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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

## 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.

## 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? ## 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

The charge remains same

Use Cm=KCo
and
V reduced to 1/8 means capacitance increased 8 times since Q=CV

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

Why there is a 4π in Coulomb’s law?

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 4πr2.

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.

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