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Electrostatics deals with the study of slowly moving or stationary electric charges. Electrostatic force is also called Coulomb’s force after the French physicist, Charles-Augustin de Coulomb. It refers to the forces of attraction or repulsion that exist between two objects because of their electric charge.

Electrostatic force examples
One common example of electrostatic force is the static electricity that is produced when there is a transfer of charged particles from one object to another. When two insulating objects are rubbed against each other, equal and opposite charges are acquired by them. These equal and opposite charges result in a force of attraction between the two objects. The object that loses electrons is now positively charged and the object that gains electrons is negatively charged. These positive and negative charges attract each other.

Coulomb’s law can also be written in a mathematical form:

If q1 and q2 represent the amount of electrical charges at two points and r is the distance between their centers then the Electrostatic force F between these two charge points can be represented as

F = k  (q1 * q2)/ r2
Where k is the proportionality factor known as the Coulomb’s law constant. It is dependent on the material in which the charged particles are present. In vacuum k is equal to 1. In air, the value of the constant k is approximately 9.0 x 109 N m2 / C2. If the charged points are in water, then the value of Coulomb’s constant k can be reduced by a factor of 80.

Charges can be positive or negative hence q1 and q2 may hold positive or negative values.  This will result in a net positive or negative electrostatic force. A positive electrostatic force implies it is a force of repulsion as it means both points have the same charge (could be positive or negative). A negative electrostatic force implies it is a force of attraction as the points have one positive and one negative charge resulting in a net negative charge.

The units of the Electrostatic force F are Newtons.

It is important to note that the objects in Coulomb’s law act as point charges that is the charge can be considered to be at the center of the object. The electrostatic force is inversely proportional to the square of the distance between the centers of the two objects.

Example:

Question: What is the electrostatic force between two objects with charges of +3.5 x 10-8 C and -2.9 x 10-8 C respectively, if the distance between their centers is 0.63m.

q1 = +3.5 x 10-8 C
q2 = -2.9 x 10-8 C
r = 0.63
k = 9.0 x 109 N m2 / C2
F= k *q1 * q2/ r2
= -2.3 x 10-5 Newtons
As the force is negative, it is an attractive force.

Question: What is the electrostatic force between two balloons with equal and opposite charges of 4.0 x 10-6 C. The distance between the centers of the balloons is 0.7m. Is it a repulsive or an attractive force?

Since charges on the two balloons are equal and opposite
q1 = 4.0 x 10-6 C
q2 = -4.0 x 10-6 C
r = 0.7
k = 9.0 x 109 N m2 / C2
F    = k *q1 * q2/ r2
= 9.0 x 109 * -4.0 x 10-6 * 4.0 x 10-6 / (0.7 * 0.7)
= - 0.29 Newtons

As the force is negative, it is an attractive force.

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