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Transfer of Heat

Convection. – The simplest way in which heat may be transferred from one place to another is by the motion of the heated substance. Such a transfer is known as convection. It is caused by the change in density that takes place when the substance is heated. For example, when a gas or a liquid is heated, it expands and becomes lighter than the cold gas or liquid. When water is heated in a vessel on a stove, the liquid in the bottom of the vessel is hotter than that on the top. The density at the bottom is less than that near the top. The cool liquid sinks down and forces the warmer liquid to a higher level. The currents of water thus set up in the liquid are known as convection currents.

Conduction. - When a metal rod is held in the fire, the heat trav­els along the rod and after a time the rod becomes too hot to hold. In this process the vibrations of the molecules are handed on from molecule to molecule. The layer of molecules in contact with the fire is heated first and thus made to vibrate more rapidly. This layer hands the motion on to the adjacent layer, because each layer is bound to the adjacent layer by certain cohesive forces. It is thus impossible for the molecules in one layer to vibrate without setting the molecules in the neighboring layers in vibration. As this process goes on, the entire medium is heated after a time. When heat, as in this case, is trans­ferred from one part of the body to another without any progressive motion of the parts of the substance, the heat is said to be trans­ferred by conduction. The conductivity differs widely according to the nature of the substance. Steam pipes are covered to reduce heat losses [2, С. 91].

2.12.5 Read the text Heat and Work, translate it and choose the best ending to the sentences:

a) By supplying heat to a gasoline engine we transform…

· heat into a work;

· work into a heat;

b) While transforming heat into work and inversely work into heat…

· an enormous energy can be created;

· energy remains the same;

c) Due to the second law of thermodynamics heat always flows…

· from higher to lower temperatures;

· from higher place to the lower one;

d) The slide valve is used in steam engines.…

· to prevent heating;

· to cut off the supply of steam;

e) The gasoline engine is..…

· the most sophisticated type of engine;

· the commonest type of engine;

f) The vertical axis and the horizontal axis in the diagram of gas work represent..…

· pressure and volume;

· pressure and temperature;

g) The diagrams represent­ing the work done by a gas expanding at constant pressure and at variable pressure…

· are similar;

· do not differ;

h) To find the useful work done in one complete cycle it is required…

· to subtract negative work from the positive work;

· to subtract positive work from the negative work.

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