Convection and Breezes

Class 7 Science · Chapter 7: Heat Transfer in Nature · NCERT Curiosity section 7.2

Learn

Smoke from the stove curls upwards. To find out why, Neel hangs two paper cups upside down from the ends of a balanced stick and holds a candle under one, at a safe distance. That cup rises! The air inside warms, expands, becomes lighter and goes up, just as a balloon left in the sun swells.

With a teacher, they drop one grain of a purple crystal at the bottom of a beaker of water and heat it from below. A coloured streak rises up the middle and comes down the sides. Warm water rises, cooler heavier water sinks to take its place, and the cycle repeats. Heat carried by the actual movement of particles is convection: it is how liquids and gases are heated.

At the beach, Zoya measures bowls of soil and water in the sun every 5 minutes. Soil heats up faster than water, and indoors it cools faster too. By day, air over the hot land rises and cool air flows in from the sea: a sea breeze. At night the land is cooler, warm air over the sea rises, and the breeze blows from land to sea: a land breeze.

Explore: what moves the heat?

1. Sort each one.

Remember

• Convection: heat moves by the actual movement of particles in liquids and gases; warm fluid rises, cool fluid sinks.
• Land heats and cools faster than water, causing sea breezes by day and land breezes by night.

Quick check

A sea breeze blows during the day because …

1. land heats faster, so warm air over it rises and cool sea air moves in
2. the sea heats faster
3. the Sun pushes the air
4. there is no convection by day

Think about it

Why are room heaters placed low and air conditioners high on a wall?

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.