Charging by Contact
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Objects become electrically charged in either of two ways: by contact or by induction.
A charged object transfers electric charge to an object with lesser charge if the two touch. When this happens, a charge flows from the first to the second object for a brief time. Charges in motion form an electric current. When charge flows between objects in contact, the amount of charge that an object receives depends on its ability to store charge. The ability to store charge is called capacitance and is measured in units called farads.
Charging by contact can be demonstrated by touching an uncharged electroscope with a charged comb. An electroscope is a device that contains two strips of metal foil, called leaves, that hang from one end of a metal rod. A metal ball is at the other end of the rod. When the charged comb touches the ball, some of the charges on the comb flow to the leaves, which separate because they now hold like charges and repel each other. If the comb is removed, the leaves remain apart because they retain their charges. The electroscope has thus been charged by contact with the comb.
This flow of charge between objects with different amounts of charge will occur whenever possible. However, it requires a pathway for the electric charge to move along. Some materials, called conductors, allow an electric current to flow through them easily. Other materials, called insulators, strongly resist the passage of an electric current.
Under normal conditions, air is an insulator. However, if an object gains a large enough charge of static electricity, part of the charge may jump, or discharge, through the air to another object without touching it directly. When the charge is large enough, the air becomes a conductor. Lightning is an example of a discharge.
A charged object transfers electric charge to an object with lesser charge if the two touch. When this happens, a charge flows from the first to the second object for a brief time. Charges in motion form an electric current. When charge flows between objects in contact, the amount of charge that an object receives depends on its ability to store charge. The ability to store charge is called capacitance and is measured in units called farads.
Charging by contact can be demonstrated by touching an uncharged electroscope with a charged comb. An electroscope is a device that contains two strips of metal foil, called leaves, that hang from one end of a metal rod. A metal ball is at the other end of the rod. When the charged comb touches the ball, some of the charges on the comb flow to the leaves, which separate because they now hold like charges and repel each other. If the comb is removed, the leaves remain apart because they retain their charges. The electroscope has thus been charged by contact with the comb.
This flow of charge between objects with different amounts of charge will occur whenever possible. However, it requires a pathway for the electric charge to move along. Some materials, called conductors, allow an electric current to flow through them easily. Other materials, called insulators, strongly resist the passage of an electric current.
Under normal conditions, air is an insulator. However, if an object gains a large enough charge of static electricity, part of the charge may jump, or discharge, through the air to another object without touching it directly. When the charge is large enough, the air becomes a conductor. Lightning is an example of a discharge.
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