Coulombs Law Coulombs Law Coulomb's law or Coulomb's inverse-square law is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism. Coulomb's law has been tested heavily and all observations are consistent with the law. Coulomb's law states that: "The magnitude of the Electrostatics force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distances between them." There are scalar and vector forms of the equation. The scalar expression assumes that the distance between the charges is large compared to the size of the charge, which means that the two charges in the scalar equation are point charges at any distance. In the more useful vector-form statement, the force in the equation is a vector force acting on either point charge, so directed as to push it away from the other point charge; the right-hand side of the equation, in this case, must have an additional product term of a unit vector pointing in one of two opposite directions, e.g.,
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from q 1 to q 1 if the force is acting on q 2 ; the charges may have either sign and the sign of their product determines the ultimate direction of that force. Thus, the vector force pushing the charges away from each other (pulling towards each other if negative) is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. The square of the distance part arises from the fact that the force field due to an isolated point charge is uniform in all directions and gets "diluted" with distance as much as the area of a sphere centered on the point charge expands with its radius. The law of superposition allows this law to be extended to include any number of point charges, to derive the force on any one point charge by a vector addition of these individual forces acting alone on that point charge. The resulting vector happens to be parallel to the electric field vector at that point, with that point charge (or "test charge") removed. Coulomb's law can also be interpreted in terms of atomic units with the force expressed in Hartrees per Bohr radius, the charge in terms of the elementary charge, and the distances in terms of the Bohr radius. Magnitude of Electric forces between the charged objects was observed by the coulomb by using the torsion balance. We can directly come to the law but first we will discuss that how the coulomb law was derived by coulomb using the Torsion Balance. Torsion balance was used by the coulomb for measuring the electrostatic force. Torsion balance is shown in the figure 1 below. There are two pith balls present in the torsion balance. One ball is mounted and present on the end of a glass rod. This ball is shown in red color and is unmovable. The other ball is shown in blue color. Blue ball can spin around and can move. Electrostatic Charge was given to both the balls. We can give electrostatic charge to the balls firstly by rubbing the rod of plastic with fur and then touching the plastic rod with the balls.
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Now what happened was, when similar type of charge was given to both the red and blue balls, the blue ball moved away from the red ball. But on the other hand, when different electrostatic charges were given to the red and blue balls, blue ball which is movable start moving towards the red ball which is unmovable. This shows that when both the balls were given same charge they repel each other but on the other hand when they were give different kind of charges they attracted each other. Electrostatic force between the balls was measured with the help of distance moved by the blue ball. Coulomb measured the torsion on the fiber with the help of scale near the top of the device and the distance between the two balls on the scale wrapped around the base of the jar. Mathematical equation was derived by him which showed the relationship between these two measurements.
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