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# Category Archives: Electrostatics

## Coulomb’s law

at each of the four corners of a sqare of side A, a charge +q is placed freely. what charge should be placed at the centre of the sqare so that whole system be in equilibrium

Asked Shashank

## Electrostatics Concept Map and Question Bank

## Electrostatics Concept Map

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## Value based questions from Electrostatics

## Numerical from Electrostatics

Three point charged lie along the x axis in a vacuum: charge 1 in the middle, with charge 2 to the left, and charge 3 to the right. The magnitude of charge 1 is +3 microcoulomb, the magnitude of charge 2 is -4 microcoulomb, and the magnitude of charge 3 is -7 microcoulomb. The distance between charges 1 and 2 is 20 cm, while the distance between charges 1 and 3 is 15 cm. Determine the magnitude and direction of the net electrostatic force on charge 1.

Asked Tammy

## A question from electric field

An insulating sphere of mass m and positive charge q is attached to a spring with length h and spring constant k and is at equilibrium. An infinitely long wire with positive linear charge density λ is placed a distance l away from the charged mass at equilibrium (note that the position of the top of the spring is fixed). The previous length of the spring was h. What is the new length of the spring in terms of h, q, k, λ, l as needed.

## An interesting question from electrostatics

Two equal negative charge -q are fixed at the point (0,a) & (0,-a) on yhe y-axis. a positive charge Q is released from rest at the point (2a,0) on the x-axis. the charge Q will execute oscillatory motion how?

Asked Kritika

## Capacitance, voltage and Potential difference

the capacity of parallel plate condenser is 5 micro Faraday. When a glass plate is placed between the plates of the conductor its potential become 1/8th of the original value. The value of dielectric constant will be

(A)1.6

(B)5

(C)8

(D)40

Answer:

The charge remains same

## Why there is a 4(pi) in Coulomb’s law?

Why there is a 4π in Coulomb’s law?

Asked Rick Ghosh

Answer:

This is a consequence of geometry.

Please remember that Coulomb’s law is stated originally for **point charges ** at rest. By symmetry, a point charge should exert equal force on a test charge at all points equidistant from it, which constitute a spherical shell of surface area **4πr ^{2}**.

This consideration itself suggests the inverse square relation ship as well as the need for **4π.**

That was actually my explanation for it.

The term 4π was not there when Coulomb’s law was first stated in cgs system. The term 4π was introduced in SI on rationalization of units based on Maxwell’s equations of electromagnetism.

But I feel that my earlier explanation is simple and convincing.

Hope you got it.

Further discussions are welcome.