ELECTRICITY: Static Electricity Static Electricity Atomic Model Electrical Charges Electrical Forces

LIGHTNING IS ELECTRICAL Charging by induction occurs when the lower, negatively charged regions of thunderclouds induce a positive charge on the Earth's surface. If the charges become large enough, the resistance of the air is overcome and lightning occurs.

STATIC ELECTRICITY Static electricity can be seen at work when hair is combed on a cold, dry day. As the comb is pulled through the hair, strands of hair stand out stiffly. Some kind of force seems to pull the hair upward toward the comb. To understand the nature of this force it is necessary to know something about the structure of atoms and the concept of electric charge.

CHARGES An electroscope is used to detect the presence of electric charges, to determine whether these charges are positive or negative, and to measure and indicate their intensity. This schematic drawing shows the basic parts of the device: (a, a-) are thin leaves of metal foil, suspended from (b), a metal support; (c) is a glass container, while (d) is a knob that collects electric charges. Electric charges (either positive or negative) are conducted to the leaves at the bottom via the metal support. Because like charges repel one another, the leaves fly apart. The amount of the charge is calculated by measuring the distance the leaves are forced apart.

ELECTRICAL CHARGES MAKE FIELDS In “A,” like charges produce field lines that repel and veer away from each other. In “B,” unlike charges are attracted, and field lines move towards each other.

ELECTRICAL FORCES Electrical Forces are proportional to the product of the charges, and inversely proportional (reduced), by the square of the distance between charges.

ELECTRICAL FORCES Electrical Forces are proportional to the product of the charges, and inversely proportional (reduced), by the square of the distance between charges.

CHARGES and MATERIALS

Three objects demonstrate the way in which electrical charges affect conductors and nonconductors. A negatively charged rod, A, affects the way charges are distributed in a nearby conductor, B, and a nonconductor, C. A positive charge is induced on the sides of B and C that are nearest A; a negative charge is induced on the sides of B and C that are farthest from A. In the conductor, B, the separation of charge involves the entire object because the electrons are free to move. In the nonconductor, C, the separation of charge is limited to the way in which the electrons redistribute themselves within an atom. This effect is most noticeable if the nonconductor is close to the charged object.

ELECTRICAL CHARGES and FORCES When a charge is stationary or static, it produces forces on objects in regions where it is present, and when it is in motion, it produces magnetic effects. Electric and magnetic effects are caused by the relative position and movement of positively and negatively charged particles of matter. So far as electrical effects are concerned, these particles are either neutral, positive, or negative. Electricity is concerned with the positively charged particles, such as protons, that repel one another and the negatively charged particles, such as electrons, that also repel one another. Negative and positive particles, however, attract each other. This behavior may be summarized as follows: Like charges repel, and unlike charges attract.

ELECTRICAL FORCES Two rods that carry the same kind of charge repel each other. To observe this, obtain two rods that are made of the same kind of material (glass stirring rods, for example). Rub both rods in the same way (with a piece of silk, for example). If they are made of the same material and have been rubbed in the same way, the rods should carry the same kind of charge. Hang one rod from a thread so that it is free to rotate. Bring the other rod near. The first rod should rotate away from the second, demonstrating that like charges repel. If the rods had different kinds of charges, the first rod would rotate toward the second, demonstrating that unlike charges attract.

ATOMIC MODEL and ELECTRICITY How does the simple atomic model relate to the static electricity experiments? Rubbing action creates charged objects because it tears electrons loose from some kinds of atoms and transfers them to others. In the case of plastic rubbed with wool, electrons are taken from the wool and pile up on the plastic, giving the plastic a net negative charge and leaving the wool charged positively. When glass is rubbed with silk, the glass loses electrons and the silk gains, producing glass that is charged positively and silk that is charged negatively.