Question

if force of repulsion between tow electrons is 90N find distance between them(e=1.6×10 –19c) in adjoint fig find Ē at point 'p'​

Answer 1

This is the most basic question of the electrostatic section, it merely requires the knowledge of the formula of the electrostatic force between two charged particles of point size separated by a distance r.

The formula :

• F = KQ1Q2/r²

Here,

• K = 1/4πε0 = 9 × 10⁹ Nm²/C²
• ε0 = 8.85 × 10-¹² C²/Nm²
• Q1,Q2 = Charged particles
• r = distance b/w Q1 and Q2

So, that's the one thing you need to know to solve this question but however I suppose you had difficulty figuring out where are the charged particles given in the question,so you actually been said very clearly about two electrons in the question and of course electrons are charged particles, negatively charged, so both Q1 and Q2 are supposed to be taken as electrons and your work has been made simpler by giving you the value of charge on electron, so Q1 = Q2 = 1.6 × 10-¹⁹C.

Just substitute the values you have in your question and solve for the unknown quantity, in this case distance between them r.

90 = K × 1.6 × 10-¹⁹ × 1.6 × 10-¹⁹/r²

Substitute value of K,

90 = 9 × 10⁹ × (1.6 × 10-¹⁹)²/r²

Cross multiply,

90r² = 9 × 10⁹ × 1.6² × 10-³⁸

Divide by 90,

r² = 9 × 10⁹ × 1.6² × 10-³⁸/ 90

r² = 10⁹ × 1.6² × 10-³⁸ / 10

r² = 10⁸ × 1.6² × 10-³⁸

r = 10⁴ × 1.6 × 10-¹⁹

r = 1.6 × 10-¹⁵

So,the distance between the two electrons to have a repulsive force of 90N is 1.6 × 10-¹⁵ m.

Revision (upto EFL) :

Electrostatic is the branch of physics dealing with charge at rest and various properties associated with it.

Charge : Charge is the intrinsic property of matter carried by the fundamental particles. It is denoted by Q or q.

Types of charges :

• Positive charge
• Negative charge

Properties of charges :

1. Quantization of charge :

Charge on any object can be an integer multiple of a smallest

charge,e = 1.6 × 10-¹⁹ C.

Mathematically,

Q = ± ne ,n ∈ I

• 2. Additivity of Charge

Suppose you have charges q1,q2,q3...qn so the net charge with you will be algebraic sum of all the charges.

• 3. Conservation of charge

Charge can neither be created nor be destroyed, but can be

transferred from one object to another object.

• Coulomb's law :

This law It states that “the force of interaction between any

two point charges is directly proportional to the product of the

charges and inversely proportional to the square of the distance

between them.

Mathematical form :

F = KQ1Q2/r²

• Continuous charge distribution :

There are basically three types of CCD :

1. Linear charge distribution (λ) = Charge/Length = Q/L
2. Surface charge density (σ) = Charge/Surface Area = Q/S
3. Volume charge density (ρ) = Charge/Volume = Q/V