Heat Transfer in Nature – Class 7 Science Chapter 7 Notes & NCERT PDF

Introduction

Pema and her brother Palden live in Gangtok. Sitting by a fireplace on a cold winter evening, Palden describes his winter trip to Kerala, which was warm and humid. Their grandfather, a retired science teacher, explains that Kerala is closer to the equator and has a long coastline. Watching her grandmother cook thukpa in a metal pan, Pema asks why utensils are made of metal. This chapter explains how heat travels — by conduction, convection and radiation — and how the Sun's heat drives the water cycle.

Key Concepts

7.1 Conduction

In Activity 7.1, four pins are stuck with wax about 2 cm apart on a 15 cm metal strip (aluminium or iron). When one end is heated, pin I (closest to the flame) falls first, then II, III and IV. Heat moves along the strip, melting the wax.

Conduction: transfer of heat from the hotter part of an object to the colder part. Each heated particle passes heat to its neighbour, but the particles do not move from their positions.

7.2 Convection

Pema asks why smoke rises from the firewood. In Activity 7.2, two paper cups hang upside down from a balanced stick; the cup above a candle rises. Air near the flame heats up, expands, becomes lighter and rises. A partly inflated balloon kept in the Sun also grows as its air expands. Smoke from an agarbatti rises because it is warmer than the air around it.

In Activity 7.3, a grain of potassium permanganate is placed at the centre of a half-filled 500 mL beaker of water heated from below. A coloured streak rises in the middle and comes down the sides — hot water rises, cooler heavier water sinks to take its place, until all the water is heated.

Convection: heat transfer by the actual movement of particles of liquids and gases from one place to another.

#### Land and sea breeze

In Activity 7.4, bowls of soil and water in sunlight show that soil heats up faster than water, and it also cools faster.

7.3 Radiation

The warmth Pema and Palden feel from the fire reaches them directly by radiation. The Sun's heat reaches Earth this way.

Radiation: heat transfer that does not require any medium. All objects radiate heat — a hot utensil kept away from the flame cools by radiating heat.

All three together: heating water in a pan — flame to pan by conduction, water heats by convection, and the warmth felt near the flame is radiation. The bukhari, an iron stove used in upper Himalayan regions for warmth and cooking, uses all three.

Summary of the three ways:

7.4 Water cycle

Heat from the Sun melts some snow and ice in summer, and evaporates water from oceans, rivers and lakes; plants release water by transpiration. Water vapour rises, cools and condenses into clouds, which bring rain, snow and hail — precipitation.

Water cycle: the continuous movement of water — upward as water vapour and downward through precipitation, passing through soil, rocks and plants, and returning to water bodies.

The water cycle redistributes and replenishes water and conserves the total water on Earth. Varahamihira (6th century CE, Ujjaini) gave methods in Brihatsamhita for predicting seasonal rainfall from clouds, winds, stars and the moon.

#### Seepage of water beneath the Earth

In Activity 7.5, 200 mL of water is poured through clay, sand and gravel in cut bottles. Water seeps fastest through gravel, slower through sand and slowest through clay, because the spaces between gravel particles are widest.

Groundwater is not unlimited: over-extraction, less vegetation and more concrete reduce infiltration. Rainwater harvesting and recharge pits help. In Ladakh, ice stupas — cone-shaped ice towers made by spraying stream water into freezing air in winter — melt slowly in spring to supply water.

Activities in the Chapter

Key Terms

Real-life Connections

Common Misconceptions

Keep the Curiosity Alive (exercise hints)