CONDUCTORS AND INSULATORS

All substances have some ability to conduct electric cur­rent, however, they differ greatly in the ease with which the current can pass through them. Substances through which electricity is easily transmitted are called conductors.

Metal is a good conductor of electric current, copper being our most commonly used conductor. That is why the electri­cally operated appliances in your home are connected to the wall socket by copper wires. Indeed, if you have turned on the light and are reading this book by an electric lamp and some­body pulls the metal wire out of the socket, the light will go out, at once. The electricity has not been turned off but it has no path to travel from the socket to your electric lamp. The flow of electrons cannot travel through space and get into an electrical device when the circuit is broken.

Not all substances can be used to conduct electric current. For example, if we use a piece of string instead of a metal wire, we find that the current stops flowing. A material like string, which does not permit electric current to flow, is called an insulator. Some commonly used insulators are air, glass, porcelain, plastics, paper, and oil. Some insulating materials used to cover wire are rubber, asbestos, lacquer and plastics.

Air is the most important of these insulators because it is the one we often rely upon. Surround your wire with plenty of space and you have built a reliable insulation. That is why transmission lines are bare wires depending on air to keep their thousands of volts in place. However, even these lines must be supported at intervals and it is the porcelain insulator that supports the wire of the transmission lines. The higher the voltage, the better should be the insulation.

Only when there is a very high voltage (high electrical pressure), which gives a strong push to the flow of electrons, they can be made to jump the air gap from one wire to another. When this does happen, a "spark" jumps the gap. Ordinarily even a small gap of air in the circuit is enough to cut off the current.

HEAT TRANSFER

Heat is a form of power transported from one body to another because of a temperature difference. When two bodies at different temperatures are brought into contact, the warmer body will be cooled and the colder body will be warmed. In this case, heat is said to flow from the hot body to the cold body by conduction. The molecules of the hot body, being at a high­er temperature, have a higher level of kinetic power than do the molecules of the cold body. Thus, power is transferred through the molecules from the hot body to the cold body. The power being transferred is called heat while it is flowing from the hot body to the cold body.4 The power received by the cold body increases its temperature and may be stored in its molecules as an increase in molecular or internal power.

Heat may also be transferred from one body to another through space by means of radiation. Heat is received from the Sun by any body which is exposed to the rays of the sun. The molecules of any substance at high temperature emit waves of power which travel through the air with the speed of light. They are known to differ from light waves only in their wave lengths. The radiant power may be absorbed and reflected by a body upon which it falls. Upon absorption, the radiant power is stored as molecular power in the body and a rise of temperature takes place.

Heat may be transferred from one body to another body at a distance from the first one by means of convection. In this case, a fluid is heated or cooled by conduction through contact with one body after which the fluid flows to another place where heat is transferred between moving fluid and the second body. For instance heat is transferred by conduction from the hot metal surface of a stove to a stream of moving air,, the temperature of which is increased. The heated air is then transported to a room in which the moving warm air transfers heat by conduction to the objects in the room. When heat is transferred by convection, a fluid transports the power from one body to another through the movement of the fluid.