Conduction - Applications of Thermal conductivity

1. Cooking Utensils — Metal Body with Insulating Handles

Cooking utensils are made of metals because metals are good conductors of heat — heat is easily conducted through the base of the utensil to cook food.​

The handles of utensils are made of bad conductors of heat, such as wood or ebonite, so that they cannot conduct heat from the utensil to our hands.

2. Cold Storage Rooms — Thick Walls Using Bad Conductors

Thick walls are used in the construction of cold storage rooms. Brick is a bad conductor of heat, so it reduces the flow of heat from the surroundings into the room.​

Still better heat insulation is obtained by using hollow bricks. Air, being a poorer conductor than brick, further prevents the conduction of heat from outside.

3. Wrapping Ice in a Gunny Bag

Stacked jute gunny bags with drawstrings
To prevent ice from melting, it is wrapped in a gunny bag. A gunny bag is a poor conductor of heat and reduces heat flow from outside to the ice.​

Moreover, the air filled in the interspaces of a gunny bag, being a very bad conductor of heat, further prevents the conduction of heat from outside.

4. Disadvantage — Cracking of Glass Beaker

Low thermal conductivity can also be a disadvantage.

When hot water is poured into a glass beaker, the inner surface of the glass expands on heating. Since glass is a bad conductor of heat, the heat from inside does NOT reach the outer surface quickly. Hence, the outer surface does not expand, thereby causing a crack in the glass.

Problem: The temperature difference between two sides of an iron plate, 2 cm thick, is 10°C. Heat is transmitted through the plate at the rate of 600 kcal per minute per square metre at steady state. Find the thermal conductivity of iron.

Given:

Formula:

From Fourier's Law:

Q = \[\frac{k\cdot A\cdot(T_1-T_2)\cdot t}{x}\]

Rearranging for k:

k = \[\frac{Q}{A\cdot t}\times\frac{x}{T_1-T_2}\]

Substitution:

k = \[10\times\frac{2\times10^{-2}}{10}\]

k = 0.02 kcal m⁻¹ s⁻¹ °C⁻¹

Problem: Calculate the rate of loss of heat through a glass window of area 1000 cm² and thickness of 4 mm, when temperature inside is 27 °C and outside is −5 °C. The coefficient of thermal conductivity of glass is 0.022 cal/s·cm·°C.

Given:

Formula:

\[\frac{Q}{t}=\frac{k\cdot A\cdot(T_1-T_2)}{x}\]

Substitution:

\[\frac{Q}{t}=\frac{0.022\times10^2\times1000\times10^{-4}\times32}{0.4\times10^{-2}}\]

\[{\frac{Q}{t}=1.76\times10^3\mathrm{~cal/s}=1.76\mathrm{~kcal/s}}\]

• Metals are good conductors — used for the base of cooking utensils for fast heat transfer to food.​
• Wood, ebonite, brick, gunny bag, and air are bad conductors — used wherever heat flow must be restricted.
• Air is one of the worst conductors of heat — its presence in hollow bricks and gunny bag fibres enhances insulation significantly.​
• Low thermal conductivity is a disadvantage in glass — it causes cracking due to uneven thermal expansion when heated suddenly.