8. Resistance and resistivity.

Every material offers some resistance to the flow of an electric current through it. The resistance of a conductor depends on its material. It also depends on its temperature. Materials change the value of resistance with change in their temperature. Different materials also have different melting points. Good conductors, like the metals copper, silver, and aluminium, offer very little resistance, while non-conductors, like glass, wood and paper, offer a very high resistance. The resistance of nichrome is rather high.

Resistance of conductors and their resistivity have different units. The unit by which resistance is measured is called the ohm, in honour of the German physicist Ohm.

The unit, of resistance is the ohm while the unit of resis­tivity is the ohm • m. The standard international ohm is de­fined as the resistance offered to a steady electric current by a column of mercury 1 sq.mm in cross-section and 106.3 cm long at a temperature of 0°.

There are several factors that determine the electrical re­sistance of any wire: a) the material which it is composed of; b) the size of the wire; c) its temperature.

In more general terms, the resistance of a wire is propor­tional to its length and inversely proportional to its cross-sectional area (provided the temperature of a conductor re­mains constant). This is Ohm's law.

9. Electric power plants.

Electric power is generated at electric power plants. The main unit of an electric power plant comprises a prime mover and the generator which it rotates.

In order to actuate the prime mover energy is required. Many different sources of energy are in use nowadays. To these sources belong heat obtained by burning fuels, pressure due to the flow of air (wind), solar heat, etc.

According to the kind of energy used by the prime move: power plants are divided into groups. Thermal, hydraulic (water-power) and wind plants form these groups. According to the kind of prime mover, electric power plants are classed as:

1. Steam turbine plants, where steam turbines serve as prime movers. The main generating units at steam turbine plants belong to the modern, high-capacity class of power plants.

2. Steam engine plants, in which the prime mover is a piston-type steam engine.

Nowadays no large generating plants of industrial importance are constructed with such prime movers. They are used only for local power supply.

3. Diesel-engine plants; in them diesel internal combustion engines are installed. These plants are also of small capacity, they are employed for local power supply.

4. Hydroelectric power plants employ water turbines as prime movers. Therefore they are called hydroturbine plants. Their main generating unit is the hydrogenerator.

Modern wind-electric power plants utilize various turbines: these plants as well as the small capacity hydroelectric power plants are widely used in agriculture.

10. Electric shock safety electric system.

The strength of current depends on both the voltage and on the resistance in a circuit. A current of 50ma is dangerous for a man, it may result in an electric shock. One gets an electric shock in case one touches live conductors when the power is on. And a current of l00ma and higher is lethal. Thus, before working on a circuit, deenergize it and work on it with the power off.

Earthing system serves to protect attending personnel from electric shocks when voltage appears on parts that are nor­mally dead. The risk of an electric shock decreases with de­creasing voltage. In wet and hot atmosphere the risk of elec­tric shock increases. Safe voltage for circuits used in dry atmosphere is under 36V. When the power is on contacts with live conductors are dangerous for life. When a live con­ductor is touched with both hands the resistance of the con­ductor is from 10,000 to 50,000 ohms. When a live conduc­tor is touched with one hand the resistance is much higher. The higher is the body resistance, the smaller is the current that flows through the body. Take it into consideration and work with one hand if the power is on! Or work on the circuit with the power off!

Thus measures are taken to protect attending personnel from contacts with live parts of installations under voltage.

The danger of electric shock disappears provided the met­al parts of installations under voltage are connected with ground by means of safety earthing. Connecting to ground is made by means of measuring devices. The faulty parts should be detected, eliminated, and replaced by new ones.