Electrostatics is the science of electric charge at rest.

There are two types of electric charge: positive and negative.

The nucleus of an atom consists of neutral neutrons and positive protons, giving it an overall positive charge.
The electrons surrounding the nucleus are negatively charged.
An atom is neutral or without net charge if the number of electrons balances the number of protons in the nucleus.

Electrons may be removed from a neutral atom, leaving the atom (now called an ion) with a net positive charge. Atoms may also acquire excess electrons and become negatively charged ions.

Like charges repel, and opposite charges attract.

Charging by friction

Lessons in electrostatics invariably begin with demonstrations of ordinary objects being charged by friction and either attracting or repelling other objects, which have been polarized.

Two balloons on strings are rubbed against a volunteer's hair. The friction of the rubbing draws electrons from the hair onto the surfaces of the balloons, because the material the balloon is made of has a greater affinity for electrons than the hair does.

The balloons, which previously were neutral, each acquire a net negative charge from the negatively charged electrons. (The hair has a net positive charge, so the sum of the hair and balloon charges is zero.)

Credit: NASA

If the balloons are suspended from one point by their strings, they repel each other. This shows that like charges repel.

Even as the balloons repel each other, however, they attract small pieces of paper. The paper has no overall charge, but the charged particles in the paper will move around or molecules will align themselves so that the paper becomes polarized, with a positive charge at one end and negative at the other. The positive charges in the paper are attracted to the negatively charged balloons: opposite charges attract.

Eventually, if a balloon and piece of paper stick together, the paper falls off. Water vapor and other electrically polarized molecules in the surrounding air can remove excess charge.

Charging by friction works best with two materials that have a different affinity for electrons. Polyester and rubber will draw electrons out of materials that have a lower affinity for electrons, such as silk, wool, and glass. Cotton and other natural fibers are not easily charged.

In this diagram, the relative position of any two items is what is important. For example: rubber and polyester are close together. But if you rub rubber with polyester, the only thing that is important is that rubber is above polyester, so the rubber will tend to give up electrons to the polyester.

Materials higher up in the list have a tendency to give up electrons and be positive, while materials lower down acquire electrons and become negative. A cloth made of natural fiber like cotton tends to quickly lose any charge it gains because it can draw a neutralizing charge from the ground through the body.

Charging by conduction

A neutral object may be charged by touching it with a charged object. This is sometimes called charging by contact.

For charging by conduction or contact to work, the initially neutral object must be insulated from ground. If it is not insulated, the charge it acquires from the charging object will just be neutralized.

Charging by induction

Charging by induction is like charging by conduction, except that the two objects never actually touch. The inductively charged object gets its charges through a grounding wire.

In this example, electrons leave the grounded conductor through the grounding wire, leaving a positive charge. When the grounding wire is cut, the conductor remains positive, even when the original charged object is removed.

Ouch! I got a shock!

The spark of static electricity is caused by a sudden electrostatic discharge. Our feet tend to draw electrons out of a carpet as we shuffle across it. Then, when we touch a metal object (such as a doorknob), the excess electrons flow between finger and doorknob. The brief current through a very small area of the body (the fingertip) is felt as a tingling sensation.

We can reduce this “zap” by grasping a set of metal keys, then using the keys to touch the doorknob. This works because the keys make contact with a large area of the hand, which dilutes the tingling sensation.

Did you know?

A good way to remove excess charge from your body before touching sensitive equipment is to touch a water faucet. The metal pipe acts as a ground or a limitless supply of balancing charge.
Having hairy legs can cause more static electricity to build up than if the legs are shaved.
Be safe: It is best not to get into or out of your vehicle while gas is being pumped — the static spark could ignite the fuel.

The classic story of electrostatics

Around 600 BCE, the Greek philosopher and mathematician Thales knew that rubbing a piece of amber (hardened tree sap) against wool or animal fur would cause it to attract light objects such as dust, pieces of straw, and feathers.

Thales apparently mistook this case of electrostatic attraction for a certain kind of magnetism induced by friction. What he actually witnessed was the buildup of electrical charge on the amber, which then attracted the light objects by polarizing them.

Comparison of electrostatics and magnetism

The attraction between rubbed amber and feathers appears similar to that between a lodestone and an iron nail. Not until the 17th century did scientists distinguish between the electrostatics and magnetism.

Only a few elements can be magnetized: mainly, iron, cobalt, and nickel. However, almost any substance can be electrified or acquire a charge.

A magnet always has at least two opposite poles, north and south, and its magnetic field is concentrated at these poles. However, an object may have just one electric charge, positive or negative.

Insulators and conductors

Materials that hold on to built-up electrical charge are called insulators. Amber, latex balloons, dry air, and silk are insulators.

Materials that can quickly carry off a buildup of electrical charge are called conductors. Metals are generally good conductors, because electrons in metals are loosely bound to their atoms and can transmit charge through the material. Other conductors include salt solutions and graphite.

There is no sharp line between insulators and conductors. Dry wood, paper, and water are poor electrical conductors, but not completely insulating. A relatively poor conductor such as silicon may be called a semiconductor.

When an insulator is charged, the charge may remain in one spot. When a conductor is charged, the charge spreads out over the whole surface.

Storing energy in an electric field: the capacitor

A capacitor (sometimes called a condenser) is a device to store energy. Fundamentally, it consists of two parallel metal plates with air or an insulating material in between. The insulating material is called a dielectric.

Capacitors come in a variety of shapes and sizes. Credit: Wikipedia

The capacitor is charged by connecting it to a battery or power supply. This causes a negative charge (excess electrons) to build up on one plate, and a corresponding positive charge (a dearth of electrons) on the other plate. Between the two plates we find an electric field, and a force pulling the plates toward each other.

The charge builds up until the capacitor reaches a saturation point, which depends on the size of the plates and their separation. (Increasing the plate size or decreasing their separation increases the charge and electric field that can be built up.)

The capacitor can release stored charge back into an electric circuit. A current quickly flows from the capacitor until the charge is drained.

A simple capacitor consists of two metal plates separated by a distance d. The space between the plates may be filled with air or with an insulating material. When the capacitor is charged, one plate has positive charge Q and the other has an equal negative charge Q, which creates an electric field E between the two plates. If a wire were used to connect the two plates, or if the capacitor were part of a closed electrical path, the charge on the plates would drain away.