8. Speak on the topic «cIasses of eIectricity»;

Part3

Electric Currents and Their Properties

1. Remember the words and word combinations:

Conduction

Charge

Ion

Gaseous conductor

Liquid conductor

Mobile

A tiny crystal

Visible

A complex particle

Nucleus

At random

Driving force

To cause

To tend

Circuit

Electric current

To pass through

To measure

A direct current

An alternating current

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A pulsating current

2. Read and translate the text:

Conduction is the name normally given to a movement or flow of charges. The charges are usually electrons, but may also be ions when the conduction takes place in gaseous or liquid conductors, in which the ions are mobile.

How does the current flow through a wire? A metal is made up of tiny crystals which are visible under a microscope. A crystal is a regular and orderly arrangement of atoms. As it was explained, an atom is a complex particle in which tiny electrons move around nucleus. When the atoms are tightly packed as they are in a metallic solid, some of the electrons move freely between the atoms. These are called free electrons. Ordinarily, the free electrons move at random through the metal. There must be some driving force to cause the electrons to move through the metal conductor. This driving force tending to produce the motion of electrons through a circuit is called an electromotive force or e.m.f. that moves electric charges from one point in the circuit to another.

When an electromotive force is applied to the ends of a wire the free electrons move in one direction. It is the movement of the free electrons in a conductor that includes an electric current. The greater the number of participating electrons, the greater is the flow of current.

No one has ever seen an electric current. We only know of the existence of a current by its effects. A

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current can heat a conductor, it can have a chemical action when passing through a solution, or it can produce a magnetic effect. We can measure currents by observing their heating, their chemical, or their magnetic effects.

There are some kinds of current: a direct current (d.c.), an alternating current (a.c.), a pulsating current.

3. Answer the questions:

1. What is the conductor?

2. In what case may the charge be ions?

3. What is the metal made up of?

4. When do the electrons move?

5. What are the free electrons called?

6. How do the free electrons move?

7. When do the free electrons move in one direction?

8. When can current have a chemical action?

9. How can we measure current?

10. How many kinds of current do you know?

4. Correct the mistakes:

The charges are usually electrons.

A metal is made up of big crystals.

A crystal is irregular arrangement of atoms.

Once an atom was a complex particle.

The free electrons do not move at random through the

metal.

When an electromotive force is applied to the ends of

a wire the free electrons stay at their own places.

The greater the number of participating electrons, the

smaller is the flow of current.

A current cannot heat a conductor.

We can measure currents by observing their freezing.

There are only two kinds of current.