C H A P T E R   12Temperature and Heat

12.1 Common Temperature

Scales

Temperature Conversion

  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,

  Fahrenheit scale

Celsius scale

Boiling point 212 100

  Unknown temperature 

  

Tf

  

Tc

  

Freezing point 32 0

Temperature Conversion

  Fahrenheit scale

Celsius scale

Boiling point 212 100

  Unknown

  

Tf

  

Tc

  

Freezing point 32 0

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

,0100

0

32212

32

cf TT

Temperature Conversion

,0100

0

32212

32

cf TT

,0100

3221232 cf TT

,32100

180 cf TT

325

9 cf TT

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.

Constant-volume Gas Thermometer

Absolute Zero 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,

.15.273 ck TT

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).

Converting Temp Differences

325

9 cf TT

.5

9cf TT

Converting Temp Differences

.ck TT

.15.273 ck TT

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.

Thermometric Properties

Thermometer Thermometric Property

Mercury thermometer Length of mercury column

Constant-volume gas thermometer Pressure of the gas

Thermocouple Voltage

Thermogram or Ear thermometer Infrared radiation

Thermocouple

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

12.4 Linear Thermal Expansion

.0 TLL

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

Area Expansion

.2 0 TAA

The area A0 of an object changes by an amount when its

temperature changes by an amount T:

where α is the coefficient of linear expansion.

Volume Expansion

The volume V0 of an object changes by an amount V

when its temperature changes by an amount T:

where is the coefficient of volume expansion.

     Coefficients of Thermal Expansion for Solids and Liquidsa

Coefficient of Thermal Expansion, (C°)-1

SubstanceLinear

()Volumetric ()

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

Lead 29 × 10-6 87 × 10-6

Nickel 13 × 10-6 39 × 10-6

Volumetric Thermal Expansion Coefficient (C°)-1

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

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.

EX-3 Buckling of a Sidewalk

Expansion Joint in a Bridge.

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.

The Bimetallic Strip

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?

Answer: Expand.

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.