Question Bank Unit I: ELECTROSTATICS. VERY SHORT ANSWER TYPE QUESTIONS (1, 2, 3 MARKS) 1. How many electrons must be removed form a conductor so that it acquires a 2.
3. 4.
5. 6.
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charge of 3.5 nC. [Ans: 2.1875 X 1010 ] Two electrons have been removed from each atom. Find the distance between two such atoms, if they repel each other with a force of 8.8 X 10-9 N, when placed in free space. It two point charges q1 and q2 are 3m apart and their combined charge is 20µC. If one repels the other with a force of 0.075 N, What are the two charges? Two charged particles having charges 5 µC each one is joined by an i nsulating string of length 1m and the system is placed p laced on a smooth horizontal table. Find the tension in the string. Calculate the electric field strength required to just support water drop of mass 10-7kg and having a charge 1.6 X 10 -19C. [Hint: Ec = mg, E = 6.125 X 1012 NC-1 ] A system has two charges qa and qb; qa = 2.5 X 10-7 C and q b = -2.5 X 10-7 C located at the points A (0, 0, -15cm) and B (0, 0, +15cm). What is the total charge and electric dipole moment? Fig shows tracks of +ve charged particles crossing uniform electric field with same velocities along with horizontal. Give the signs si gns of the three charges. Which particle has the highest charge to mass ratio?
8. In the electric field shown, the electric field lines on the l eft have twice the
separation as that between those on the right. If the magnitude of the field at point A is 40NC-1. Calculate the force experienced by a proton placed at point A. Also find the magnitude of electric filed at the point B.
9. Two charges ± 10µC are placed 5.0mm apart. Determine the electric field at (a) a
point on the axis of the dipole d ipole 15 cm away from its centre – O on the side of the
positive charge, (b) a point 15 1 5 cm away from point – O on a line passing through ‘O’ and normal to the axis of the th e dipole. [(a) 2.6 X 105 NC-1 (b) 1.33 X 105 NC-1 ] 10. The flux of the electric field, through the closed surface S’ is found to be four times that through the closed spherical surface S. Find the magnitude of the charge Q. Given q1 = 1µC, q2 = - 2µC, and q3 = 9.854µC. [Ans: 26.562 µC]
11. An electric dipole AB consists of charges ± 5 nC and separated separated by a distance of 2
X 10-3 m as shown in fig. The Dipole is placed near a long line l ine charge having linear -4 -1 charge density 4.5 X 10 cm , such that the -ve charge is at a distance di stance A = 2.5 cm from the +ve charge. Find the force acting on the dipole. [Ans:0.12 N towards the line of charges].
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12. Two charges 3 X 10 C and -2 X 10 C are located 15 cm apart. At what point on
the line joining the two charges is the electrical potential to be zero. 13. A parallel plate capacitor with air between plates has a capacitance of 8PF. The separation between the plates is now reduced by half and the space between them is filled with a medium of dielectric diel ectric constant 5. Calculate the value of capacitance of the capacitor in the second case. [Ans: 20 PF] 14. The effective capacitance of two capacitors is 3µF, when they are connected in series and parallel respectively, Compute the capacitance of each capacitor. 15. Find the effective capacitance between the terminals X and Y of the network shown. Given that C1 = 5 µF, C2 = 10 µF, C3 = 2 µF, C4 = 4 µF and C5 = 10 µF.
16. Fig. Shows the variations of voltage V across the plates of the capacitors A and B
versus increase of charge Q stored on them. Which of the two capacitors has higher capacitance? Give reason for your answer.
17. Fig shows the variation of charges Q versus potential difference V for two
capacitors C1 and C2. The two capacitors have same plate separation but the plate area of C2 is double that of C1 and C2 and why?
18. A charged particle Q is placed at the centre O of the cube at a distance L from O.
Then the electric flux through ABCD is?
19. A square surface of side L meters is in the plane of the Paper, is limited only to
the lower half of the square surface as shown in fig. What is the flu x associated with the surface.
20. A copper sphere of mass 2g contains nearly 2 X 10
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electrons. The change on the nucleus of each atom is 29e. What fraction of the electrons must be removed from the sphere to give it a charge of +2 µC? µ C? [Ans: 2.16 X 10-11 ] 21. An attractive force of 5N is acting between two charges of +2 µc placed at some distance. If the charges are mutually touched and placed again at the same distance, what will be the new force, between them? [Ans: Zero] 22. Suppose that the earth has a net charge that is not zero. Is it still possible to adopt the crash as a structural reference point of potential and assign the potential V=0 to it?
23. A test charge q is moved without acceleration from A to C over the path ABC as
shown. Calculate potential difference between A and C.[ Hint: ] 24. Electric lines of force are shown in the fig. At which point (P, Q, or R) is electric field minimum?
