Reference Reference Book isBook is

TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS

* Two objects are in * Two objects are in Thermal contact Thermal contact with each with each other if energy can be exchanged between them.other if energy can be exchanged between them.* Thermal equilibrium * Thermal equilibrium is a situation in which two is a situation in which two objects in thermal contact with each other cease objects in thermal contact with each other cease to exchange energy by the process of to exchange energy by the process of heat.heat.

Some important definitionsSome important definitions

HeatHeat is defined as the transfer of energy across the is defined as the transfer of energy across the boundary of a system due to a temperature boundary of a system due to a temperature difference between the system and its difference between the system and its surroundingssurroundings.

Units of HeatUnits of Heat1- 1- The calorie (cal)The calorie (cal), which is defined as the amount , which is defined as the amount of energy transfer necessary to raise the of energy transfer necessary to raise the temperature of 1 g of water from 14.5°C to 15.5°Ctemperature of 1 g of water from 14.5°C to 15.5°C

This relation is known as This relation is known as mechanical equivalent of heatmechanical equivalent of heat

The specific heat The specific heat c of a substance c of a substance is defined as the is defined as the amount of energy needed to raise the temperature amount of energy needed to raise the temperature of unit mass by 1°C.of unit mass by 1°C. Thus, if Thus, if energy energy Q transferred Q transferred by heat to mass m of a substance changes the by heat to mass m of a substance changes the temperature of temperature of the sample by ∆the sample by ∆T, then the specific T, then the specific heat of the substance isheat of the substance is

Zeroth law of Zeroth law of thermodynamicsIf objects A and B are separately in thermal If objects A and B are separately in thermal equilibrium with a third object C, then objects A equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other.and B are in thermal equilibrium with each other.

Two objects in thermal equilibrium with each Two objects in thermal equilibrium with each other are at the same other are at the same temperature.temperature.

A B

C

THERMOMETERS AND THERMOMETERS AND THE CELSIUS TEMPERATURE SCALETHE CELSIUS TEMPERATURE SCALE

Thermometers are devices that are used to Thermometers are devices that are used to define and measure temperaturesdefine and measure temperatures

Some physical properties that change with Some physical properties that change with temperature are temperature are (1) the volume of a liquid, (1) the volume of a liquid, (2) the length of a solid,(2) the length of a solid,(3) the pressure of a gas at constant volume, (3) the pressure of a gas at constant volume, (4) the volume of a gas at constant pressure, (4) the volume of a gas at constant pressure, (5) the electric resistance of a conductor, (5) the electric resistance of a conductor, (6) the color of an object.(6) the color of an object.

A common thermometer in everyday A common thermometer in everyday use consists of a mass of liquid—use consists of a mass of liquid—usually mercury or alcohol—that usually mercury or alcohol—that expands into a glass capillary tube expands into a glass capillary tube when heated. when heated. In this case the physical In this case the physical property is the change in volume of a property is the change in volume of a liquid. liquid.

As a result of thermal expansion, the As a result of thermal expansion, the level of the mercury in the level of the mercury in the thermometer rises as the mercury is thermometer rises as the mercury is heated by water in the test tube.heated by water in the test tube.

The thermometer can be calibrated by placing it in The thermometer can be calibrated by placing it in thermal contact with some natural systems that thermal contact with some natural systems that remain at constant temperature. On the remain at constant temperature. On the Celsius Celsius temperature scale temperature scale a mixture of water and ice in a mixture of water and ice in thermal equilibrium is defined to have a temperature thermal equilibrium is defined to have a temperature of zero degrees Celsius, which is written as 0°C. of zero degrees Celsius, which is written as 0°C. Another commonly used system is a mixture of water Another commonly used system is a mixture of water and steam in thermal equilibrium its temperature is and steam in thermal equilibrium its temperature is 100°C, the distance between the two points is divided 100°C, the distance between the two points is divided into 100 equal segments to create the Celsius scale.into 100 equal segments to create the Celsius scale.

