Monday, 4 April 2016



1. Two charges +q and –q are located at points A (0,0,-2) and B(0,0,2) respectively How much work will be done in moving a test charge form point P(4,0,0) to Q(-5,0,0)

2.If 100J of work must be done to move electric charge =4C from a place where potential is -10 V to another place where potential is V volt, find the value of V.
VA – VB = WAB/q The value of V is 15 V.

3.If 10J of work is to be done in moving a charge of -200C from A to B., which of the 2 points is at higher potential? What is potential difference?
A; -0.05V

4. Two point charges of 4μC and -2μC are separated by a distance of 1m in air. Calculate at what point on the line joining the 2 charges is the electric potential zero?
2/3 m from 4 C

5. What is potential at the centre of a square of each side 1.0m when 4 charges +1 x 10 -8C, -2 x10 -8 ,
+3 x10 -8 and +2 x 10-8 C are placed at 4 corners of the square.
potential is 5.1 x 102 V

6. A point charge of 10 -8 C is situated at the origin of coordinates. Find the potential difference between the point A (4, 4,2) and B(1,2,2).
potential is -15 V

7. 27 charged water droplets each with diameter of 2mm and a charge of 10 -12 C coalesce to form a single drop. Calculate the potential of bigger drop.
Potential of the bigger drop is 81 V

8.An electric field E = (20i+30j)N/C. exist in the space. If the potential at the origin is taken to be 0 , find the potential at (2m,2m) .
potential at (2m, 2m) is -100 V

9.Two point Charges of A and B of value +15μC and +9μC are kept 18cm apart in air. Calculate the work done when charge B is moved by 3 cm toward A.
Work done is 1.35 J

10.2 electrons each moving with a velocity of 10 6 m/s are released towards each other. What will be the closest distance of approach between them?
2.56 x 10-10 m

11. Setup an arrangement of 3 point charges +q , +2q and xq are separated by equal finite distances so that electric PE of the system is 0. What is x?
Charges +q, +2q and -2q/3 at vertices of an equilateral triangle; x = -2/3

12. Three charges –q ,Q and –q are placed at equal distances on a straight line. If the potential Energy of system of 3 charges is 0 then what is the ratio of Q: q?


Q 1. An electric dipole with dipole moment 4 x 10 -9 Cm is aligned at 30O with direction of a uniform electric field of magnitude 5 x 10 4 N/C. calculate the magnitude of torque acting on the dipole.

Total charge of the system is zero . Dipole moment of system is given by q x d =7.5x 10-8 Cm

Q 2. A system has two charges qA = 2.5 x 10 -7 C and qB = -2.5 x 10 -7 C are located at points A(0,0,-15 cm) and B(0,0,+15 cm) respectively. What are the charge and electric dipole moment of the system?

Torque =pE Sin θ =10-4 Nm

Q 3. In a certain region of space, electric field is along the z direction throughout. The magnitude of electric field, however not constant but increases uniformly along the positive z direction, at the rate of 10 5 N/C/m. What are the force and torque experienced by a system having a total dipole moment equal to 10-7 Cm in the negative z-direction?

Force along the – ve Z axis 10-2 N Torque = pE sin 180O = 0

Q 4. An dipole of length 2 cm is placed 300 w.r.t a uniform electric field of 105 N/C experiences a torque of magnitude 10√3 Nm. Calculate (i) magnitude of charge and (ii) PE of electric dipole.

i)√3 x 10-2 C (ii) U = -30 J.
Q5. An electric dipole consist of two opposite charges of magnitude q=1 x 10 -6 C separated by 2 cm. The dipole is placed in an external field of 1 x 10 5 N/C.What maximum torque does the field exert on the dipole? How much work must an external agent do to turn the dipole end for end, starting from position of alignment (θ=0O)?

2 x 10-3 Nm is max torque and work done

Q6. An electric dipole consist of charges +2 e and -2 e separated by 0.78 nm. It is an electric field of strength 3.4 x 106 N/C. Calculate the magnitude of the torque on the dipole when the dipole moment is
i) Parallel to
ii) Perpendicular to
iii) Antiparallel to electric field

(i) Zero (ii) 8.5 x 10-22 Nm (iii) Zero

Q7.An electric dipole of length 10 cm having charges ±6 x 10 -3C, placed at 30O with respect to a uniform electric field experiences a torque of magnitude 6(3)1/2 Nm. Calculate
i) Magnitude of electric field
ii) Potential energy of dipole.
 (i)E = 2 √3x104N/C                    (ii)U = -18 J

Q8.An electric dipole of length 4 cm, when placed with its axis making an angle of 60◦ with a uniform electric field experiences a torque of 4(3)1/2 Nm. Calculate the potential energy of the dipole, if it has charge of ± 8 nC.

-4 J