Static Electricity

Before we explore the electricity, lets review the fundamental forces of in nature.  Fundamental means that, every other force that we observe the consequences of,  can be identified as one of the or a combination of the only a finite number of basic forces.  These basic forces, or fundamental forces are

1)      gravitational force : such as Earth’s pull on Moon, or Earth’s pull on an object weight),

2)      electromagnetic force : electric and magnetic forces are intimately related, therefore they are identified as electromagnetic,

3)      strong nuclear force : forces which are dominant at the subatomic level that hold the nucleus together,

4)      weak nuclear force : forces which are dominant at subatomic level that is responsible for radioactive decay.

We will learn about the second one, electromagnetic force.  We will first start with electric force, whose origin dates back to ancients.  The word electricity comes from Greek word electron, which means amber.  Amber is petrified tree resin, and the ancients knew that when you rub an amber rod with a piece of cloth, the amber attracts small pieces of leaves or dust.  This amber effect, namely when rubbed with a cloth, the ability to attract some other objects is due to static electricity. When you walk across a carpet during a winter day, you probably noticed that you got shocked when you touched a doorknob.  In this case, both you and carper acquired static electricity due to rubbing just like amber.  Therefore both you and carpet possess a net electric charge.  If one observes carefully, she/he will notice that, electric force can be attractive or repulsive.  To explain these two opposite feature of electric force, scientist proposed the existence of two types of electric charges; positive and negative (due to Benjamin Franklin, yes the same famous American statesman, philosopher, and scientist, the inventor of famous Benjamin Franklin Stove…).  The rule is that: unlike charges attract; like charges repel.

Characteristics of the force (called Coulomb Force):

• Attractive or repulsive:  unlike charges attract; like charges repel
• The direction of the force is along the line joining the centers of two charges.
• The magnitude of the force (how strong it is..) depends on the distance inversely, that is, as the charges are separated more, the force between them decreases.  If the charges are brought closer, the force increases.

See the skethes below which show the direction of the force (direction of the arrow) and its magnitude (size of arrow),

Choice of the names negative or positive is arbitrary.  If you rub a glass rod with a cloth, the charge produced on the glass rod is called positive, whereas when a plastic rod is rubbed with a cloth, the charge produced on the plastic rod is negative.  Charge is conserved: when a certain amount of one king charge is produced on a body, an opposite type and equal amount of charge is produced on another body.  For example, see the following sketch and count the opposite charges.  We assume that, before the rubbing each object is neutral which means that there are equal amounts of positive and negative charges on each object.

How does a rod get charged when rubbed with a cloth?

This happens when a negative charge (later we will see that negative charges are associated with electrons) is transferred from first body to the second, the first body charged positively, and the second body is charged negatively.  Which body looses the negative depends on the force with which the negative charge is held in its place in that particular body. In the case of glass rod, the negative charges of the glass are not as tightly bound as the negative charges of the cloth.  Therefore, it is the negatives of the glass that are moved over to the cloth.  The sketch below illustrates this process.

Electric Charge in the Atom:

In the past century, it has become clear that a fundamental understanding of electricity begins inside the atom.  A simplified model atom includes a tiny but heavy positively charged core (protons and neutrons) surrounded by negatively charged electron cloud.  Protons are positively charges, neutrons have zero charge, and electrons have negative charge.  All protons and electrons have exactly the same magnitude of electric charge, but their signs are opposite.  Atoms are neutral under normal conditions.  Therefore, a neutral atoms contains the same number of electron and protons.  A charged rod may not remain charged forever.  It is because, its charges may leak off into air which contains a polar water molecule.  A polar water molecule is shown in

the sketch.

Its charge is not distributed uniformly.

One side (Hydrogen side) is positive, and

the other side (Oxygen side) is positive

even though the net charge is zero.  Such

an object is called a dipole, meaning a definite

charge separation.  Leakage happens when

for example a negatively charged rod’s

negative charge combines with the positive

side of the water molecule.

Instructor will hold a charged rod near a stream

of water to show the polar nature of water!

Insulators and Conductors

The electrons of some materials such as metals are able to move around freely when experience an external force, whereas the electrons of some other materials such as wood are tightly bound to local atoms therefore, are not able to move around freely.  First type of materials are called conductors, the second types are called insulators.