Temperature Scales (Copy)

Measuring Temperature with Physical Properties

• Temperature is a physical quantity that indicates the thermal state of a body — whether it will gain or lose heat when in contact with another body.
• Since temperature cannot be measured directly, we rely on temperature-dependent physical properties.

A suitable physical property for temperature measurement must:

1. Vary continuously with temperature
2. Be measurable and reproducible
3. Be independent of other unrelated variables (as much as possible)

Examples of Physical Properties Used for Thermometry

• These properties are used to calibrate and construct thermometers, such as:
  • Mercury-in-glass thermometer
  • Thermistor (temperature-sensitive resistor)
  • Thermocouple thermometer

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

Thermodynamic Temperature Scale

• The thermodynamic (Kelvin) temperature scale is based on fundamental physical laws (e.g. ideal gas laws) and not on the properties of any specific substance.

Key Properties:

• Independent of the physical properties of materials
• Defined using absolute zero and the triple point of water
• Used in scientific thermodynamics and gas law calculations
• Thermodynamic temperature (T) is measured in kelvin (K)
• Absolute zero = 0 K is the lowest possible temperature, where:
  • Molecular motion theoretically stops
  • Internal energy is at its minimum

Conversion Between Celsius and Kelvin

• The Celsius scale (°C) is based on:
  • 0 °C = freezing point of water
  • 100 °C = boiling point of water (at 1 atm)
• The Kelvin scale (K) is an absolute scale:
  • Begins at absolute zero
  • One degree on Kelvin scale is equal in size to one degree Celsius

Conversion Formula:

T/K = θ/°C + 273.15

• Examples:
  • 0 °C = 273.15 K
  • 100 °C = 373.15 K
  • –273.15 °C = 0 K (absolute zero)

Understanding Absolute Zero (0 K)

• Absolute zero is the zero point of the thermodynamic temperature scale.
• At absolute zero:
  • Particles have minimum internal energy
  • Molecular motion approaches zero
  • It is not attainable in practice, but is a theoretical limit
• Absolute zero provides:
  • The baseline for the Kelvin scale
  • A reference for calculating thermal energy, entropy, and ideal gas behaviour

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

Comparison Between Celsius and Kelvin Scales

Thermometers and Their Calibration

• Primary thermometers are based on physical laws and do not require calibration (e.g. constant volume gas thermometer)
• Secondary thermometers require calibration using fixed reference points such as:
  • Ice point (0 °C)
  • Steam point (100 °C)
  • Triple point of water (273.16 K)
• Modern thermometers often use:
  • Electronic sensors (e.g. thermistors)
  • Infrared detectors for non-contact temperature measurement

Practical Uses of Kelvin Scale

• Ideal Gas Law calculations: PV = nRT → T must be in Kelvin
• Thermodynamics and entropy calculations
• Astrophysics: measuring cosmic background temperature (~2.73 K)
• Low temperature physics: study of superconductivity, Bose–Einstein condensation

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course

Common Questions and Clarifications

Q1: Why can’t temperature be measured directly?
Because it is not a physical substance — we measure properties (like resistance or expansion) that change with temperature.

Q2: Why is Kelvin used in science instead of Celsius?
Because Kelvin is absolute and avoids negative temperatures, making it consistent with thermodynamic laws.

Q3: Can something have a temperature below 0 K?
No. 0 K is the lowest theoretical temperature possible. Temperatures below that are not physically meaningful.

Summary Table

Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change A2 Level Physics Full Scale Course