Conduction of Heat
Class 7 Science · Chapter 7: Heat Transfer in Nature · NCERT Curiosity section 7.1
Learn
On a chilly evening in the hills, Zoya and Neel sit by their aunt's stove while she stirs soup in a big steel pot. Neel touches the steel ladle that has been resting in the pot and quickly lets go: "Ouch, the handle is hot!" The wooden spoon beside it is cool. "How did the heat climb up the ladle?" asks Zoya.
They try a test: stick four pins with wax along a metal strip, about 2 cm apart, and heat one end. The pin nearest the flame falls first, then the next, and so on. Heat travels from the hotter part to the colder part: each particle passes heat to its neighbour without leaving its place. This is conduction, the main way heat moves in solids.
Metals are good conductors of heat, so pots and tawas are metal. Wood, plastic, glass, clay and porcelain are poor conductors (insulators): handles stay cool and tea in a clay cup stays hot longer. Air is a poor conductor too. Woollen clothes trap air in their fibres, and two thin blankets with air between them keep us warmer than one thick one.
Builders use this idea. In very cold mountain villages, walls may be two layers of wood filled with mud and dung, both poor conductors, so heat stays inside. Hollow bricks trap air and keep homes cooler in summer and warmer in winter.
Explore: which materials conduct heat?
- Test each object and sort it.
Remember
- Conduction: heat passes from particle to particle in a solid, from hot to cold, without the particles moving away.
- Metals are good conductors; wood, plastic, clay, glass, wool and air are poor conductors.
Quick check
Why does tea stay hot longer in a clay cup than in a steel tumbler?
- clay is a poor conductor of heat
- clay is a good conductor
- steel makes tea cold
- clay adds heat
Answer
clay is a poor conductor of heat. Less heat escapes through an insulator.
Think about it
Birds fluff up their feathers on cold mornings. How does this help them?
One answer
Fluffed feathers trap more air, and air is a poor conductor of heat, so less body heat escapes.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.