Unit 2 conductors, insulators and semiconductors

II. READING COMPREHENSION If we connect a battery across a body, there is a movement of free electrons towards the positive end. This movement of electrons is an electric current. All materials can be classified into three groups according to how readily they permit an electric current to flow. These are: conductors, insulators and semiconductors. In the first category are substances which provide an easy path for an electric current. All metals are conductors; however some metals do not conduct well. Manganin, for example, is a poor conductor. Copper is a good conductor, therefore it is widely used for cables. A non metal which conducts well is carbon. Salt water is an example of a liquid conductor.

UNIT 2 CONDUCTORS, INSULATORS AND SEMICONDUCTORS

 Contrast the characteristics of conductors and insulators

 List some common conductors and insulators

 Explain how insulators provide protection from electricity

I VOCABULARY

Exercise 1: Match English terms and Vietnamese translation

1 Battery a Chất, khối chất

2 Positive b Nhiệt điện trở

4 Permit d Điều khiển, dẫn điện

6 Insulator f Chất dẫn điện

10 Thermistor j Chất cách điện

12 Detector l Bóng bán dẫn

Exercise 2: Complete the sentences by selecting the correct word from the box to fill in the gap

Conductors Metal Conduct Insulators Current

1 are materials that permit electrons to flow freely from particle to particle

Exercise 3: Make definition

Transistor a device consisting of one or more electrochemical cells that convert

stored chemical energy into electrical energy

Thermistor a device that triggered electronically to hear sound waves

Detector a semiconductor device used to amplify and switch electronic signals

and electrical power Capacitance a type of resistor whose resistance varies significantly with

temperature, more so than in standard resistors Battery the ability of a body to store an electrical charge

II READING COMPREHENSION

If we connect a battery across a body, there is a movement of free electrons towards the positive end This movement of electrons is an electric current All materials can be classified into three groups according to how readily they permit an electric current to flow These are: conductors, insulators and semiconductors In the first category are substances which provide an easy path for an electric current All metals are conductors; however some metals do not conduct well Manganin, for example, is a poor conductor Copper is a good conductor, therefore it is widely used for cables A non -metal which conducts well is carbon Salt water is an example of a liquid conductor

A material which does not easily release electrons is called an insulator Rubber, nylon, porcelain and air are all insulator There are no perfect insulators All insulators will allow some flows of electrons, however this can usually be ignored because the flow they permit is so small (See Fig 1.1)

Semiconductor is mid -way between conductors and insulators Under certain conditions they allow a current to flow easily but under others they behave as insulators Germanium and silicon are semiconductors These are known as thermistors The resistance of thermistors falls rapidly as their temperature rises They are therefore used in temperature sensing devices

Exercise 1: Describe if the statements are true or false Quote from the passage to support your decision

1 Electrons flow from positive to negative

2 Copper provides an easy path for an electric current

3 All metals are good conductors

4 Materials which provide a path for an electric current are conductor

5 Air is not a perfect good insulator

6 Rubber readily releases electrons

7 All insulators permit some flow of electrons

8 Semiconductors are known as thermistors

Exercise 2: Answer the questions below:

1 What happen if we connect a battery across a body?

2 Can all materials be classified into three groups?

3 What kind of material doesn’t easily release electrons?

4 What is semiconductor?

5 Does the resistance of thermistors fall rapidly as their temperature rise?

Exercise 3: Put the following words in the correct column:

Copper Air Glass Silver Steel Rubber Diamond Glass Gold Aluminum Wood Plastic

………

………

………

………

………

………

III USE OF LANGUAGE

1 Relative clauses

The relative pronouns are:

Subject Object Possessive Adverb of

time

Adverb of place

Adverb of reason

We can use relative clauses to join two English sentences, or to give more information about something

Defining and Non-defining

A defining relative clause tells which noun we are talking about.

A non-defining relative clause gives us extra information about something We don't

need this information to understand the sentence

Study these sentences:

Eg.1: Starter motor brushes are made of carbon The carbon contains copper

→ Starter motor brushes are made of carbons which contain copper.

Eg.2: 33kV lines are fed to intermediate substations In the intermediate substations the voltage is stepped down to 11kV

→ 33 kV lines are fed to intermediate substations where the voltage is

stepped down to 11kV.

Now find the relative clauses in the reading comprehension Then link these following sentences, make the second sentence in each pair a relative clause

1 The coil is connected in a series with a resistor

The resistor has a value of 249 ohms

2 The supply is fed to the distribution substation

The supply is reduced to 415 V in the distribution substation

3 Workers require a high degree of illumination

The workers assemble very small precision instrument

4 Manganin is a metal

This metal has a relatively high resistance

The workers assemble heavy machinery.

8 Armored cables are used in places There is a risk of mechanical damage in these places

2 Reason and result connectives

Connectives are words that join two or more ideas in the sentence We can use connectives to indicate result and reason

- W e can link a statement and a reason using because When the reason is a noun or a noun phrase, we can use because of.

- We can link a statement and a result using therefore Note that a comma is used before therefore.

Study these sentences:

Eg: 1 Copper is used for cables

2 Copper is a good conductor

1+2 → Copper is used for cables because it is a good conductor

Sentence 1 tells us what copper is used for Sentence 2 tells us why it is used; sentence 2 provides a reason for sentence 1

Now link these ideas using because and therefore to make shorten two sentences.

1 Soft iron is used in electromagnets

Soft iron can be magnetized easily

2 The voltage is 250 V and the current 5 A

The resistance is 50 ohms

3 Pvc is used to cove r cables

Pvc is a good insulator

4 Transistors can be damaged by the heat

Care must be taken when soldering transistors

5 Capacitance is usually measured in microfarads or Pico -farads

The farad is too large a unit

7 It is easy to control the speed of DC motors.

DC motors are used when variable speeds are required

8 A cathode ray tube screen glows when an electron beam strike it The screen is coated with a phosphor

IV FURTHER READING

Translate the reading passage in to Vietnamese then discuss with your partner about how insulators provide protection from electricity.

Insulators are used to protect us from the dangerous effects of electricity flowing through conductors Sometimes the voltage in an electrical circuit can be quite high and dangerous If the voltage is high enough, electric current can be made to flow through even materials that are generally not considered to be good conductors Our bodies will conduct electricity and you may have experienced this when you received an electrical shock Generally, electricity flowing through the body is not pleasant and can cause injuries The function of our heart can be disrupted by a strong electrical shock and the current can cause burns Therefore, we need to shield our bodies from the conductors that carry electricity The rubbery coating on wires is

an insulating material that shields us from the conductor inside Look at any lamp cord and you will see the insulator If you see the conductor, it is probably time to replace the cord