History of Electrostatics :: CBSE Physics

History of Electrostatics

Origin

When we talk about history of electrostatics or electricity, names like Benjamin Franklin, Thomas Edison and Alessandro Volta come to mind.

In fact, these are not the people who discovered electricity

  • Electricity was first mentioned in the works of a Greek scientist named Thales of Miletus in about 600BC!
  •  He noticed that if amber (hardened tree sap) was rubbed, it had the ability to pick up dust and leaves. What he was seeing is what we now call “static electricity”.
  • Another Greek named Theophrastus noticed in 300BC that other substances had static electricity if rubbed.
  • Unfortunately, neither Thales nor Theophrastus had any scientific explanation for it. They just thought it was interesting.

What they did realize was that sometimes two objects would attract each other and sometimes they would repel.

  • This developed into the idea that there are two kinds of charges (we call them positive and negative).
    • Like charges repel
    • Opposite charges attract.
  • This is usually called the Law of Charges.

In 1600AD an Englishman named William Gilbert started studying these phenomena.

  • He wanted to come up with a good scientific explanation for these ancient discoveries.  He was actually the first person to use the word “electric,” which is a variation of the Greek name for amber.
  • Although he had only some success in describing electricity, he was able to show that there were differences between magnetism and electricity that seemed to indicate that they were completely different things.
    • For example, an amber rod had to be rubbed to have electric effects; a magnet was always a magnet (didn’t need to be rubbed).
  • Up until that point most scientists had believed electricity and magnetism were simply different versions of the same thing
history of electrostatics


William Gilbert was the Court Physician to both Queen Elizabeth I and King James I. This meant that he acted as an adviser in scientific matters.  

 

The Franklin Era

Benjamin Franklin’s Experiments

Benjamin Franklin (1706-1790) started his investigations after Gilbert.  He used to fly a kite on an overcast day (no actual lightning), but he wasn’t the first person to do so

  • Several people had tried to do it before him to prove that lightning was electrical, but they’d all been killed.
    • Most people thought he was crazy to do it. In fact, he had his son set up most of the equipment while he stood back.
    • Franklin was able to prove that lightning was a discharge of static electricity

Most of Franklin’s research actually focused on amber rod.

  • It had been found that if a rubbed amber rod was dangling from a string, and another rubbed amber rod was brought near, the dangling one would move away.
  • If a dangling rubbed glass rod is brought near another rubbed glass rod, the dangling one would move away.
  • If a rubbed glass rod and amber rod were brought near to each other, they were attracted.  Therefore, the charge on the glass must be different from the charge on the amber

Franklin decided to say that

  • The glass rod had a positive charge
  • The amber rod (or the plastic ebonite used today) had a negative charge

Franklin’s Single Fluid Theory

Franklin developed what he called a “single-fluid” theory to explain the results he was getting.

  • According to this theory, all matter contains an “electric fluid”, a substance that Franklin thought all matter in the universe had. His electric fluid had a positive charge.
    • An object with a positive charge has an excess of this positive electric fluid.
    • An object with a negative charge has a deficiency of this positive electric fluid.

Franklin backed up his theory with the observation that if a certain amount of charge is produced in one object, an equal amount of opposite charge is produced on another object.

  • For example, let’s say you rub a balloon on your head. The balloon will gain just as much negative charge as your hair will gain positive charge. According to this model, the electric fluid flows from one object to the other.
  • Franklin used the idea of negative and positive to figure out algebra problems, since if you charged anything, the two objects’ charges would add up to zero.
    • This would be like if you rub a plastic ruler with a paper towel. The ruler has a negative charge, and the paper towel has an equal positive charge. The charges are separate from each other but add up to zero.

 Modern Theories

Although Franklin’s single fluid theory is not exactly right, it did lead him to a law that we still use today in physics

“Law of Conservation of Charge”

The net amount of electric charge produced in any process is zero.

  • This means that even though you can move around charges, you can’t create or destroy them.
  • In the past 100 years it has become clear that these charges depend on the makeup of the atom itself, not on some “fluid
  • The nucleus is made up of protons (which are positive) and neutrons (which are neutral), surrounded by electrons (which are negative) in orbit.
  • In a “normal” state the electrons and protons balance each other out, so the charge is neutral. Sometimes the atom may lose or gain electrons.
  • Nothing happens to the stable nucleus made up of protons and neutrons.
  • It is the electrons that are being stripped off or added on because they are on the far outside edge of the atom.
  • If the atom loses electrons it will have a positive charge. if it gains electrons it has a negative charge.
  • Either way, it is now called an ion

 Usually when an object is charged by rubbing, the charge only lasts a little while.

  • Most of the charge “leaks off” to water molecules in the air
    • Water is a polar molecule, which means one end is more negative and the other is more positive.
    • The positive end can temporarily pick up electrons.
  • Therefore, there is more static electricity in winter.
    • The air is drier, so the electrons aren’t picked up as often.

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