 |
8. Complete the sentences using while. Follow the model on page 13.
|
|
|
|
8. Complete the sentences using while. Follow the model on page 13.
1. Conductors have a low resistance ….
2. Current passes through insulators with great difficulty ….
3. Metals are common conductors ….
4. To make insulators conduct, currents of great value must be applied ….
3. Carbon decreases its resistance when the temperature increases ….
6. Metals have a positive temperature coefficient of resistance ….
9. Pair work. Put these questions to your groupmate, and ask him/her to answer them:
1. What is the difference between conductors and insulators?
2. How does current pass through insulators?
3. What materials are commonly used to produce insulators?
4. What materials are commonly used to produce conductors?
5. In what case do insulators conduct current?
6. How does resistance change when the temperature decreases?
Unit Twelve [12]
1. a) Cover the right column and read the English words. Translate them into Russian and check your translation.
b) Cover the left column and translate the Russian words back into English.
| core | сердечник | |
| winding | [ˈ waɪ ndɪ ŋ ] | обмотка |
| turn | виток | |
| to step up | повышать | |
| to step down | понижать | |
| frequency | [ˈ friː kwə nsɪ ] | частота |
| due to | [ˈ djuː tə ] | благодаря, из-за |
2. Put down the Russian for:
| iron core | _____________ | primary winding | _____________ |
| closed core | _____________ | secondary winding | _____________ |
| input voltage | _____________ | step-up transformer | _____________ |
| output voltage | _____________ | step-down transformer | _____________ |
Transformers
A transformer is used to transfer energy. Due to the transformer electric power may be transferred at a high voltage and reduced at the point where it must be used to any value. Besides, a transformer is used to change the voltage and current value in a circuit.
Fig. 10 Fig. 11
A two-winding transformer consists of a closed core and two coils (windings). The primary winding is connected to the voltage source. It receives energy. The secondary winding is connected to the load resistance and supplies energy to the load.
The value of voltage across the secondary terminal depends on the number of turns in it. In case it is equal to the number of turns in the primary winding the voltage in the secondary winding is the same as in the primary.
In case the secondary has more turns than the primary the output voltage is greater than the input voltage. The voltage in the secondary is greater than the voltage in the primary by as many times as the number of turns in the secondary is greater than the number of turns in the primary. A transformer of this type increases or steps up the voltage and is called a step-up transformer. In case the secondary has fewer turns than the primary the output voltage is lower than the input. Such a transformer decreases or steps down the voltage, it is called a step-down transformer.
|
|
|
Compare T1 and T2 in the diagram. T1 has an iron core. For this reason it is used for low-frequency currents. T2 has an air core and is used for high frequencies.
Common troubles in transformers are an open in the winding, a short between the primary and the secondary, and a short between turns. In case a transformer has a trouble it stops operating or operates badly. A transformer with a trouble should be substituted.
3. Complete the sentences using the correct variant:
| 1. A transformer is used | a) to store charge. b) to prevent the change of energy. c) to transfer energy. d) to change the voltage and current value in a circuit. |
| 2. Electric power is transferred at a high voltage and reduced to any value | a) due to resistors. b) due to capacitors. c) due to transformers. |
| 3. A transformer consists of | a) cores only. b) the primary and the secondary windings. c) a core and the primary and the secondary windings. |
| 4. The function of the primary is | a) to prevent the change of voltage. b) to supply energy. c) to receive energy. |
| 5. The function of the secondary is | a) to receive energy. b) to supply energy. c) to transfer energy. d) to decrease the value of charge. |
| 7. A step-down transformer is used | a) to step down the secondary voltage. b) to step down the primary voltage. |
| 8. A transformer with an iron core | a) is used for high-frequency currents. b) is used for low-frequency currents. |
| 9. A transformer with an air core is used | a) for high-frequency currents and for low-frequency currents. b) for high-frequency currents only. |
| 10. In a step-up transformer | a) the number of turns of the secondary winding is greater than the number of turns of the primary. b) the number of turns of the primary winding is greater than the number of turns of the secondary. |
| 11. A transformer should be substituted | a) in case it has an open in the winding. b) in case it has a short between the primary and the secondary. c) in case it has a short between turns. |
|
|
|
