Question
Why do we wear woollen clothes in winter? Explain your answer using the concept of conduction of heat.
Solution — Step by Step
Conduction is the transfer of heat through a material without the bulk movement of the material itself. Heat flows from a region of higher temperature to a region of lower temperature through the material.
Different materials conduct heat at different rates. Good conductors (metals like copper, iron) allow heat to flow rapidly. Poor conductors (insulators) resist the flow of heat — examples include wood, plastic, glass, and wool.
The key property is thermal conductivity — the higher the value, the faster the material conducts heat.
Wool fibres trap a large amount of air between them. Air is an excellent insulator — it has very low thermal conductivity. The air pockets in wool are small and stationary, so convection is also minimised.
Because wool + trapped air conducts heat very poorly, it acts as a thermal barrier between your body and the cold surroundings.
Our bodies continuously generate heat through metabolism. In winter, the outside temperature is lower than our body temperature (37°C). Heat tends to flow from our warm body to the cold surroundings.
Wool (with its trapped air) slows down this heat loss. Your body heat stays close to your skin rather than escaping to the surroundings. This is why you feel warm.
Important: Wool does not produce heat — it only reduces the rate at which heat escapes from your body. The warmth you feel is your own body heat being retained.
Cotton fibres are packed more tightly and have less trapped air. Cotton is a relatively better conductor of heat than wool. Heat flows more easily from your body through cotton to the cold air.
That’s why a thin cotton shirt is comfortable in summer (allows heat to escape) but inadequate in winter (can’t retain body heat). Wool is preferred in winter precisely because it is a poor conductor.
Why This Works
The physics is simple: heat always flows from hotter to colder. The rate of flow depends on the thermal conductivity of the material separating the two temperatures. Wool’s effectiveness lies entirely in its ability to trap still air — the fibres themselves create the pockets, but it’s the trapped air that does the real insulating work.
This principle is used in many technologies: double-glazed windows (trapped air between glass panes), thermos flasks (vacuum between walls — no air at all), and building insulation (fibreglass, which traps air like wool does).
Alternative Method
Think of it as a resistance analogy. Just as electrical resistance opposes current flow, thermal resistance (R-value) opposes heat flow. Wool has a high thermal R-value — it presents a large resistance to heat flow. Your body heat is “stuck” on the warm side of this resistance, keeping you comfortable.
Common Mistake
Many students write “wool produces heat to keep us warm.” This is wrong. Wool itself does not generate any heat. It only retains the heat already produced by your body by slowing its escape to the cold environment. If you put a woollen blanket over a cold object, it won’t warm the object — it will just slow the object losing any remaining warmth it has. The source of heat is always your body, not the wool.
This question often appears in CBSE Class 7 science exams. The model answer always has two parts: (1) wool is a poor conductor because it traps air; (2) it slows heat loss from the body, not generates heat. Including both parts earns full marks.