Coulomb’s Law

by on November 17, 2009

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coulombs law

Coulomb’s Law, shown in the image, is an expression of the electrostatic force between two charged particles. The two q’s represent the magnitudes of the charge of each particle while r is the distance between the two. The last symbol, k, is a proportionality constant known as Coulomb’s constant and takes the value approximately equal to 8.99×109 N m2 C-2.

The electrostatic force is a vector quantity. As such, when solving for the net electrostatic force on a particle due to, say, 4 other charged particles, vector addition is used. That is, the positions of the other particles relative to the charged particle in question is factored into the solution.

In a basic physics problem, you may solve for the net electrostatic force , also known as a the resultant electrostatic force, using either the component method or the graphical method – just like any force problem. Of course, when one considers the net force exerted by an infinitely large number of charged particles (known as a charge distribution), an integral over the region is necessary.

The direction of the electrostatic force of one charged particle on another depends on the signs of both charges. For like charges (positive and positive or negative and negative), the force is repulsive. Therefore, the force will point away from the particle exerting the force. For unlike charges (positive and negative), the force is attractive. Therefore, the force will point towards the particle exerting the force.

In both cases, the direction of the force will lie along an imaginary line representing the shortest distance between the two charged particles.

The behavior of the magnitude is easy to determine if we take a look at Coulomb’s Law above. As you can see, the magnitude of the force is directly proportional to the magnitudes of the two charged particles. The larger their charges are, the larger the magnitude becomes.

On the other hand, the magnitude of the force is inversely proportional to the square of the distance between the two. Hence, a slight change in the distance between the two will automatically correspond to a huge change in the electrostatic force.

Readers who are familiar with classical mechanics will be quick to observe the similarity between Coulomb’s Law and the Law of Universal Gravitation. Just change the proportionality constant to G (the gravitational constant) and both q’s to m’s. However, while Coulomb’s Law allows for both attractive and repulsive forces, the Law of Universal Gravitation only results in a purely attractive force.

Coulomb’s Law deals with charges. Universe Today has articles talking about the charge of the proton and the charge of the electron.

NASA also has some related stuff. Check out the following articles:
Charges
Killer Electrons

Here are two episodes at Astronomy Cast that you might want to check out as well:
Antimatter
The Search for Dark Matter

Source: The Physics Classroom

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