## Assignment on charge, electric force, electric field and dipole

### Question 1

A charged oil drop has a mass 2.2x10-15kg. It remains suspended in an electric field of intensity 4.5x104 NC-1.

What is the charge on the drop. What will be the nature of the charge and is it possible for the drop to have this charge?

Ans: 4.8x10-19C, +ve charge, n=3, Yes

### Question 2

Two point charges +2x10-8 C and -2x10-8 C are placed 6 cm apart. A charge of +1x10-8 C is placed such that it is 5 cm away from each charge. Find the force acting on the third charge.

Ans: 8.64 x 10-4N

### Question 3

Two charges +q and +4q are kept at a distance d apart. Where should we keep the third charge such that the system of three charges is in equilibrium. Also fine the magnitude and sign of the charge.

Ans: At d/3 from +q, 4/9 q, -ve

### Question 4

Two charges of +10μC and +40μC are placed 12 cm apart.

(1) At what point between the two charges, the electric field intensity is zero?

(2) What is the electric field intensity at the mid point between the two charges?

Ans: (i) 4 cm from the the 10μC charge(ii) 0.75 x 108 NC-1

### Question 5

ABCD is a square of side 4 cm. Charges of +16 nC, -16 nC and +32 nC are placed at points A,C,D respectively. Find the magnitude and direction of electric field at point B.

Ans: 1.56x105 NC-1 at an angle of 9o44' with AB produced

### Question 6

An electric dipole consists of two opposite charges each of 1μC separated by 2 cm. The dipole is placed in an external uniform field of 105NC-1 intensity. Find:

(i) Maximum torque exerted by the field on the dipole, and

(ii) the work done in rotating the dipole through 180o starting from position θ=0o.

Ans: τm = 2 x 10-3 Nm, W = 4x10-3J

### Question 7

A dipole consists of two charges +4μC and -4μC separated by a certain distance. If they are located at x = 16.0 cm, y = 0 and x=-16 cm, y = 0 respectively. Find the field strength at a point x = 0, y = 12 cm.

Ans: 1.44 x 106 NC-1 parallel to dipole axis