Class 7 Science Chapter 7: Heat Transfer in Nature Notes

Why some places feel warmer

The chapter begins in Gangtok, where Pema and her brother Palden sit by a fire on a winter evening. Palden has just visited Kerala, which felt warm and humid even in winter. Kerala is closer to the equator and has a long coastline, and the Sun is the main source of heat for the Earth. Watching their grandmother cook thukpa in a metal pan, they wonder why cooking pans are made of metal. The answers lie in the ways heat travels.

Conduction

When one end of a metal strip with wax-stuck pins is heated (Activity 7.1), the pins drop one by one, the nearest first. Heat moves along the strip from the hot end to the cold end: each particle passes heat to its neighbour, while the particles stay in place. This is conduction, the main way heat moves in solids.

Materials that let heat pass easily, such as metals, are good conductors, which is why we cook in metal pans. Wood, glass, clay and porcelain are poor conductors or insulators, so tea stays hot longer in a clay or porcelain cup. Air is a poor conductor too. Woollen clothes trap air in their pores, two thin blankets hold a layer of air between them, and some houses keep out heat and cold with hollow bricks or with walls of two wooden layers filled with mud and cow dung, as in the Mori block of Uttarkashi.

Convection

A paper cup hung over a candle rises (Activity 7.2), because the air inside warms, expands, becomes lighter and goes up. Smoke rises for the same reason. In water heated from below (Activity 7.3), a coloured streak rises in the middle and sinks at the sides: warm water rises and cooler water takes its place. Heat transfer by the actual movement of the particles of a liquid or gas is convection. Read more in conduction, convection and radiation.

Sea and land breezes

Soil heats up faster than water in sunlight, and also cools faster (Activity 7.4). So by day, warm air over the land rises and cooler air moves in from the sea: a sea breeze. At night the land cools first, warm air over the sea rises, and a land breeze blows from the land to the sea. Read more in land and sea breezes.

Radiation

The warmth we feel from a fire, and the Sun’s heat on Earth, reach us by radiation, which needs no medium. All objects radiate heat, which is why a hot utensil cools on its own. Light-coloured clothes reflect most of the heat falling on them, so they suit summer; dark clothes absorb more heat and suit winter. When water is heated in a pan, all three happen together: the pan heats by conduction, the water by convection, and we feel warmth by radiation. The bukhari, an iron stove used in the upper Himalayas, both cooks and warms a room using all three.

The water cycle and groundwater

The Sun’s heat evaporates water from oceans, rivers and lakes, and plants give off vapour by transpiration. Rising vapour cools and condenses into clouds, which bring rain, snow and hail: precipitation. This water cycle redistributes water and keeps the total amount on Earth the same. In the 6th century CE, Varahamihira of Ujjaini described ways to predict seasonal rainfall in his Brihatsamhita.

Some rainwater seeps into the ground: fastest through gravel, slower through sand and slowest through clay (Activity 7.5). This infiltration stores groundwater in aquifers, which wells and bore wells tap. Groundwater is being used up faster than it is refilled, so rainwater harvesting, recharge pits and Ladakh’s ice stupas help to save water. Read more in water cycle and groundwater.

Answers to every question are in the NCERT solutions for Chapter 7.