Question

The thermal conductivity is minimum for ______

• A. Copper
• B. Water
• C. Silver
• D. Air

Hint:

Still air is one of the best passive thermal insulators.
Industrial insulation materials (like fiberglass or expanded polystyrene) work by trapping pockets of air, utilizing its extremely low thermal conductivity (\( \approx 0.026 \text{ W/mK} \)) to prevent heat transfer.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the material with the lowest thermal conductivity among the given options: copper, water, silver, and air.
Thermal conductivity (\( k \)) is a physical property that measures a material's ability to conduct heat.

Step 2: Key Formula or Approach:
The magnitude of thermal conductivity varies significantly across different phases of matter:
\[ k_{\text{solids}} \gt k_{\text{liquids}} \gt k_{\text{gases}} \]
This is because thermal energy is conducted via free electrons and lattice vibrations (phonons) in solids, whereas in gases it occurs through random molecular collisions over relatively large distances.

Step 3: Detailed Explanation:

• Metals (Silver and Copper): Silver and copper are excellent electrical and thermal conductors.
Their high thermal conductivity is due to the abundance of free electrons.
Approximate thermal conductivities:
- Silver: \( k \approx 429 \text{ W/mK} \)
- Copper: \( k \approx 401 \text{ W/mK} \)

• Liquids (Water): Liquids have larger molecular spacing than solids, which limits heat transfer efficiency.
- Water: \( k \approx 0.6 \text{ W/mK} \)

• Gases (Air): Gas molecules are highly dispersed and transfer heat only through occasional collisions.
Consequently, gases are poor conductors of heat and make excellent thermal insulators when stagnant.
- Air: \( k \approx 0.026 \text{ W/mK} \)


Step 4: Final Answer:
Among the given choices, air has the lowest thermal conductivity.