25. A Conductor has a cavity in it and is given a charge +Q. What sill be the total
charge on its surface o another conductor carrying charge +q is placed in the cavity without touching the outer conductor? 26. Two charges each of 200µc, but opposite in sign are 40cm apart. Calculate the electric field at a point distance 30cm from the mid point poi nt on the axial line li ne of the 8 -1 dipole. [Ans: 10 NC ] 27. Two similarly and equally charged identical metal spheres A and B repel each other with a force of 2 X 10 -5 N. A third identical uncharged sphere C is touched with A and then placed at at a midpoint between A and and B. Calculate the net net electric force on C.
ONE MARK QUESTIONS 28. Why is the electric potential energy of a pair of like point charges positive? 29. What will be the electric potential of a charge at α ? 30. A charge q0 has been taken from R to A, R to B and R to C. In which condition
work done is maximum?
31. Devise an arrangement of three point charges separated by finite distances that
as zero electric potential energy. 32. Does an electric charge experience a force due to the field, it produces it self? 33. The test charge used to measure electric field at a point should be vanishingly small why? 34. What is an ideal electric dipole? 35. A small test charge is released at rest at a point in i n an electric field configuration. Will it travel along the th e line of force passing through that point? 36. Two capacitors C1 = 3µF and C2 = 6µF are connected in series and charged by connecting a battery of voltage V= 10volt in series with them. They are then disconnected from the battery and loose wires are connected together. What is the final charge on each? 37. On what factor does the capacitance of a capacitor depends? 38. Two capacitors 4 µF and 6 µF are connected in series with a 6V battery. Which will have more potential difference? 39. In what form is the energy stored in a charged capacitor? 40. What is the effect of temperature on dielectric constant?
TWO MARKS QUESTION 41. The electric potential at a distance 1.0m from a charge is +100 volt. Compute the
charge. 42. If a charge +Q is revolved once round another charge +q in a circle of radius R how much work is done? 43. The electric potential at 0.9m from a point charge is +50v. Find the magnitude and nature of charge.
44. The work done in moving a charge of 3C between two points is 6J. What is the
potential difference between the points? 45. A point charge q is place at O as shown in the figure. Is Vp – Vq positive or negative when (i) q>0 (ii) q<0 ? justify your answer. 46. When the plates of a charged capacitor are brought near, what is the effect on
potential difference between plates? Explain with reason. 47. Explain why the polarization of a dielectric reduces the electric field inside th e dielectric. 48. Find the length of the paper used in a capacitor of capacitance 2 µF if the dielectric constant of the paper is 2.5 and its width wid th and thickness are 50mm and 0.05mm respectively. 3 MARK QUESTIONS 49. Calculate the potential at the centre of a square of side √(4.5)m which carries at
its four corners charges of + 5 X10-9 C, +2 X 10-9 C, -5 X 10-9 C and -7 X 10-9 C respectively. po int on the equatorial line li ne of an 50. Show mathematically that the potential at a point electric dipole is zero. 51. A charge of 8Mc is located at the origin. Calculate the work cone in taking a small charge -2 X 10-9 C from a point P (0, 0, 3cm) to a point Q(0, 4cm, 0) via a point R(0, 6cm,9cm). 2 52. The electric field due to a point charge depends on the distance r as (1/r ), similarly indicate how each of the following quantities quantiti es depends on it? a. Intensity of light from a point source. b. Electric potential due to a point charge and c. Electric potential at a distance r from the centre of a charged metallic sphere of radius (r < R). 53. A capacitor of capacitance C is charged fully by connecting it to a battery e.m.f ε. It is then disconnected from the battery. If the separation between the plates of the capacitor is doubled, what will be effect on a. Potential difference across it. b. Energy stored by the capacitor? 54. A 60 PF capacitor is connected to a 20V battery and is charged to 20V. How many electrons are transferred from one plate to the other?
5 MARK QUESTIONS 55. Explain the concept of electric potential energy derive an expression for potential
energy of a system of two point charges. Generalize the expression for N discrete charges. -8 56. An electric dipole consists of two opposite charges each of magnitude 6 X 10 coulomb separated by 6.0cm. The dipole is placed in an external electric filed of 5 X 10-5 NC-1 . a. What maximum torque will the field exert on the dipole? di pole? b. How much work will an external agent have to do in turning the dipole through 180o, starting from the position θ = 0 o? 57. If one of the two electrons of a hydrogen molecule is removed, we get a hydrogen molecular ion (H2+). In the ground state of H 2+, the two protons are separated roughly by 1.5 Å and electron is roughly 1 Å from each proton. determine the potential energy of the system. Specify your choice of zero of P.E. 58. Find an expression for the energy of a charged capacitor. Where does this thi s energy reside? 59. N drops of mercury of equal radii and possessing equal charges combine to form a big drop. What is the charge, capacitance and potential of the bigger drop.
