# What Are the Applications of Coulomb's Law?

Coulomb's Law has a great many applications to modern life, from Xerox machines to laser printers, to powder coating.

The ancient peoples living around the Mediterranean Sea knew that if they rubbed a rod of amber on a cat's fur, the rod would attract light objects, such as feathers. The ancient Greeks knew that certain minerals, such as magnetite, were magnetic.

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In 1600, the English scientist William Gilbert distinguished magnetism from static electricity, and he named the static electricity force electricus from the Greek word for "amber."

It wasn't until 1767 when the English scientist Joseph Priestley formally proposed that the electrical force between two charged objects diminished with the square of the distance between them. Priestley is primarily known as the discoverer of the element oxygen, and for having invented carbonated water. So, the next time you sip a Coke, tip your hat to Joseph Priestley.

In 1785, the French physicist Charles-Augustin de Coulomb published three papers on electricity and magnetism in which he described the electrostatic force. The law has become known as Coulomb's Law, and it states:
1. Like charges repel each other, and unlike charges attract one another.
2. The attraction or repulsion acts along a line between the two charges.
3. The magnitude of the force is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

The scalar form of the equation is:

Where ke is Coulomb's constant, q1 and q2 are the signed magnitudes of the charges, and the scalar r is the distance between the charges. Force is measured in newtons, charge in coulombs, and distance is measured in meters.

Surprisingly, the inverse-square behavior of the electrostatic force is identical to that of gravity as described in Isaac Newton's law of universal gravitation.

### Haloid Becomes Xerox

Fast forward to 1938 and a patent attorney named Chester Carlson who was working in the U.S. Patent Office in New York City. Carlson was required to make copies by the hand of a large number of papers, but his hands were arthritic. Tinkering in his kitchen at night, he made the first photocopy machine.

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Between 1939 and 1944, Carlson tried to market his invention to over 20 companies, including IBM and General Electric, and they all turned him down citing no need for a copy machine. Eventually, in 1947, the Haloid Corporation, a small New York manufacturer of photographic paper, obtained a license to develop a copy machine based on Carlson's invention.

They consulted a professor of classical languages who came up with the name "xerography" from the Greek words xeros for dry, and graphos for writing, and the Haloid Corporation became the Xerox Corporation.

The electrostatic process is what makes copies. It uses a selenium-coated aluminum drum because selenium has an interesting property — it is an insulator when in the dark, and a conductor when exposed to light.

In the first stage of the xerography process, a negative charge is induced under a thin layer of positively charged selenium.

The surface of the drum is then exposed to the image to be copied, and where the image is light, the positive charge is neutralized, and where the image is dark, the positive charge remains. The image has now been transferred to the drum.

Then, a dry black powder, called toner, is sprayed with a negative charge, which will be attracted to the positive areas of the drum.

A blank piece of paper is given a greater positive charge than the drum, so that it will pull the toner from the drum, and finally, the paper and toner are passed through heated rollers that melt and permanently adhere the toner to the paper.

In laser printers, a laser beam is scanned across a photoconducting drum, which leaves a positively charged image, then the next steps are the same as in xerography. Because laser light can be very precisely controlled, laser printers can produce very high-quality images.

The electrostatic process is also used in ink jet printers where a nozzle finely sprays tiny ink droplets, which are then given an electrostatic charge. The droplets are directed using pairs of charged plates, and they form letters and images on paper. Color inkjet printers use black, cyan, magenta, and yellow jets.

### Powder Coating, It's Not Just For Motorcycles

Another use of the electrostatic process is electrostatic painting or coating, also known as "powder coating." The process uses a high voltage electrostatic charge which is applied to both the object to be coated and the sprayer mechanism.

A coating of either powdered particles or an atomized liquid is accelerated toward the workpiece by a powerful electrostatic charge. The ionic bond of the coating to the object creates a uniform coating that adheres extremely well.