Conductors, Semiconductors and Insulators

Text 1

Conductors and Insulators

Materials can be divided into three groups according to their resistance:

1) Conductors are materials having a low resistance so that current easily passes through them. The most common conductors are metals. Metals are used to make connecting wires, switch contacts and lamp filaments. Silver, copper and aluminum are the best of them. Copper is widely used to produce wire conductors. Wire conductors are used to connect a voltage source to a load resistance. Since copper wire conductors have a very low re­sistance a minimum voltage drop is produced in them.

Most materials change the value of resistance when their temperature changes. Metals increase their resistance when the temperature increases while carbon decreases its resistance when the temperature increases. Thus metals have a positive temperature coefficient of resistance while carbon has a negative temperature coefficient.

2) Insulators are materials having a very high resistance. Cur­rent passes through insulators with great difficulty. The most common insulators are air, paper, rubber, plastics, polythene and PVC (polyvinylchloride). PVC is widely used as an outer covering for wires to prevent them making contact.

Any insulator can conduct current when a high enough voltage is applied to it. The higher the resistance of an insulator, the greater the applied voltage must be.

When an insulator is connected to a voltage source, it stores electric charge and a potential is produced on the insulator. Thus, insulators have the two main functions:

1. to isolate conducting wires and thus to prevent a short between them and

2. to store electric charge when a voltage source is applied.

Here is a list of conductors and insulators:

Conductors: Insulators:

gold [gould] золото Teflon ['teflOn] тефлон (политетрафторэтилен)

silver ['sIlvə] серебро mica ['maIkə] слюда

copper ['kOpq] медь glass [glRs] стекло

aluminum [ə'lHmInəm] алюминий porcelain ['pLsəlIn] фарфор

brass [brRs] латунь polyvinylchloride (PVC) ["pOlI'vaInIl'klOraId] поливинилхлорид (ПВХ)

bronze [brOnz] бронза fiberglass ['faIbqglRs] стекловолокно

iron ['aIqn] железо rubber ['rʌbə] резина

steel [stJl] сталь ceramic [sI'rxmIk] керамика

mercury [ 'mWkjurI] ртуть oil [OIl] масло

graphite['grxfaIt] графит quartz [kwLts] кварц

water ['wLtə] вода plastic ['plxstIk] пластик

concrete ['kOŋkrJt] бетон air [Fq] воздух

salt, acid, alkali solutions diamond ['daIəmənd] алмаз

[sOlt]['xsId] ['xlkqlaI] dry wood

3) Semiconductors are materials having a moderate resistance.
Examples of semiconductors are germanium and silicon.
Semiconductors are used to make diodes, LEDs, transistors and integrated circuits (ICs, chips).

Text 2

Diodes

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow.

Diodes can be of two types:

Signal diodes which pass small currents of 100mA or less.

Rectifier diodes which can pass large currents.

There are also LED s. Their symbol is .

Transistors

Transistors amplify current, for example they can be used to amplify small currents. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

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