Conductors,Insulators, Semiconductors

Conductors,Insulators, Semiconductors

Conductors

    In some materials, electrons move easily from atom to atom.  In others, the electrons move with difficulty.And in some materials, it is almost impossible to get them to move. 

An electrical conductor is a substance in which the electrons are mobile.The best conductor at room temperature is pure elemental silver. 

Copper and aluminum are also excellent electrical conductors. 

Iron, steel, and various other metals are fair to good conductors

of electricity. 

In most electrical circuits and systems, copper or aluminum wire is used. (Silver is impracticalbecause of its high cost.)

Some liquids are good electrical conductors.

 Mercury is one example. 

Salt water is a fair conductor.

Gases or mixtures of gases, such as air, are generally poor conductors of electricity. 

This is because the atoms or molecules are usually too far apart to allow a free exchange of electrons. But if a gas becomes ionized, it can be a fair conductor of electricity.

Electrons in a conductor do not move in a steady stream, like molecules of water through a gardenhose. Instead, they are passed from one atom to another right next to it . This happens to countless atoms all the time. As a result, literally trillions of electrons pass a given point each second in a typical electrical circuit.

Insulators

An insulator prevents electrical currents from flowing, except occasionally in tiny amounts. 

Most gases are good electrical insulators. 

Glass, dry wood, paper, and plastics are other examples. 

Pure water is a good electrical insulator, although it conducts some current with even the slightest impurity. 

Metal oxides can be good insulators, even though the metal in pure form is a good conductor.

Electrical insulators can be forced to carry current. 

Ionization can take place; when electrons are stripped away from their atoms, they move more or less freely. 

Sometimes an insulating material gets charred, or melts down, or gets perforated by a spark. 

Then its insulating properties are lost, and some electrons flow. 

An insulating material is sometimes called a dielectric. 

   This term arises from the fact that it keeps electrical charges apart, preventing the flow of electrons that would equalize a charge difference between two places. 

Excellent insulating materials can be used to advantage in certain electrical components such as capacitors, where it is important that electrons not flow.

Porcelain or glass can be used in electrical systems to keep short circuits from occurring.

 The  insulators, come in various shapes and sizes for different applications. 

We can see them on high-voltage utility poles and towers. 

They hold the wire up without running the risk of a short circuit with the tower or a slow discharge through a wet wooden pole.

Resistors

Some substances, such as carbon, conduct electricity fairly well but not really well. 

The conductivity can be changed by adding impurities like clay to a carbon paste, or by winding a thin wire into a coil. Electrical components made in this way are called resistors.

 They are important in electronic circuits because they allow for the control of current flow. 

The better a resistor conducts, the lower its resistance; the worse it conducts, the higher the resistance.

Electrical resistance is measured in units called ohms. 

The higher the value in ohms, the greater the resistance, and the more difficult it becomes for current to flow. 

For wires, the resistance is sometimes specified in terms of ohms per unit length (foot, meter, kilometer, or mile).

 In an electrical system, it is usually desirable to have as low a resistance, or ohmic value, as possible. 

This is because resistance converts electrical energy into heat.

Semiconductors

In a semiconductor, electrons flow, but not as well as they do in a conductor. 

Some semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost

as bad as insulating materials.

Semiconductors are not the same as resistors. 

In a semiconductor, the material is treated so that it has very special properties.

Semiconductors include certain substances such as silicon, selenium, or gallium, that have been “doped” by the addition of impurities such as indium or antimony. 

 Electrical conduction in these materials is always a result of the motion of electrons. 

But this can be a quite peculiar movement, and sometimes engineers speak of the movement of holes rather than electrons. 

A hole is a shortage of an electron—you might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons

When most of the charge carriers are electrons, the semiconductor is called N-type, because electrons are negatively charged.

 When most of the charge carriers are holes, the semiconductor material is known as P-type because holes have a positive electric charge. 

But P-type material does pass some electrons, and N-type material carries some holes.

 In a semiconductor, the more abundant type of charge carrier is called the majority carrier. The less abundant kind is known as the minority carrier.

 Semiconductors are used in diodes, transistors, and integrated circuits.