Thermometry Fundamentals: Temperature Measurement & Heat Transfer

Thermometry: Measuring Temperature

Learn the different procedures to determine or measure the temperature of bodies.

Understanding Thermometers

A thermometer is an instrument or device used for the measurement or determination of temperature. Their operation is based on the variations in the dimensions of a body with temperature. The dilation should be significant enough to provide a measurable indication of temperature.

Thermometer Principles

It is feasible to measure temperature increases by noting the volume changes experienced by a reference substance.

Expansion Thermometers

An expansion thermometer is independent of the substance used (solid, liquid, or gaseous). Like any measuring instrument, it must have a suitably graduated scale.

Mercury's Properties in Thermometers

Mercury possesses several properties that make it suitable for thermometers:

• It is highly visible in a capillary tube due to its metallic luster and characteristic color.
• It is a liquid that does not wet the walls of a container.
• It is an excellent conductor of heat.
• It can be obtained from nature very easily with a high degree of purity.

Mercury Thermometer Components

A mercury thermometer is composed of a glass bulb containing mercury, which extends into a capillary tube, also made of glass. The mercury moves within this tube, which has a graduated scale. In the case of precision thermometers, the scale is etched directly onto the glass tube.

Thermometric Scales

Celsius Scale

The Celsius scale (formerly Centigrade) is defined by two fixed points:

• Superior Point: Corresponding to the boiling point of distilled water at normal atmospheric pressure, defined as 100 degrees Celsius (100°C).
• Inferior Point: Corresponding to the solidification or fusion point of distilled water at normal atmospheric pressure, defined as 0 degrees Celsius (0°C).

Fahrenheit Scale

The Fahrenheit scale is also defined by two fixed points:

• Lower Point: The solidification point of a mixture of salt and ice in certain proportions, defined as 0 degrees Fahrenheit (0°F).
• Superior Point: The boiling point of distilled water at normal atmospheric pressure, defined as 212 degrees Fahrenheit (212°F).

On the Fahrenheit scale, the range between the freezing and boiling points of water (32°F to 212°F) encompasses 180 degrees. A temperature difference of 100 degrees Celsius is equivalent to a temperature difference of 180 degrees Fahrenheit.

Thermal Expansion in Solids

The atoms that make up most solids are distributed in a regular and orderly manner, forming what is called a crystal lattice. They are held together by electric forces, resembling very small and rigid 'springs'. When the temperature increases, the average distance between the atoms also increases, leading to the dilation (expansion) of the entire solid body in all directions.

Calorimetry: Heat Measurement

Calorimetry aims to measure the amount of heat resulting from various processes and to determine specific heat capacities.

What is Heat?

Heat is a form of energy transferred between systems due to a temperature difference. It can be generated from chemical reactions, nuclear processes, or electromagnetic interactions.

What is Temperature?

Temperature is a physical quantity that expresses the degree of hotness or coldness of a substance. It is related to the average kinetic energy of the particles within a system.

The temperature a substance reaches depends on the amount of heat supplied and the duration of heating. For instance, if you heat one liter, two liters, or three liters of water, a greater amount of heat and time will be required to reach the same temperature for larger volumes. Similarly, if you heat a container of water and another container with the same amount of a different substance (like mercury), you will observe that mercury heats up more quickly in the same time interval, indicating a higher temperature increase.

Calorie and Specific Heat

The Calorie Unit Defined

A calorie is the amount of heat required to raise the temperature of one gram of water by one degree Celsius.

Specific Heat Capacity Explained

Specific heat capacity is the amount of heat absorbed by one gram of a substance to vary its temperature by one degree Celsius. The specific heat of substances allows us to distinguish between good and poor heat conductors.