Coulombs Law, Electrostatic Field & Electric Dipole

1 Mark Questions

1. Why do the electrostatic field lines not form closed loop? [All India 2014, Delhi 2012] Ans. The electrostatic field lines do not form closed loop because no electric field lines exist inside the charged body
2. Why do the electric field lines never cross each other? [All India 2014]

Ans Ans. At the intersection point, there would be two directions of electric field which is not possible so lines of forces never cross each other

1. Why must electrostatic field at the surface of a charge every point? Give reason. [Foreign 2014, Delhi 2012]

Ans. As, electric field inside a conductor is always zero. The electric lines of forces exert lateral pressure on each other leads to explain repulsion between like charges. Thus, in order to stable spacing, the lines are normal to the surface.

4.Two point charges g_x and q₂ are placed at a distance d apart as shown in the figure. The electric field intensity is zero at the point P on the line joining them as shown. Write two conclusions that you can draw from this. [Delhi2014c]

<math>q_1</math> d <del> </del>

Ans.(i) The two point charges <math>(q^{\Lambda})</math> and <math>(q^{\Delta})</math> should be of opposite nature.

1. Magnitude of charge <math>q_1</math> must be greater than magnitude of charge <math>q_2</math>.

5.Define dipole moment of an electric dipole. Is it a scalar quantity or a vector quantity? [Foreign 2012; All India 2011]

Ans. Electric dipole moment of an electric dipole is equal to the product of its charges and the length of the electric dipole. It is denoted by p. Its unit is coulomb-metre. р <math>\mathbf{p} = q \times 2\mathbf{I}</math>

It is a vector quantity and its direction is from negative charge towards positive charge

6.Draw a plot showing the variation of electric field (E) with distance r due to a point charge

1. [Delhi 2012]

Ans. The plot showing the variation of electric field and electric potential with distance r due to a point charge q is shown as below

4.5 4 4πε₀ /2 †<br>E 3.5 3 2.5 2 1.5 1 0.5 00 0.5 1 1.5 2 2.5 3 3.5 4

7.A proton is placed in a uniform electric field directed along the position X-axis. In which direction will it tend to move? [Delhi 2011 c]

Ans. Proton will tend to move along the X-axis in the direction of a uniform electric field.

1. In which orientation, a dipole placed in a uniform electric field is in (i) stable (ii) unstable equilibrium?[Delhi 2011; All India 2008] Ans.
2. For stable equilibrium, the angle between p and E is <math>0^{\circ}</math>. (1/2)

-a +q

1. For unstable equilibrium, the angle between p and E is 180°.

+ E +q (1/2)

1. Two point charges having equal charges separated by Im distance experience a force of 8
2. What will be the force experienced by them if they are held in water at the same distance?

(Given, K_water = 80).[Ail India 2010 C]

Ans. Two point charges system is taken from air to water keeping other variable (e.g., distance, magnitude of charge) unchanged. So, only factor which may affect the interacting force is dielectric constant of medium

Force acting between two point charges

<math display="block">F = \frac{1}{4\pi \, \epsilon_0 K} \frac{q_1 \, q_2}{r^2} \quad \text{or} \quad F \propto \frac{1}{K}</math>

medium

8 <math>\Rightarrow</math> <math>= 80</math> <math>F_{\rm water}</math>

<math>F_{\text{water}} = \frac{1}{80}</math>

<math>F_{\text{water}} = \frac{1}{10} \text{ N}</math>

10.A metallic shpere is placed in a uniform electric field as shown in the figure. Which path is followed by electric field lines and why?[HOTS; Foreign 2010]

Ans. Path d is followed by electric field lines. Electric field intensity inside the metallic sphere will be zero, therefore, no electric lines of force exist inside the sphere, also lines fall normally on the surface. Electric field lines are always perpendicular to the surface of the conductor.

1. Point out right or wrong for the following statement. The mutual forces between two

charges do not get affected by the presence of other charges.

Ans. Right, because mutual force acting between two point charges is proportional to the product of magnitude of charges and inversely proportional to the square of the distance between them, i.e. independent of the other charges.

12.A dipole of dipole moment p is present in a uniform electric field E. Write the value of the