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An electric cell supplies electric energy provided its electrodes are of different materials. In case the electrodes are of the same material they become charged but there is no difference of potential across the terminals. Iron and zinc plates are commonly used for producing negative electrodes since these materials produce a high charge. Carbon is commonly used to produce positive electrodes.

The voltage output of cells in use nowadays is from 1 to 2 V. The value of the output depends only on the materials used for the electrodes. Besides, it depends on the electrolyte of a cell. It does not depend on the size of a cell and its construction, while the current capacity of a cell depends on the size of the electrodes. The larger the size of the electrodes, the more current capacity they can supply. When the size of the electrodes is increased the current capacity also increases while the voltage output does not increase. Such is the relation between the size of the electrodes and the current capacity.

What element is described in the text?

In what case does a cell supply energy?

What materials are commonly used for producing negative electrodes?

Explain why iron and zinc are used.

What is the voltage output of cells in use nowadays?

What does the value of the output depend on?

What is the relation between the size of the electrodes and the current capacity?

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The capacity of a capacitor is measured in farads. A capacitor has a capacity of one farad when a charge of one coulomb increases the potential between its plates by one volt.

The capacity depends on four things:

first, the higher the voltage used to charge the capacitor the more energy it will store;

second, the larger the size of plates and the greater their number the more energy will be stored;

third, the closer are the positive and negative plates the greater is the charge;

fourth, some insulators store greater charge than others.

What does the capacity of a capacitor depend on?

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Wattmeter

A wattmeter is used to measure the value of power. It is connected to the circuit directly. A wattmeter consists of coils: two fixed coils and a coil which moves in the magnetic field produced by the fixed coils. Wire used for the coils must have a high resistance; the fixed coils are in series with the load, the moving coil is connected across the line in series with a resistance. When a wattmeter is used, the readings on its scale show the value of power being used.

What is the wattmeter used for?

What does it consist of?

In what way are the elements connected?

What do the readings on the scale show?

 

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Rheostat

A rheostat is a resistor whose resistance value may be varied. Thus, a rheostat is a variable resistor.

It is used to change the resistance of circuits, and in this way to vary the value of current.

A rheostat consists of a coil and a switch. Take into consideration that wire used for the coil must have a very high resistance. When a rheostat is used its terminals are connected in series with the load. The switch is used to change the length of the wire through which the measured current passes. The resistance may be changed to any value from zero to maximum.

The longer the rheostat wire used in the circuit, the greater is the resistance.

What type of resistor is a rheostat?

What is a rheostat used for?

In what way does a rheostat vary the value of current?

What elements does a rheostat consist of?

In what way are the terminals connected with the load?

What is the function of the switch?

 

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Voltage Values

Voltages up to about 250 V are called low. The common electric lighting circuit operates either at about 127 or 200 V, and the voltage used on the main circuit of large houses is usually the same. One can get an electric shock, when one touches an uninsulated wire of such a circuit.

Voltages above 250 V are high voltages. They are used in industry. Medium-powered motors are usually operated at 380 V. Large motors are supplied by voltages of from about 500 up to 6,000 V.

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