Thermometric properties and temperature scale

To understand heat and temperature in thermal physics, we first need to explore some basic concepts including thermometric properties and temperature scales. These concepts help us measure temperature, which is one of the essential parameters in thermal physics and daily life.

What are the thermometer properties?

Thermometric properties are physical properties of substances that change in a predictable manner with a change in temperature. Using these properties, we can measure temperature accurately. Several thermometric properties are used in designing thermometers:

• Volume or length: In a liquid-in-glass thermometer, the volume of the liquid (usually mercury or alcohol) changes with temperature.
• Electrical resistance: Metals such as platinum have resistance that changes with temperature, which is used in resistance thermometers.
• Voltage: Thermocouples produce voltage when two different metals are connected and there is a difference in their temperatures.
• Pressure: The pressure of a gas at constant volume changes with temperature, which is used in gas thermometers.

Volume or length expansion: A simple example

Imagine a mercury thermometer. Here, the liquid (mercury) expands as the temperature increases and contracts as it cools. Suppose you have a simple glass tube filled with mercury:

    | | |______________|

As the temperature rises, mercury expands:

    | | |______________|  / __________/

This predicted expansion is then calibrated on a scale to accurately measure temperature.

Normal temperature scale

To express temperature, we use different temperature scales. Each scale provides a system of parallels that are numerically different and based on fixed points. The most common temperature scales are Celsius, Fahrenheit, and Kelvin.

Celsius scale

The Celsius scale is widely used around the world, especially in scientific contexts. It is based on the melting point of ice and the boiling point of water at standard atmospheric pressure. On the Celsius scale:

• 0 degrees Celsius (°C) is the melting point of ice.
• 100 degrees Celsius (°C) is the boiling point of water.

The interval between these two points is divided into 100 equal parts, increasing by 1°C for each part. Here is a simple visual representation:

    0°C |-----|-----|-----|---- ... ----|-----|-----| 100°C

Celsius is practical because of its direct relationship to the phase change points of water, making it intuitive in many real-world scenarios.

Fahrenheit scale

The Fahrenheit scale is used primarily in the United States. Its calibration is more complex than Celsius:

• 32 degrees Fahrenheit (°F) is the melting point of ice.
• 212 degrees Fahrenheit (°F) is the boiling point of water.

The interval between these points is divided into 180 equal parts, resulting in each degree on this scale being smaller than that of Celsius. Here's an example:

    32°F |-----|-----|-----|---- ... ----|-----|-----| 212°F

The following formula is used to convert temperature from Celsius to Fahrenheit:

    °F = (°C × 9/5) + 32

And to convert Fahrenheit to Celsius:

    °C = (°F - 32) × 5/9

Kelvin scale

The Kelvin scale is the standard unit of temperature in scientific measurements, because it starts at absolute zero, the lowest temperature theoretically possible where the motion of particles theoretically stops. There are no negative numbers on this scale, which simplifies many scientific calculations.

In this scale:

• 0 Kelvin (K) is absolute zero.
• 273.15 K is the freezing point of water.
• 373.15 K is the boiling point of water.

The formula to convert Celsius to Kelvin is:

    K = °C + 273.15

Kelvin to Celsius conversion is done using the following:

    °C = K - 273.15

Measurement instruments

Different thermometers are designed based on specific thermometric properties, and these instruments are calibrated to meet different scales. Let us check out the commonly used types of thermometers:

Liquid-in-glass thermometer

These thermometers are among the simplest thermometers. The expansion of a liquid, usually mercury or alcohol, in a narrow glass tube corresponds to a temperature change. Because of their simplicity they are most commonly used in both household and field applications.

Gas thermometer

Gas thermometers vary the pressure based on changes in temperature while maintaining a constant volume. They are extremely accurate and are mainly used for precise measurements in laboratories.

Resistance thermometer

Using metals such as platinum, the functionality of an electrical resistance thermometer is based on the change in metal resistance with temperature. Its accuracy makes it popular in industrial applications.

Thermocouples

These temperature sensors are based on the voltage generated by two dissimilar metals connected at one end, relative to the temperature difference at the other end. Due to their wide temperature range and fast response time, they are beneficial in a variety of fields.

Conclusion

Understanding thermometric properties and temperature scales is important for studying heat and temperature in thermal physics. These principles not only help us measure temperature but also connect various scientific concepts, giving us valuable information about temperature-dependent phenomena in nature and industry.

By learning how different materials and devices achieve these measurements, we become better equipped to apply thermal concepts in practical and theoretical contexts.

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