2.12.6 Look through the text and answer the questions: For what purpose should we know work efficiency? How can we calculate it? Efficiency

Since heat and work are convertible, the most important thing to know about any device for this puprose is its efficiency, which gives a measure of the amount of heat which can be transformed into work under a given set of conditions. The ratio of the work obtained from the machine to heat put into it is called the efficiency of the machine. Both the heat and the work must be meas­ured in the same units. In finding the efficiency of a burner used to heat a kettle of water, it is necessary to find the amount of gas con­sumed by the burner and the amount of heat thus developed. It is next necessary to find the amount of heat that gets into the water in the kettle. The ratio obtained by dividing the heat that gets into the water by the heat developed by the burning of the gas is called the efficiency of the burner. The efficiency may be defined as the fraction which tells what portion of the total heat supplied is used for the purpose for which it was intended.

heat used

Efficiency = –––––––––––

heat supplied

2.12.7 Read the text, translate it and answer the questions: What type of energy: potential or kinetic is more widespread? What is a mechanical energy? What do we call the appliances for energy converting?

Energy Transformation. Varieties of Energy

There are many sources of energy in the world both potential and kinetic. One source consists in water falling from high level such as an upland lake. Another source is wind or moving air. Others are in tides in the sea, also heat from subterranean sources and lastly coal deposits and oil wells yielding mineral oil. But all these sources and stores of energy are not equally useful to mankind. Moreover, some stores of energy such as coal and oil can never be replaced by us when once used up. On the other hand stores of high level water are continually being replaced by rain and wind and tides will not, as far as we know, ever cease to exist.

Hence a very important matter is the conversion of energy from one form to another. The form most required by us is mechanical energy.

We require to rotate shafts in a factory for driving various ma­chines and also for driving the wheels of vehicles of automobiles or locomotives. The energy of water power is very unequally distributed. Some countries such as Norway and Switzerland are rich in it and some such as England are poor.

Appliances for converting energy from one form to another are called engines. Thus a heat engine is a machine for converting heat into mechanical energy by the combustion of coal or oil and a water engine (water turbine) can convert the kinetic energy of falling water into mechanical energy. Another important matter to all such engines and transformation is the question of their efficiency or the ratio of the energy delivered in the desired form to that given to the engine in the available form. Thus, for instance, in the case of a heat engine we give it so much heat by combustion of coal or oil, and we take from it so much mechanical energy in kinetic form rotational or mo­tional and the important question is the ratio of the latter to the form­er. In most cases it is very small.