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Equals average pressure times change of volume

Positive and Negative Work. — In the preceding cases the work done on a piston by the expanding gas was discussed. Such work must be considered positive. In order to force the dead steam or the burned gases out of the cylinder, the piston on its return stroke must exert a force and must therefore do work. This is the work done to return the piston to its initial position ready for a new forward stroke. It represents a waste, or loss, that must take place in order to make the working stroke of the piston possible. To distinguish this work from the work done on the piston by the gases in the forward stroke, it is called negative. This work done on the gases by the piston must be subtracted from the work done on the piston by the gases in order to obtain the useful work done by the piston in one complete cycle [2, С. 93 - 97].

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 purpose 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, С. 97 - 98].

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?

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