Thermodynamics - Laws

S.Gunabalan Associate Professor Mechanical Engineering Department Bharathiyar College of Engineering & Technology Karaikal - 609 609. e-mail : gunabalans@yahoo.com

1. Zeroth law of Thermodynamics a) Temperature Measurement b) Thermal Equilibrium

Temperature

• Thermodynamics – science of temperature • Temperature – is associated with ability to

differentiate the hot from cold. • Thermal equilibrium – When two bodies at

deferent temperature are brought into contact, some time later they attain a common temperature – now they are in thermal equilibrium.

Zeroth law of Thermodynamics

• Base for all temperature measurement. • Thermal equilibrium is the key word for zeroth law

Definition: When a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other.

Zeroth law of Thermodynamics

• Temperature measurement procedure – A reference body is used, and a certain physical

characteristic of this body which changes with temperature is selected.

– The changes in the selected characteristic may be taken as an indication of change in temperature. The selected characteristic is called the thermometric property,

• expansion of the mercury in the tube is used as the thermometric property

– The reference body which is used in the determination of temperature is called the thermometer

Zeroth law of Thermodynamics

Zeroth law of Thermodynamics

Zeroth law of Thermodynamics

• An easily reproducible state of an arbitrarily chosen standard system is called a fixed point – (a) The ice point, the temperature at which pure ice

coexisted in equilibrium with air-saturated water at one atmosphere pressure.

– (b) the steam point, the temperature of equilibrium between pure water and pure steam at one atmosphere pressure.

– (c) the triple point of water, the state at which ice, liquid water and water vapour coexist in equilibrium (273.16Kelvin).

Zeroth law of Thermodynamics

• The use of two fixed points was found unsatisfactory and later abandoned, – (a) the difficulty of achieving equilibrium between

pure ice and air- saturated water (since when ice melts, it surrounds itself only with pure water and prevents intimate contact with air-saturated water).

– (b) extreme sensitiveness of the steam point to the change in pressure

Zeroth law of Thermodynamics

• The temperature of the triple point of water, which is an easily reproducible state, is now the standard fixed point of thermometry.

Considerable difference in temperature measurement between thermometers, the smallest difference observed in different gas thermometers. So gas is chosen as standard thermometric substance.

Zeroth law of Thermodynamics

Zeroth law of Thermodynamics

Questions

1. Fixed point ? An easily reproducible state of an arbitrarily chosen standard system is called a fixed point. Before 1954, there were two fixed points: (a) the ice point (b) the steam point • Method in Use After 1954 Triple point of water is the only one fixed point used after 1954.

Questions

2. Why gas is chosen as standard thermometic substance? There is considerable difference in temperature measurement between thermometers, the smallest difference observed in different gas thermometers. So gas is chosen as standard thermometric substance. 3. Celsius Temperature Scale?

푡 = 푇 − 273.15표

4. Applications of zeroth law of thermodynamics? temperature measurement.

Reference • Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett

Publishers, Sudbury, Mass. • Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New

Delhi. • http://www.grc.nasa.gov/WWW/BGH/thermo0.html