STATIC ELECTRICITY Static electricity can be seen at work when hair is combed on a cold, dry day. 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.

ATOMS 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.

STATIC ELECTRICITY and ELECTRICAL CHARGES When a charge is stationary, 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 position and movement of positively and negatively charged particles of matter. 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.

CONDUCTORS AND INSULATORS When some atoms combine to form solids, one or more electrons are often liberated and can move with ease through the material. Electrons are easily liberated in some materials, which are known as conductors. Metals, particularly copper and silver, are good conductors. Materials in which the electrons are tightly bound to the atoms are known as insulators, nonconductors, or dielectrics. Glass, rubber, and dry wood are examples of these materials.

BEN FRANKLIN Benjamin Franklin and his son William performed an experiment in They flew a kite during a thunderstorm. An electric current traveled down the wet string. When Franklin moved his knuckle towards a key tied on the string, a spark jumped. Franklin concluded that lightning was a form of electricity.

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.

CURRENT ELECTRICITY Current electricity is flow of electric charge. The electric charge in a current is carried by minute particles called electrons that orbit the nuclei of atoms. Each electron carries a small electric charge. When a stream of electrons moves from atom to atom—for example, inside a copper wire—the flow of the charge they carry is called electric current. Batteries and generators are devices that produce electric current to power lights and other appliances. Electric currents also occur in nature—lightning being a dramatic example.

CURRENT FLOW Electric currents flow because atoms and molecules contain two types of electrical charge, positive and negative, and these opposite charges attract each other. If there is a difference in the overall charge of atoms between two points—for example, between two ends of a wire—the negatively charged electrons will flow toward the positively charged end of the wire, creating electric current. Direct current (DC) is the flow of electricity in one direction. Alternating current (AC) intermittently reverses direction because of the way it is generated.

CIRCUITS: A Pathway for Moving Electrons When the terminals of a battery are connected with a conductor an electric circuit is produced. By means of chemical reactions within the battery, a potential difference is created between the terminals, and electrons flow in the conductor in one direction, away from the negative terminal toward the positive.

CIRCUIT DIAGRAMS The two small parallel lines in each diagram represent an electrical cell. Electrons flow through wires, represented by the straight lines. The zigzag lines represent resistance, what the circuit is operating. In a series circuit, each electron flows through each resistor (in Science class, this would be the lightbulb). The current is the same in resistor but the voltage is reduced. In a parallel circuit, the voltage is the same across each resistor, but the current can be different.