Types of Current

Current is a flow of electricity through a circuit. Let us consider two types of current: direct and alternating.

A direct current (d.c.) flows through a conducting circuit in one direction only. It flows provided a direct voltage source is applied to the circuit.

An alternating current (a.c.) is a current that changes its direction of flow through a circuit. It flows provided an alternating voltage source is applied to the circuit. Alternating current flows in cycles. The number of cycles per second is called the frequency of the current. In a 60-cycle alternating current circuit the current flows in one direction 60 times and in the other direction 60 times per second.

It is easy to transform a.c. power from one voltage to another by a transformer. Transformers are also used to step down the voltage at the receiving point of the line to the low values that are necessary for use. When necessary a.c. can be changed into d.c. but this is seldom necessary.

Most students begin their study about electricity with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. DC is the kind of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other.

As useful and as easy to understand as DC is, it is not the only “kind” of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage switching polarity or as a current switching direction back and forth, this “kind” of electricity is known as Alternating Current (AC).

With AC it is possible to build electric generators, motors and power distribution systems that are far more efficient than DC, and so we find AC used predominately across the world in high power applications. To explain the details of why this is so, a bit of background knowledge about AC is necessary.

Вправа 3. Дайте відповіді на питання до тексту.

1. What is current?

2. What types of current do you know?

3. How does current flow?

4. What is direct current? What is alternating current?

5. What is the frequency of the current?

6. When is applied AC and DC?

Вправа 4. Прочитайте речення. Знайдіть серед них вірні.

1. There are three types of current.

2. An alternating current changes its direction of flow through a circuit.

3. A direct current flows through a conducting circuit in many directions.

4. We find DC used predominately across the world in high power applications.

5. With AC it is possible to build electric generators, motors and power distribution systems that are far more efficient than DC.

Вправа 5. Знайдіть переклад поданих слів та словосполучень в тексті.

1. електрика	10. число циклів
2. ланцюг	11. частота струму
3. вимірювати	12. вимірювач
4. постійний струм	13. параллельно
5. змінний струм	14. напруга
6. напрямок
7. джерело напруги
8. величина опору
9. градуйований

Вправа 6. Закінчіть речення.

1. Current is ……………

2. The frequency of the current is…………………

3. Direct current is ………………………………..

4. It is easy to transform a.c. power from ………

5. Alternating current is …………………………

Вправа 7. Прочитайте та перекладіть текст без словника.

Дайте визначення різним видам струму.