Electrostatic Induction Simulation of Electroscope


Q1) When you brush your hair and scrape electrons from your hair, the charge of your hair is

A. positive.
B. negative.
C. both A and B
D. neither A nor B

Answer) A.
And if electrons were scraped off the brush onto your hair, your hair would have a negative charge.

Q2) How does charging an object by contact differ from charging an object by induction?

 A. An object charged by contact gains the same type of charge while an object charged by induction gains the opposite type of charge.
 B. An object charged by contact is touched while an object charged by induction is not.
 C. An object charged by contact must be a conductor while an object charged by induction is an insulator.

Answer) A.

Electrostatic Induction Simulation of Electroscope

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Electrically Charged Objects 
Matter is made of atoms, and atoms are made of electrons and protons (and neutrons as well). An object that has equal numbers of electrons and protons has no net electric charge. But if there is an imbalance in the numbers, the object is then electrically charged. An imbalance comes about by adding or removing electrons.
Although the innermost electrons in an atom are bound very tightly to the oppositely charged atomic nucleus, the outermost electrons of many atoms are bound very loosely and can be easily dislodged. 

FIGURE 01. When electrons are transferred from the fur to the rod, the rod becomes negatively charged.

How much energy is required to tear an electron away from an atom varies for different substances. The electrons are held more firmly in rubber than in fur, for example. Hence, when a rubber rod is rubbed by a piece of fur, as illustrated in Figure 01, electrons transfer from the fur to the rubber rod. The rubber then has an excess of electrons and is negatively charged. The fur, in turn, has a deficiency of electrons and is positively charged. If you rub a glass or plastic rod with silk, you’ll find that the rod becomes positively charged. The silk has a greater affinity for electrons than the glass or plastic rod. Electrons are rubbed off the rod and onto the silk.
An object that has unequal numbers of electrons and protons is electrically charged. If it has more electrons than protons, the object is negatively charged. If it has fewer electrons than protons, it is positively charged.

The TriboElectric Series
If we did a study of many materials and put them in order from those with the least desire for  electrons to those with a very strong desire for electrons we would have created a Triboelectric series. If two items from the list are rubbed together, then the item that is higher on the list will end up more positive and the lower one will end up more negatively charged.  For example, if leather were rubbed with wool, the leather becomes positive and the wool negative.  Yet if rubber is rubbed with wool, the rubber becomes negative and the wool positive.
It is important to note that this series is true only if the samples are clean and dry.  The presence of moisture, dirt, or oils may cause some of the items to interact differently.

Human Hands
Rabbit's Fur
Human Hair
Cat's Fur
Sealing wax
Rubber balloon
Hard rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Acetate, Rayon
Saran Wrap

Forces on Charged Bodies
The forces that you observed on tape strips also can be demonstrated by suspending a negatively charged, hard rubber rod so that it turns easily, as shown in Figure 02. If you bring another negatively charged rod near the suspended rod, the suspended rod will turn away. The negative charges on the rods repel each other. It is not necessary for the rods to make contact.

Figure 02. A charged rod, when brought close to another charged and suspended rod, will attract or repel the suspended rod.

The force, called the electric force, acts at a distance. If a positively charged glass rod is suspended and a similarly charged glass rod is brought close, the two positively charged rods also will repel each other. If a negatively charged rod is brought near a positively charged rod, however, the two will attract each other, and the suspended rod will turn toward the oppositely charged rod. The results of your tape experiments and these actions of charged rods can be summarized in the following way:

• There are two kinds of electric charges: positive and negative.
• Charges exert forces on other charges at a distance.
• The force is stronger when the charges are closer together.
• Like charges repel; opposite charges attract.

Neither a strip of tape nor a large rod that is hanging in open air is a very sensitive or convenient way of determining charge. Instead, a device called an electroscope is used. An electroscope consists of a metal knob connected by a metal stem to two thin, lightweight pieces of metal foil, called leaves. Figure 03 shows a neutral electroscope. Note that the leaves hang loosely and are enclosed to eliminate stray air currents.

Figure 03. An electroscope is a device used for detecting charges. In a neutral electroscope, the leaves hang loosely, almost touching one another.

Charging by conduction 
When a negatively charged rod is touched to the knob of an electroscope, electrons are added to the knob. These charges spread over all the metal surfaces. As shown in Figure 04a, the two leaves are charged negatively and repel each other; therefore, they spread apart.
The electroscope has been given a net charge. Charging a neutral body by touching it with a charged body is called charging by conduction. The leaves also will spread apart if the electroscope is charged positively. How, then, can you find out whether the electroscope is charged positively or negatively? The type of charge can be determined by observing the leaves when a rod of known charge is brought close to the knob. The leaves will spread farther apart if the rod and the electroscope have the same charge, as shown in Figure 04b. The leaves will fall slightly if the electroscope’s charge is opposite that of the rod, as in Figure 04c.

Figure 04. A negatively charged electroscope will have its leaves spread apart (a). A negatively charged rod pushes electrons down to the leaves, causing them to spread farther apart (b). A positively charged rod attracts some of the electrons, causing the leaves to spread apart less (c).

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