The Zeroth Law of Thermodynamics

Ch 1, Lesson E, Page 9 - The Zeroth Law of Thermodynamics

• The key to quantifying temperature is the Zeroth Law of Thermodynamics.
• This Law lets us calibrate temperature measuring devices, such as thermometers by bringing them into thermal equilibrium with systems that are at very well-known temperatures.
• Examples of such systems include water at its normal boiling point, melting point or the triple point.
• Ok, now let’s talk about temperature measuring devices.
• A thermometer is the temperature measuring device with which you are most familiar, right ?
• The liquid in the thermometer is brought into thermal equilibrium with the system of interest.
• The liquid expands as it is heated and contracts as it is cooled.
• When a fluid expands, its volume, molar volume and specific volume increase.
• As a result, the column of liquid pushes up farther into the shaft in the center of the thermometer.
• The glass on the thermometer has a scale.
• This scale was created by calibrating the thermometer with systems at well-known temperatures.
• You may not be quite as familiar with the operation of thermocouples.
• The key to the operation of a thermocouple is the fact that a junction potential develops when two different metals are brought into contact with each other.
• The size of the contact potential depends on which metals are used and on the temperature of the metals.
• The junction potentials are rather small, so this voltage must be multiplied before it is measured and recorded electronically.
• A thermocouple must be calibrated in the same way that a thermometer is calibrated.
• Then, the voltage from the junction when it is at an unknown temperature can be translated into the temperature of the junction using the calibration equation.