Electricity: Electric Charge and Resistance

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About 600 B.C., Thales, a Greek philosopher and scientist, observed that a piece of amber attracted small bits of “paper” or very thin wood shavings after it was rubbed with wool. He had discovered, in effect, static electricity. William Gilbert, an English scientist in the sixteenth century, found that many other different materials could be made to act like amber. Whenever these objects were rubbed against each other, they were able to attract each other light objects. He called this strange behavior electricity, from the Greek word elektron, meaning amber.

How is electricity produced? Have you ever noticed how your hair stands straight and stiff when you comb it vigorously on a dry, cold day? Or how difficult it is to remove dust particles from your comb? Your hair and the comb become charged by rubbing. This stationary electric charge is described as static electricity.

Activity 1.
Rub a plastic sheet with flannel and then hold the plastic sheet near Styrofoam bits. What do you observe? From the observations made, you can conclude that rubbing produces static electricity on the rubbed objects. The two objects, when rubbed against each other, become electrified. We can described them as being charged.

Activity 2.
Prepare a simple electroscope, using the following materials: iron stand, silk thread, small Styrofoam ball covered with aluminum foil. The electroscope is a simple instrument that can help detect the presence of static electricity. If a glass rod were rubbed with a piece of cloth then placed near the ball, the ball will be attracted to the rod. After a few seconds of contact with the rod, the ball will be repelled. Why? If a plastic rod were rubbed with flannel or wool, and the rod placed closed to the ball, the same result will be observed. Both the silk and woolen pieces of cloth also show signs of having been electrified when placed closed to the electroscope. The observations made in activity 2 indicate that a body consist of charge particles and that there are two kinds of charges. Benjamin Franklin (1706-1790) described this charges as positive and negative. He called the electrical condition that the glass acquires when it has been rubbed with silk as positive electric charge and the condition that a rubber rod acquires after rubbing it with flannel as negative electric charge. Based on experimental findings, the law of electric charges was developed. It states that like charges repel each other while unlike charges attract. Objects are said to be electrically neutral or uncharged when their positive charge is balanced by an equal negative charge in them.

A more accurate electroscope can be made of a brass rod with a brass ball or flat disc on one end. The rod is pushed through a rubber stopper which snugly fits the mouth of an Erlenmeyer flask. Two strips of any metal foil (gold, aluminum or tin), are attached opposite each other at the lower end of the rod.

Electroscopes may be charged in two ways: by contact or by induction. 1. Charging by contact. If a positively charged glass rod is placed in contact with the knob of the electroscope, it will attract the negative charges. The negative charges will be transferred to the rod. Then the rod is removed. This loss of negative charges will make the strips of the metal foil positively charged causing them to repel each other and open up. If a plastic rod rubbed with wool were used, the leaves of the electroscope will be left with a negative charge which will make them repel each other. Charging by contact gives the electroscope a charge similar to that of the charging object. 2. Charging by induction. With an electroscope with an equal number of positive and negative charges. If you now placed a negatively charged rod near the ball of the electroscope, the negative charges are repelled to the metal foils which become...
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