Temperature

Assigning

numbers

to hot and cold.

All of us know what temperature signifies in terms of our sense impressions, and we do not have to understand what temperature means in terms of the structure of matter to make use of the concept of temperature in a variety of ways. For the moment it is sufficient for us simply to regard temperature as that which gives rise to sensations of hot andcold. A thermometer is a device used to measure temperature. Most substances expand when heated and contract when cooled, and the thermometers of everyday life are based on this property of matter. More precisely, they are based on the different rates of expansion of different materials (Fig. 13.1). Thus the length of the

Figure 13.1
Three types of thermometer on thermal expansion. based

(a)Two strips of different metals that are joined together bend to one side or the other with a change in temperature owing to different rates of expansion in the two metals, which is the basis of household oven thermometers. The higher the temperature, the greater the deflection. When cooled, such a bimetallic strip bends in the opposite direction. A thermostat uses a bimetallic strip to operate aswitch that turns a heating or cooling system on and off at preset temperatures.

(b) In a constant-volume gas thermometer, which is a very sensitive laboratory instrument, the height of the mercury column at the left is adjusted until the mercury column at the right is at a fixed level. The difference in heights of the two mercury columns is a measure of the pressure needed to maintain the gas ina fixed volume, and hence a measure of the temperature.

(c) Mercury (or colored alcohol) expands more when heated than glass does, and so the length of the liquid column in a liquid-in-glass thermometer is a measure of the temperature of the thermometer bulb.

mercury column in the familiar mercury-in-glass thermometer indicates the temperature of the bulb because mercury expands more thanglass when heated and contracts more than glass when cooled. Thermal expansion is not the only property of matter that can be used to make a thermometer. The electrical resistance of most materials, for instance, varies with temperature. Another example-is the color of an object heated until it glows. A poker thrust in a fire at first becomes a dull red, then bright red, orange, and yellow. At ahigh enough temperature it becomes "white hot." The color of a really hot object is thus a measure of its temperature, a fact used by astronomers when they study stars. Before we can make a thermometer of any kind, we need a temperature scale. One way to establish such a scale is to note that water freezes into a solid (ice) and vaporizes into a gas (steam), both at definite temperatures at a givenpressure. In the Celsius scale the freezing point of water at 1 atm pressure (or, more exactly, the point at which a mixture of ice and water is in equilibrium, with as much ice melting as water freezing) is called 0°, and the boiling point of water at 1 atm pressure (or, more exactly, the point at which a mixture of steam and water is in equilibrium) is called 100° (Fig. 13.2). Temperatures inthe Celsius scale are written, for example, "40°C." In the United States the Celsius scale is sometimes called the centigrade scale. Although the Celsius scale is used in most of the world, a different temperature scale called the Fahrenheit scale is commonly used for nonscientific purposes in some English-speaking countries. In the Fahrenheit scale the freezing point of

Figure 13.2
Calibratinga thermometer on the Celsius scale. A mixture of ice and water at atmospheric pressure is, by definition, at Doe, and a mixture of steam and water at atmospheric pressure is, again by definition, at 100°e.

Mixture of ice and water

Mixture of steam and water

water is 32°F and the boiling point of water is 212°F (Fig. 13.3). This means that 180°F separates the freezing and boiling points...