C and another charge +2Q is placed
outside the shell at a distance x from the centre as shown in the figure.
Find (i) the force on the charge at the centre of the shell and at point A and (ii)
the electric flux through the shell. (5M)
,Ans. (b) (i) (ii)
2016
Q1. A point charge +Q is placed at point O as shown in the figure. Is the
potential di erence VA – VB positive, negative or zero? (1M)
Ans. positive
Q2. How does the electric flux due to a point charge enclosed by a
spherical Gaussian surface get a ected when its radius is increased? (1M)
Ans. does not get a ected
Q3. A charge is distributed uniformly over a ring of radius ‘a. Obtain an
expression for the electric intensity E at a point on the axis of the ring.
Hence show that for points at large distances from the ring, it behaves
like a point charge. (3M)
Ans.
2017
Q1. (a) Derive an expression for the electric field E due to a dipole of
length ‘2a’ at a point distant r from the centre of the dipole on the axial
line. (b) Draw a graph of E versus r for r >> a. (c) If this dipole were kept in
a uniform external electric field Eo, diagrammatically represent the
position of the dipole in stable and unstable equilibrium and write the
expressions for the torque acting on the dipole in both cases. (5M)
Ans. c) i) 0 ii) 0
Q2. (a) Use Gauss’s theorem to find the electric field due to a uniformly
charged infinitely large plane thin sheet with surface charge density σ. (b)
An infinitely large thin plane sheet has a uniform surface charge density
+σ. Obtain the expression for the amount of work done in bringing a
point charge q from infinity to a point, distant r, in front of the charged
plane sheet. (5M)
, Ans. a) b)
2018
Q1. Define electric flux and write its SI unit. The electric field components
in the figure shown are Ex = ax, Ey = 0, Ez = 0 where a = 100 N/Cm. Calculate
the charge within the cube, assuming a = 0.1m. (3M)
Ans. SI unit is Nm^2c^(− 1), q = 8.85 × 10^− 13 C
2019
Q1. Draw the pattern of electric field lines, when a point charge – Q is kept
near an uncharged conducting plate. (1M)
Q2. (a) Draw the equipotential surfaces corresponding to a uniform
electric field in the z-direction. (b) Derive an expression for the electric
potential at any point along the axial line of an electric dipole. (3M)
Q3. (a) Derive an expression for the electric field at any point on the
equatorial line of an electric dipole. (b) Two identical point charges, q
each, are kept 2m apart in the air. A third point charge Q of unknown
magnitude and sign is placed on the line joining the charges such that
the system remains in equilibrium. Find the position and nature of Q. (5M)
Ans. b) r = 1 m & Q=q/4
2020
Q1. If the net electric flux through a closed surface is zero, then we can
infer
(A) no net charge is enclosed by the surface.
(B) uniform electric field exists within the surface.
(C) electric potential varies from point to point inside the surface. (D)
charge is present inside the surface. (1M)
