Temperature and Its Measurement

Temperature is a physical property of matter that quantitatively expresses the common notions of hotness and coldness. In physics, it is more accurately defined as a measure of the average kinetic energy of the particles in a substance.

1. The Concept of Temperature

While heat is a form of energy (measured in Joules) that flows from a body of higher temperature to one of lower temperature, temperature is the state that determines the direction of that flow. The SI unit of temperature is the Kelvin (K).

2. Thermometric Properties

A thermometric property is any physical property of a substance that changes continuously and linearly with changes in temperature. Common properties include:

• Volume of a liquid: Used in mercury-in-glass or alcohol-in-glass thermometers.
• Resistance of a metal wire: Used in platinum resistance thermometers.
• Gas pressure at constant volume: Used in constant-volume gas thermometers.
• Electromotive force (e.m.f): Used in thermocouples (two different metals joined).
• Pressure of a gas at constant volume: Used in gas thermometers.

3. Calibration of Thermometers

To calibrate a thermometer, two fixed points are required:

• Lower Fixed Point (Ice Point): The temperature of pure melting ice at standard atmospheric pressure ($0^\circ C$).
• Upper Fixed Point (Steam Point): The temperature of pure boiling water at standard atmospheric pressure ($100^\circ C$).

The fundamental interval is the difference between these two points.

4. Temperature Scales and Conversions

The two primary scales required for JAMB are Celsius and Kelvin. The relationship is given by:

T(K) = θ(°C) + 273.15 (For JAMB calculations, 273 is usually sufficient).

To find an unknown temperature θ using a thermometric property X:

θ = [(Xθ - X0) / (X100 - X0)] × 100°C

5. Types of Thermometers

• Liquid-in-glass: Uses mercury (high boiling point, doesn't wet glass) or alcohol (low freezing point).
• Clinical: A mercury-in-glass thermometer with a constriction to prevent backflow, used for body temperature.
• Thermocouple: Measures high temperatures and rapidly changing temperatures due to low heat capacity.
• Resistance: Very accurate, used for a wide range of temperatures.
• Gas: Used as a standard for calibrating other thermometers.