C H A P T E R   12 Temperature and Heat

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C H A P T E R   12 Temperature and Heat. 12.1  Common Temperature Scales. Fahrenheit scale. Celsius scale. Boiling point . 212. 100. Unknown . T f. T c. Freezing point. 32. 0. Temperature Conversion. Fahrenheit scale. Celsius scale. Boiling point . 212. 100. Unknown temperature.

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Fahrenheit scale

Celsius scale

Boiling point

212

100

Unknown

Tf

Tc

Freezing point

32

0

Temperature Conversion

Fahrenheit scale

Celsius scale

Boiling point

212

100

Unknown temperature

Tf

Tc

Freezing point

32

0

Temperature Conversion

Temperature conversion equations can be obtained by comparing the temperature difference ratios as follows,

Fahrenheit scale

Celsius scale

Boiling point

212

100

Unknown

Tf

Tc

Freezing point

32

0

Temperature Conversion

Temperature conversion equations can be obtained by comparing the temperature difference ratios as follows,

12.2 The Kelvin Temperature Scale

Kelvin temperature scale has greater scientific significance.

It was introduced by Kelvin (1824–1907).

A temperature of 300 K (not 300 °K) is read as “three hundred kelvins,” not “three hundred degrees kelvin.”

The kelvin is the SI base unit for temperature.

Absolute Zero Temperature

The temperature at which the absolute pressure of the gas is zero, is the absolute temperature. It is found to be –273.150C or 0 K.

The Kelvin Temperature

The Kelvin temperature is obtained by adding 273.15 to the Celsius temperature as follows,

0C and C0

Q: What is the difference between degrees Celsius ( 0C) and Celsius degrees (C0)?

0C and C0

Q: What is the difference between degrees Celsius ( 0C) and Celsius degrees (C0)?

A: Temperature is expressed in degrees Celsius ( 0C) and temperature difference is expressed in Celsius degrees (C0).

12.3 Thermometers

All thermometers make use of the change in some physical property with temperature.

A property that changes with temperature is called a thermometric property.

Thermometer

Thermometric Property

Mercury thermometer

Length of mercury column

Constant-volume gas thermometer

Pressure of the gas

Thermocouple

Voltage

Thermogram or Ear thermometer

Thermometric Properties
Thermography

The breast in this thermograph has an invasive carcinoma (cancer) and registers colors from red to yellow/white, indicating markedly elevated temperatures.

Thermography

Thermogram showing a smoker’s forearms before (left) and 5 minutes after (right) he has smoked a cigarette. Temperatures range from over 34 °C (white) to about 28 °C (blue).

Thermography

This thermogram shows the 1997/98 El Niño, a region (red) of the Pacific Ocean that had abnormally high temperatures.

12.4 Linear Thermal Expansion

The change in length is given by the following equation, where α is the coefficient of linear expansion.

Area Expansion

The area A0 of an object changes by an amount DA when its temperature changes by an amount DT:

where α is the coefficient of linear expansion.

Volume Expansion

The volume V0 of an object changes by an amount DV when its temperature changes by an amount DT:

where b is the coefficient of volume expansion.

TABLE 12.1

Coefficients of Thermal Expansion for Solids and Liquidsa

Coefficient of Thermal Expansion, (C°)-1

Substance

Linear (a)

Volumetric (b)

Solids

Aluminum

23 × 10-6

69 × 10-6

Brass

19 × 10-6

57 × 10-6

Concrete

12 × 10-6

36 × 10-6

Copper

17 × 10-6

51 × 10-6

Glass (common)

8.5 × 10-6

26 × 10-6

Glass (Pyrex)

3.3 × 10-6

9.9 × 10-6

Gold

14 × 10-6

42 × 10-6

Iron or steel

12 × 10-6

36 × 10-6

29 × 10-6

87 × 10-6

Nickel

13 × 10-6

39 × 10-6

Liquidsb

Benzene

1240 × 10-6

Carbon tetrachloride

1240 × 10-6

Ethyl alcohol

1120 × 10-6

Gasoline

950 × 10-6

Mercury

182 × 10-6

Methyl alcohol

1200 × 10-6

Water

207 × 10-6

Volumetric Thermal Expansion Coefficient (C°)-1
Coolant Reservoir

Q: Why vehicles need a coolant reservoir?

Coolant Reservoir

Q: Why vehicles need a coolant reservoir?

A: For catching the overflow coolant from the radiator.

An antiscalding Device.

Each year thousands of children are taken to emergency rooms suffering from burns caused by scalding tap water. Such accidents can be reduced with the aid of the antiscalding device shown above.

Bimetallic Switch

A bimetallic strip controls whether this coffee pot is (a) “on” (strip cool, straight) or (b) “off” (strip hot, bent).

The Expansion of Holes

Do Holes Expand or Contract When the Temperature Increases?

The Expansion of Holes

Do Holes Expand or Contract When the Temperature Increases?

Unusual Expansion of Water

Most substances contract upon cooling. But, water expands while cooling from 4 0C until it freezes.

Water Temperature of a Frigid Cold Lake

The fact that water has its greatest density at 4 °C, rather than at 0 °C, has important consequences for the way in which a lake freezes.

When the air temperature drops, the surface layer of water is chilled. As the temperature of the surface layer drops toward 4 °C, this layer becomes more dense than the warmer water below. The denser water sinks and pushes up the deeper and warmer water, which in turn is chilled at the surface. This process continues until the temperature of the entire lake reaches 4 °C. Further cooling of the surface water below 4 °C makes it less dense than the deeper layers; consequently, the surface layer does not sink but stays on top. Continued cooling of the top layer to 0 °C leads to the formation of ice hat floats on the water, because ice has a smaller density than water at any temperature.

Below the ice, however, the water temperature remains above 0 °C. The sheet of ice acts as an insulator that reduces the loss of heat from the lake, especially if the ice is covered with a blanket of snow, which is also an insulator. As a result, lakes usually do not freeze solid, even during prolonged cold spells, so fish and other aquatic life can survive.

Bursting Water Pipes

As water freezes and expands, enormous pressure is applied to the liquid water between the ice and the faucet.