1. What is dynamo?
2. What is dynamo used for?
3. When does current flow in the conductor?
4. What does a dynamo usually include?
Relays
Relays are electromagnetic devices widely used in various branches of industry. Relays are designed for rather low currents and for operation in control circuits at low voltages. The relay is placed close to the motor which is connected to its secondary circuit. By means of a relay electric current flowing in one circuit can open or close a second circuit and thus control the switching on and off a circuit. The main components of a relay are an electromagnet, an armature and a spring. As to contacts, they belong to its auxiliary elements. When a current starts flowing in the electromagnetic winding, the armature moves and the spring closes the contacts. When there is no current in the primary circuit, the spring pulls the armature and the contacts open. The primary circuit is opened either manually or automatically. Every evening and morning street lights are switched on and off from the main panel by means of relays.
(780)
Test. Choose the correct variant
1. Relays are designed for …
a) low currents
b) high currents
2. A relay consist of
a) a spring and an armature
b) an electromagnet, an armature, and a spring
3. The relay is placed
a) close to the motor
b) far from the motor
4. The motor is connected to
a) the primary circuit
b) the secondary circuit
5. Street lights are switched on and off
a) by means of relays
b) by means of controllers
Answer the questions
1. Where are relays used?
2. What does a relay do in a circuit?
3. What are the main components of a relay?
4. How are street lights switched on and off?
Electric cells
An electric cell is used to produce and supply electric energy. It consists of an electrolyte and two electrodes. Electrodes are used as terminals, they connect the cell to the circuit – current passes through the terminals and the bulb lights.
Cells can be connected in series, in parallel and in series-parallel. In order to increase the voltage output cells should be connected in series. In case a battery has a large current capacity and a large voltage output, its cells are connected in series-parallel.
When cells are connected in series the positive terminal of one cell is connected to the negative terminal of the second cell, the positive terminal of the second cell – to the negative terminal of the third … and so on.
When cells are connected in parallel their negative terminals are connected together and their positive terminals are also connected. In case a cell has a trouble it stops operating or operates badly. This cell should be substituted by another one.
(820)
Complete the sentences using the correct variant
1. A cell is used
a) to increase the voltage output
b) to reduce the current capacity
c) to supply electric energy
2. The terminals of a cell are used
a) to conduct current
b) to increase voltage
c) to connect battery to a circuit
3. When cells are connected in series
a) all the positive terminals are connected together
b) all the negative terminals are connected together
c) the positive terminal of one cell is connected to the negative terminal of the second
4. Cells are connected in series in order
a) to increase the current capacity
b) to increase the voltage output
5. In order to increase the current capacity
a) cells are connected in series
b) cells are connected in parallel.
Answer the following questions
1. What is a cell used for?
2. What does a cell consist of?
3. What is the function of the terminals?
4. In what case does a cell stop operating?
Capacitors
A capacitor is one of the main elements of a circuit. It is used to store electric energy. The main parts of a capacitor are metal plates and insulators. The function of insulators is to isolate the metal plates and in this way to prevent a short.
There are two common types of capacitors in use nowadays: a fixed capacitor and a variable one. The plates of a fixed capacitor cannot be moved; for this reason its capacity does not change. The plates of a variable capacitor move; its capacity changes. Variable capacitors are commonly used by radiomen; their function is to vary the frequency in the circuit. Fixed capacitors are used in telephone and radio work.
Fixed capacitors have insulators produced of paper, ceramics and other materials; variable capacitors have air insulators. Paper capacitors are commonly used in radio and electronics; their advantage is their high capacity: it may be higher than 1,000 picofarad.
Besides, electrolyte capacitors are highly in use. They also have a very high capacity: it varies from 0.5 to 2,000 microfarad. Their disadvantage is that they change their capacity when the temperature changes. They can operate without a change only at temperatures not lower than - 40C.
(1010)
11.1 Complete the sentences using the correct variant:
1. A capacitor is used a) to supply voltage
b) to increase the voltage output
c) to store energy
2. The main parts of a capacitor are a) insulators only
b) metal plates only
c) metal plates and insulators
3. The function of insulators is a) to store energy
b) to isolate the metal plates
c) to prevent a short
4. The capacity of a fixed capacitor a) is constant
b) is varied
5. The plates of a variable capacitor a) can be moved
b) cannot be moved
6. Electrolyte capacitors have a) a very low capacity
b) a very high capacity