In the International System of Units (SI), what is the base unit of temperature?

Introduction / Context:
The International System of Units (SI) defines base units for fundamental physical quantities such as length, mass, time, and temperature. Knowing the correct SI unit for temperature is important in physics, chemistry, engineering, and many scientific calculations involving thermodynamic equations and gas laws. Everyday scales like Celsius and Fahrenheit are useful in daily life, but the SI base unit is specific and must be used in scientific work.

Given Data / Assumptions:
• We are asked specifically for the SI base unit of temperature. • Options include kelvin, joule, Celsius, Fahrenheit, and calorie. • We assume standard SI definitions adopted in science and engineering.

Concept / Approach:
In SI, the base unit of thermodynamic temperature is the kelvin (symbol K). The Celsius scale is related to the kelvin by a simple offset: temperature in kelvin = temperature in degrees Celsius + 273.15. Fahrenheit is an older scale mainly used in some countries for weather and cooking. Joule is the SI unit of energy, and calorie is a non SI unit of energy often used in nutrition. None of these except kelvin is a base unit of temperature in SI.

Step-by-Step Solution:
Step 1: Recall that SI uses seven base units, one of which is for thermodynamic temperature. Step 2: Identify that this base unit is kelvin, with symbol K. Step 3: Recognise that degrees Celsius and degrees Fahrenheit are temperature scales, but not SI base units. Step 4: Note that joule and calorie measure energy, not temperature directly. Step 5: Therefore, the correct SI unit of temperature is kelvin.

Verification / Alternative check:
Gas law equations such as PV = n * R * T use absolute temperature in kelvin. The Stefan Boltzmann law for radiation and many thermodynamic relations also require temperature in kelvin. Scientific references, data tables, and fundamental constants list temperatures in kelvin, reinforcing that kelvin is the standard SI unit. Celsius is often used for weather reports and laboratory readings, but those values are converted to kelvin for theoretical calculations.

Why Other Options Are Wrong:
Option b (Joule): This is the SI unit of energy or work, not temperature. Option c (Celsius): Degree Celsius is a commonly used temperature scale but not the SI base unit; it is derived from the kelvin scale. Option d (Fahrenheit): Degree Fahrenheit is an older temperature scale used mainly in a few countries, not part of SI. Option e (Calorie): This is an energy unit often used in nutrition and heat calculations, not a temperature unit.

Common Pitfalls:
Students sometimes answer Celsius because it is familiar from daily life and laboratory thermometers. To avoid confusion, remember that SI base units are defined for fundamental quantities and appear in most theoretical formulas. Whenever you see T in gas law equations, it is assumed to be in kelvin, not in degrees Celsius or Fahrenheit.

Final Answer:
The SI base unit of temperature is the Kelvin.