Temperature & Thermal Expansion Temperature Zeroth Law of - - PowerPoint PPT Presentation

Temperature & Thermal Expansion

• Temperature
• Zeroth Law of Thermodynamics
• Temperature Measurement
• Thermal Expansion
• Homework

Temperature & Thermal Equilibrium

• Temperature

– Fundamental physical quantity – Measure of average kinetic energy of molecular motion

• Thermal equilibrium

– Two objects in thermal contact cease to have an exchange of energy

The Zeroth Law of Thermodynamics

If objects A and B are separately in thermal equilibrium with a third object C (the thermometer), the A and B are in thermal equilibrium with each other.

Temperature Measurement

• In principle, any system whose physical properties change with tempera-

ture can be used as a thermometer

• Some physical properties commonly used are

– The volume of a liquid – The length of a solid – The electrical resistance of a conductor – The pressure of a gas held at constant volume – The volume of a gas held at constant pressure

The Glass-Bulb Thermometer

• Common thermometer in everyday use
• Physical property that changes is the volume of a liquid - usually mercury
• r alcohol
• Since the cross-sectional area of the capillary tube is constant, the change

in volume varies linearly with its length along the tube

Calibrating the Thermometer

• The thermometer can be calibrated by putting it in thermal equilibrium

with environments at known temperatures and marking the end of the liquid column

• Commonly used environments are

– Ice-water mixture in equilibrium at the freezing point of water – Water-steam mixture in equilibrium at the boiling point of water

• Once the ends of the liquid column have been marked for a couple differ-

ent environments, a scale must be defined

The Celsius Scale

• A common temperature scale is the Celsius scale
• On this scale the freezing point is at zero degrees (0

C) and the boiling point is at 100 degrees (100

C)

The Constant-Volume Gas Thermometer

• The standard thermometer, against which all other thermometers are calibrated
• Measures the pressure of the gas contained in the flask immersed in the bath
• The volume of gas in the flask is kept constant be raising or lowering reservoir B such

that the mercury level in column A remains constant

• Pressure is determined from the height h of the mercury in the reservoir (P = P

+

gh)

Absolute Zero

• Experiments show that the results obtained with the constant-volume gas thermometer

are nearly independent of the type of gas used

• If the measurements are performed with the gas in the flask at different starting pressures

at 0

C, different straight-line calibration curves are obtained

• All of these curves extrapolate to zero pressure at the unique temperature of -273.15

C

The Kelvin Scale

• Absolute zero is used as the basis for the Kelvin temperature scale in which -273.15

C is set as the zero point (0 K)

• The size of a degree on the Kelvin scale is the same as the size of a degree on the Celsius

scale

• The conversion between the Kelvin and Celsius scales is given by T

= T - 273.15

The Fahrenheit Scale

• The most common temperature scale
• The ice point is at 32

F and the steam point is at 212

F

• The relationship between the Fahrenheit scale and the Celsius scale is given by T

=

T

+ 32

F

• The relationship between changes in temperature on the two scales is given by

T

=

T

Thermal Expansion

• If the temperature of a metal rod of length L

is raised by an amount

T, its length is found to increase by an amount

where

is a constant called the coefficient of linear expansion.

• If the temperature of a solid or liquid whose volume is V

is increased by an amount

T, the increase in volume is found to be

where

is the coefficient of volume expansion. (Note that

= 3

.)

Expansion Coefficients for Some Materials

Example

On a hot day in Las Vegas, an oil trucker loaded 37,000 L of diesel fuel. He encountered cold weather on the way to Payson, Utah, where the temperature was 23.0 K lower than in Las Vegas, and where he delivered his entire load. How many liters did he deliver? The coefficient of volume expansion for diesel fuel is 9.50

10

/

C.

The Unusual Behavior of Water

• At temperatures above 4

C, water behaves like most liquids and contracts as it is cooled

• But, at temperatures below 4

C it expands as it is cooled

• This is why bodies of water freeze at the surface and why freezing water

can break sealed containers

Homework Set 1 - Due Wed. Jan 7

• Read Sections 16.1 - 16.3
• Answer Questions 16.1, 16.4 & 16.5
• Do Problems 16.1, 16.3, 16.8, 16.13 & 16.14