## FSc Notes: Physics XII: Chapter 12 Electrostatics Exercise Short Questions:

**Question 12.1 The potential is constant throughout a given region of space. Is the electrical field zero or non-zero in this region? Explain**

Answer 12.1Electric field is defined as “

Answer 12.1

**the negative of the potential gradient**”

i.e.

**E= - ΔV / Δr.**

If electric potential is constant over a given region of space then potential difference between any two points is zero.

**ΔV = 0 As ΔV / Δr=0 .**

So E=0 Hence the electric field is zero in that region of sphere.

**Question 12.2 Suppose that you follow an electric field line due to positive point charge. Do electric field and the potential increase or decrease?**

Answer 12.2The electric due to positive charge is always directed outward from the charge, so if we are following a field due to such a charge our distance r from the charge is increasing.

Answer 12.2

Since

**V=kq/r and E=kq/r2**.

This implies

**E∝1/r2 and V∝1/r.**

Hence Both E and V will decrease in such a case.

**Question 12.3 How can you identify that which plate of a capacitor is positively charged?**

Answer 12.3The presence of charge is detected by a device called “Gold leaf Electroscope” on bringing charged plate or disc close to the plate of gold leaf electroscope.

Answer 12.3

- If divergence of plates increases then the plate is positively charged.
- If divergence of plates decreases then the plate is negatively charged.

**Question 12.4 Describe the force or forces on a positive point charge when placed between parallel plates.**

a. With similar and equal changes.

b. With opposite and equal charge.

Answer 12.4 (a)When a positive charge is placed between two parallel plates having similar and equal charges, the net force acting on the charge will be zero.

a. With similar and equal changes.

b. With opposite and equal charge.

Answer 12.4 (a)

**(b)**When the charge is placed between the plates having equal but opposite charge, the positive charge will move from positive plate to negative pate.

**Question 12.5 Electric lines of force never cross. Why?**

Answer 12.5Actually, the direction of electric field lines at a certain point represents the direction of motion of a unit positive charge when placed at that point. Now if two lines cross each other then, at that point of intersection, a positive charge will have two directions of motion when placed there. It is not possible at all. So, electric lines of force never cross each other.

Answer 12.5

**Question 12.6 If a point charge q of mass m is released in a non-uniform electric field with field lines pointing in the same direction, will it make a rectilinear motion?**

Answer 12.6Non uniform field means that lines of force are neither parallel nor equally spaced.

Answer 12.6

- Case 1: When electric field is due to a single isolated charge, the charge ‘q’ of mass ‘m’ will make rectilinear motion.
- Case 2: When the field is due to two oppositely charged particles then q will not make rectilinear motion.

**Question 12.7 Is E necessarily zero inside a charged rubber balloon if balloon is spherical? Assume that charge is distributed uniformly over the surface.**

Answer 12.7Yes, E is necessarily zero inside a rubber balloon if balloon is spherical and charge is distributed uniformly over the surface. According to Gauss’s Law. Flux density = φe = 1 / ∈(Charge enclosed)

Answer 12.7

**φe = 1 / ∈(0)-------------I**

φe = 0

**As φe = E • A------------II**

From (i) and (ii)

here A is not zero E=0

**Hence, electric intensity E is zero inside a spherical balloon.**

**Question 12.8 Is it true that Gauss’s law states that the total number of lines of force crossing any closed surface in the outward direction is proportional to the net positive charge enclosed within surface?**

Answer 12.8Yes, it is true that total number of lines of force crossing any closed surface in the outward direction is proportional to the net positive charge enclosed within the surface

Answer 12.8

i.e. Flux density = φe = 1 / ∈(Charge enclosed) or Flux = Constant (Total Charge Enclosed)

that is Flux ∝ Total charge enclosed.

**Question 12.9 Do electrons tend to go to region of high potential or of low potential?**

Answer 12.9As e- carries negative charge, it will move from a point at lower electric potential to a point at higher potential in an electric field.

Answer 12.9

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