Unit 7. Heat

What is heat? There have been many theories to explain the nature of heat. About 2 centuries ago heat was considered to be an invisible substance that passed from a hot to a cold object. This supposed substance was called caloric.

According to the caloric theory, when an object was heated in a flame it became full of caloric. The object would transfer part of its caloric to a colder substance when placed next to it. Only Robert Boyle and Isaac Newton began to think about heat in terms of molecular motion. Our great scientist M. V. Lomonosov was among the first to state that heat phenomena were due to the motion of molecules. That statement of his was the result of many carefully performed laboratory experiments, of study and observations. Lomonosov’s theory laid the foundation of the present-day molecular-kinetic theory of heat.

Now we know that heat is a form of energy. It means that it can be used to do work. When we see an automobile speeding down a road, we can be sure that it is being driven by the heat of burning gasoline.

We receive some heat from a number of different sources. The sun, the interior of the earth, chemical action, mechanical, electrical, and nuclear energy, are the chief sources of heat. Heat is the sum of the kinetic energies of all the molecules of a body.

Heat can produce different effects. Solids expand when heated and contract when cooled. The increase per unit length per unit degree is called the coefficient of linear expansion. The increase in area, or the coefficient of superficial expansion, is approximately twice the linear coefficient. The increase in volume, or the coefficient of cubical expansion, is three times the linear coefficient. Liquids have a much higher coefficient of expansion than solids. A change in temperature or a change in pressure will change the volume of a gas.

How is the heat measured? Quantities of heat are measured scientifically in terms of the calorie, which is defined as the heat required to raise the temperature of 1 gram of water through 1 degree Celsius.

Heat can be transmitted by conduction, metals being the best conductors; convection, which is taking place only in liquids and gases; and radiation.

What is conduction? When one end of an iron rod is held in flame, the other end of the rod will soon be too hot to be held comfortable. In some way, the heat is conducted, or led, from one end of each solid to the other end. The molecules of the rod which are adjacent to the flame receive some of this heat energy, which they transmit along the rod to other molecules. Such transmission of heat from molecule to molecule is called conduction. Metals generally are much better conductors of heat than wood or stone.

What is convection? When the portion of water near the bottom of a vessel is heated, it expands and becomes less dense. The denser water from the opposite side of the vessel flows across and pushes up this less dense water thus setting up convection currents. Convection currents may be set up in either liquids or gases by unequal heating. By means of such currents heat is transmitted by the movement of the heated masses.

What is radiation? Radiant heat is the name given to heat in the form of rays. A hand held in front of a fireplace is quickly warmed by radiation. We receive heat from the sun the same way. This heat radiation is probably due to the internal motion of the atoms and molecules of the heat-producing body, and travels through space as a wave. Heat radiation is one of several types of wave motion studied in physics, called electromagnetic waves.

Radiation can travel through a vacuum. Radiant heat warms only objects on which it falls and which absorb it; it does not heat the “medium” through which it passes.