## 29 April 2012

### XII Physics Electrostatics

SYLLABUS :

Electric charges; conservation of charge ; Coulomb’s law- forces between two point charges ,Forces between multiple electric charges: Superposition principle and continuous charge distribution.

Electric field, Electric field due to a point charge, electric field lines; Electric dipole , electric field due to dipole ; torque on dipole in uniform electric field.

Electric flux, statement of Gauss’s theorem and its applications to find field due to infinitely long straight wire, uniformly charged infinite plane sheet and uniformly charged thin spherical shell (field inside and outside.)

Electric potential –potential difference, Electric potential due to point charge ,a dipole and system of charges ;equipotential surfaces ,electrical potential energy of a system of two point charges and of electric dipole in an electrostatic field. Conductors and insulators ,free charges and bound charges inside a conductor. Dielectrics and electric polarization, Capacitors and capacitance, combination of capacitors in series and parallel, capacitance of a parallel plate capacitor with and without dielectric medium between the plates, energy stored in a capacitor. Van deGraff generator.

1. Explain briefly (i) charge quantization (ii) conservation of charge.

2. Write vector form of coulomb’s law. How force depends on intervening medium.

3. Define Electric field intensity and electric potential due to a point charge. Give their S.I.units.

4. Derive expression for electric potential due to a point charge.

5. Define Electric dipole moment and give its unit and direction.

6. Find expression for electric field intensity on axial and equatorial line of a electric dipole.

7. State Gauss’ theorem . Calculate electric flux due to surface enclosing charge of 1C.

8. Obtain the expression for the electric field intensity E due to infinite plane sheet of charge .

9. Derive the expression for the E at any point outside and inside a uniformly charged spherical shell.

10. Derive an expression for the E at a point near an infinitely long , thin ,uniformly charged straight wire.

11.Find expression for torque on electric dipole in uniform electric field. Give its equilibrium state.

12. Derive expression for capacitance of a parallel plate capacitor i)without dielectric ii)with dielectric slab.

13.Derive expression for energy stored and energy density (½εοE2) of a charged capacitor.

14.How Voltage, charge , capacity ,energy and electric field changes when dielectric is introduced  between the plates of capacitor i)with battery connected and ii)without battery.

15. Draw lines of forces for i) q>0 ii)q<0 .Why lines are not discontinuous and never intersect .

16. What is equipotential surface? Find work done on moving charge on such surface.

17.Give principle ,construction, working and labeled diagram of a Vande graff generator. How

charge leakage is prevented.

19. A charge q is placed at the centre of line joining two equal charges Q. Show that the system of three charges will be in equilibrium if q = -Q/4.

20. Two point charges 3µC and -3µC are located 20cm apart in vacuum. Find electric field and electric potential at midpoint of line. Also find force and its direction experienced by charge 1.5nC kept at midpoint.

21. A 10µF capacitor is charged by a 30V d.c. and then connected across an uncharged 50µF

capacitor. Calculate (i) the final potential difference of the combination, and (ii) the initial and final

energies. How will you account for the difference in energy?