Buds Public School, Dubai
NAME:
PHYSICS
GRADE-12
Topic - Electric Potential and Capacitance
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1. What is the electrostatic potential due to an electric dipole at an equatorial point?
2. For any charge configuration, equipotential surface through a point is normal to the electric
field." Justify.
3. (a) Why does the electric field inside a dielectric decrease when it is placed in an
external electric field?
3. A point charge +Q is placed at point O as shown in the figure. Is the potential
difference VA – VB positive, negative or zero
4. Draw 3 equipotential surfaces corresponding to a field that uniformily increases in
magnitude but remains constant along Z- direction. how are thes surface different from
that of a constant electric field along Z-direction
5. A positive point charge (+q) is kept in vicinity of an uncharged conducting plate.
Sketch electric field lines originating from the point on to the surface of the plate
6. Why must electrostatic field be normal to the surface at every point of charged
conductor?
7. Draw a plot showing the variation of (i) electric field (E) and (ii) electric potential (V)
with distance r due to a point charge Q.
8. Explain, using suitable diagrams, the difference in the behaviour of a (i) conductor
and (ii) dielectric in the presence of external electric field. Define the term dielectric
polarization
9. Deduce an expression for the capacitance of a parallel plate capacitor with air as the
medium between the plates.
10. A dielectric slab of thickness ’t’ is kept in between the plates, each of area 'A', of a
parallel plate capacitor separated by a distance ’d’. Derive an expression for the
capacitance of this capacitor for t << d.
11. a) Explain briefly how a capacitor stores energy on charging. Obtain an expression
for the energy thus stored.
12. Define ‘dielectric constant’ of a medium. Briefly explain why the capacitance of a
parallel plate capacitor increases, when introducing a dielectric medium between the
plates.
13. Deduce an expression for the electric potential due to an electric dipole at any point
on its axis. Mention one contrasting feature of electric potential of a dipole at a point as
compared to that due to a single charge.
14. A parallel plate capacitor, each with plate area A and separation d, is charged to a
potential difference V. The battery used to charge it is then disconnected. A dielectric
slab of thickness d and dielectric constant K is now placed between the plates. What
change, if any, will take place in
(i) charge on the plates
(ii) electric field intensity between the plates
(iii) capacitance of the capacitor.
Justify your answer in each case.
15. Explain the underlying principle of working of a parallel plate capacitor.
If two similar plates, each of area A having surface charge densities and are σ and -σ
separated by a distance d in air, write expressions for
(i) the electric field at points between the two plates.
(ii) the potential difference between the plates.
(iii) the capacitance of the capacitor so formed.
16. Derive the expression for the energy stored in a parallel plate capacitor of
capacitance C with air as medium between its plates having charges Q and — Q. Show
that this energy can be expressed in terms of electric field as (1/2 έE2 Ad) where A is
the area of each plate and d is the separation between the plates.
b. How will the energy stored in a fully charged capacitor change when the separation
between the plates is doubled and a dielectric medium of dielectric constant 4 is
introduced between the plates?
17.(a) Obtain the expression for the energy stored per unit volume in a charged parallel
plate capacitor.
(b) The electric field inside a parallel plate capacitor is E. Find the amount of work done
in moving a charge q over a closed rectangular loop a b c d a.
18. Figure shows a configuration of the charge array of two dipoles.Obtain the
expression for the dependence of potential on r for r >> a for a point P on the axis of this
array of charges
19.Figure shows the field lines due to a positive point charge. Give the sign of potential
energy difference of a small negative charge between the points Q and P.
20. A point charge (+Q) is kept in the vicinity of uncharged conducting plate. Sketch
electric field lines between the charge and the plate.
(b) Two infinitely large plane thin parallel sheets having surface charge densities 1 and
2 (1 > 2) are shown in the figure. Write the magnitudes and directions of the net
fields in the regions marked II and III.
21. Three concentric metallic shells A, B and C of radii a, b and c (a < b < c) have
surface charge densities + , – and + respectively as shown in the figure.
If shells A and C are at the same potential, then obtain the relation between the radii a,
b and c.
22. (a) Depict the equipotential surfaces for a system of two identical positive point
charges placed a distance ‘d’ apart.
(b) Deduce the expression for the potential energy of a system of two point charges
q1 and q2 brought from infinity to the points and respectively in the presence of external
electric field.
23. Explain the principle of a device that can build up high voltage of the order of a few
million volts.
Draw a schematic diagram and explain the working of this device.
Is there any restriction on the upper limit of the high voltages set up in this machine?
Explain.
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