This temperature scale used to be called the This temperature scale used to be called the centigrade scale because there are 100 gradations centigrade scale because there are 100 gradations between the between the ice and steam points of waterice and steam points of water

1- Because mercury and alcohol have different 1- Because mercury and alcohol have different thermal expansion properties.thermal expansion properties.2- Because the limited range of temperatures 2- Because the limited range of temperatures over which it can be used. A mercury over which it can be used. A mercury thermometer, for example, cannot be used below thermometer, for example, cannot be used below the freezing point of mercury, which is -39°C, and the freezing point of mercury, which is -39°C, and an alcohol thermometer is not useful for an alcohol thermometer is not useful for measuring temperatures above 85°C, the boiling measuring temperatures above 85°C, the boiling point of alcohol.point of alcohol.

Thermometers calibrated in this way present Thermometers calibrated in this way present problems when extremely accurate readings are problems when extremely accurate readings are needed.needed.

THE CONSTANT-VOLUME GAS THERMOMETER THE CONSTANT-VOLUME GAS THERMOMETER ANDAND

THE ABSOLUTE TEMPERATURE SCALETHE ABSOLUTE TEMPERATURE SCALEA constant-volume gas A constant-volume gas thermo- meter measures the thermo- meter measures the pressure of the gas contained pressure of the gas contained in the flask immersed in the in the flask immersed in the bath. The volume of gas in the bath. The volume of gas in the flask is kept constant by raising flask is kept constant by raising or lowering reservoir or lowering reservoir B to keep B to keep the mercury the mercury level in column level in column A A constant.constant.

A typical graph of pressure versus A typical graph of pressure versus temperature taken with a constant-temperature taken with a constant-volume gas thermometer. The two volume gas thermometer. The two dots represent known reference dots represent known reference temperatures (the ice and steam temperatures (the ice and steam points of water).points of water).

If you extend the curves If you extend the curves toward negative tempe- toward negative tempe- ratures, you find, in every ratures, you find, in every case, that the pressure is case, that the pressure is zero when the temperature zero when the temperature is -273.15°C. This tempe- is -273.15°C. This tempe- rature is often referred to as rature is often referred to as absolute zero.absolute zero.

Because the ice and steam points are Because the ice and steam points are experimentally difficult to duplicate, an absolute experimentally difficult to duplicate, an absolute temperature scale based on a single fixed point temperature scale based on a single fixed point was adopted in 1954 by the was adopted in 1954 by the International International Committee on Weights and MeasuresCommittee on Weights and Measures. In this . In this new scale, the triple point of water was chosen new scale, the triple point of water was chosen as the reference temperature for this new scale. as the reference temperature for this new scale. The triple point of water is the single The triple point of water is the single combination of temperature and pressure at combination of temperature and pressure at which liquid water, gaseous water, and ice (solid which liquid water, gaseous water, and ice (solid water) coexist in equilibrium. This triple point water) coexist in equilibrium. This triple point occurs at a temperature of approximately 0.01°C occurs at a temperature of approximately 0.01°C and a pressure of 4.58 mm of mercuryand a pressure of 4.58 mm of mercury

Fahrenheit scaleFahrenheit scale. This scale sets the temperature . This scale sets the temperature of the ice point at 32°F and the temperature of the of the ice point at 32°F and the temperature of the steam point at 212°F. The relationship between steam point at 212°F. The relationship between the Celsius and Fahrenheit temperature scales isthe Celsius and Fahrenheit temperature scales is

The kelvin scale The kelvin scale , which is defined to be 1/273.16 , which is defined to be 1/273.16 of the difference between absolute zero and the of the difference between absolute zero and the temperature of the triple point of water.temperature of the triple point of water.

Thermal expansion is a consequence of the change Thermal expansion is a consequence of the change in the average separation between the constituent in the average separation between the constituent atoms in an object.atoms in an object.

THERMAL EXPANSION OF SOLIDS AND LIQUIDS

Suppose that an object has an initial length Suppose that an object has an initial length LLii along some direction at some temperature and along some direction at some temperature and that the length increases that the length increases by an amount ∆by an amount ∆L for a L for a change in temperature ∆T. change in temperature ∆T. we define the we define the average average coefficient of linear expansioncoefficient of linear expansion as as

This equation is usually rewritten asThis equation is usually rewritten as