You have seen what happens when you touch a metal bar with a positively charged rod Some of the charge the rod is transferred and the bar be- comes charged. Suppose that instead of touching the bar with the rod, you only bring the positively charged rod near to the bar In that case, elec-trons in the bar would be attracted to the point nearest the rod, causing negative charge at that point.
The opposite side of the bar would again lack electrons and be charged positive Three charges would then exist the positive charge in the rod, the negative charge in the bar at the point nearest the rod and a positive charge in the bar on the side opposite the rod. By allowing electrons from an outside source (your finger for in-stance) to enter the positive end of the bar, you can give the bar a nega-tive charge.
If the rod is negatively charged when brought near to the bar, it will in-duce a positive charge into that end of the bar which is near the rod. Electrons in that portion of the rod will be repelled and will move to the opposite end of the bar. The original negative charge of the rod then causes two additional charges, one positive and one negative, in the bar.
Removing the rod will leave the bar uncharged since the excess electrons in the negatively charged end will flow back to neutralize the bar. How-ever, if before the rod is moved a path is provided for the electrons in the negatively charged portion of the bar to flow out of the bar, the entire bar will be positively charged when the rod is removed.
You have discovered that static charges can be caused by friction and contact, or induction. Now you should see how the excess or lack of electrons in the charged body may be neutralized, or discharged.
Discharge of Static Charges:-
Whenever two materials are charged with opposite charges and placed near one another, the excess electrons on the negatively charged material will be pulled toward the positively charged material. By connecting a wire from one material to the other, you would provide a path for the electrons of the negative charge to cross over to the positive charge, and the charges would thereby neutralize.
Instead of connecting the materials with a wire might touch them together (contact) and again the charges would disappear you use materials with strong charges, the electrons may jump from the negative charge to the positive charge before the two materials are actually in contact. In that case, you would actually see the discharge in the form of an arc.
With very strong charges, static electricity can dis-charge across large gaps, causing arcs many feet in length. Lightning is an example of the discharge of static electricity resulting from the accumulation of a static charge in a cloud as it moves through the air. Natural static charges are built up wherever friction occurs be- tween the air molecules, such as with moving clouds or high winds, and you will find that these charges are greatest in a very dry climate, or elsewhere when the humidity is low.
Review of Friction and Static Electric Charges:-
You have now found out about friction as a source of electricity, and you have seen and participated in a demonstration of how static electric charges are produced and their effect on charged and uncharged materials. You have also seen how static charges can be transferred by contact or in-duction, and you have learned about some of the useful applications of static electricity. Before going on to learn about the other basic sources of electricity, you should review those facts which you have already learned.
1. NEGATIVE CHARGE - An excess of electrons.
2. POSITIVE CHARGE- electrons of An excess of A lack
3. REPULSION OF CHARGES- charges repel each other.
4. ATTRACTION OF CHARGES charges attract each other.
5. STATIC ELECTRICITY charges at rest.
6. FRICTION CHARGE- - Like - Unlike Electric A charge caused by rubbing one material against another.
7. CONTACT CHARGE - Transfer of a charge from one material to another by direct contact.
8. INDUCTION CHARGE - Transfer of a charge from one material to another without actual contact.
9. CONTACT DISCHARGE - Electrons crossing over from a negative charge to positive through contact.
10. ARC DISCHARGE - Electrons crossing over from a negative charge to positive through an arc.
When you have completed your review of friction and static electric charges, you will go on to learn about pressure as a source of electricity.
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