Chapter 7: Heat Transfer in Nature

Heat always moves from a hotter place to a colder place. In solids, heat mostly travels by conduction. When one end of a solid is heated, its particles vibrate faster and pass the heat on to the neighbouring particles, which pass it on to the next ones, and so on. The particles themselves do not travel along the solid; only the heat moves. Conduction goes on until all parts of the object are at the same temperature.

Materials that allow heat to pass through them easily are good conductors of heat. Most metals such as copper, aluminium and iron are good conductors. Materials that do not allow heat to pass easily are poor conductors or insulators, such as wood, plastic, glass, cloth, rubber and air. Cooking vessels are made of metals because they heat the food quickly, while their handles are made of wood or plastic so that we can hold them. Woollen clothes keep us warm because they trap air, which is a poor conductor. Houses built with hollow bricks, thick mud walls or thatched roofs stay cooler in summer and warmer in winter because trapped air and mud do not let heat pass easily.

In liquids and gases, heat travels mainly by convection. When a gas such as air is heated, it expands, becomes lighter and rises. Cooler, heavier air moves in to take its place. This cooler air is then heated, rises and the cycle continues. This circulation of air carries heat from one place to another. In convection, the particles themselves move from place to place and carry the heat with them. This is the main difference between convection and conduction. Smoke rises because it is mixed with hot gases that are lighter than the surrounding air. A chimney, a ventilator near the roof and a hot-air balloon all use this idea.

Liquids also transfer heat by convection. When water at the bottom of a vessel is heated, it expands, becomes lighter and rises. The cooler, heavier water from the top and sides sinks to take its place. This water is then heated and rises. The cycle repeats and the whole liquid gets heated by the actual movement of the water. If a small crystal of a coloured substance such as potassium permanganate is placed at the bottom of a beaker and the beaker is heated below its centre, the coloured streak rises in the middle and moves down along the sides. This shows the convection current in water. Heating from below is more effective than heating from the top.

Land (soil, sand and rocks) and water do not heat and cool at the same rate. In sunshine, land heats up faster than water, and at night land cools faster than water. During the day, the air above the land gets warmer, becomes lighter and rises. Cooler air from above the sea moves in to take its place. This wind blowing from the sea to the land is the sea breeze. At night the land cools faster, so the air above the sea is warmer and rises, and cooler air from the land moves toward the sea. This is the land breeze. These breezes are examples of convection on a large scale. Because of them, coastal places have moderate temperatures, and fishers use the breezes for sailing.

Radiation is the transfer of heat without the need for any material medium. It is how heat reaches us from the Sun through space, and how we feel warmth from a fire or a hot object kept at a distance. All objects give out (radiate) heat, and hotter objects radiate more. A hot vessel kept away from the flame slowly cools by radiating heat to its surroundings. Surfaces that are dark and dull absorb more radiated heat, and light, shiny surfaces reflect more. That is why we prefer light-coloured clothes in summer and dark clothes in winter. Conduction and convection need a medium; radiation does not.

The three ways in which heat moves are conduction, convection and radiation. In conduction, heat passes from particle to particle in contact and the particles stay in place. In convection, heat is carried by moving particles of liquids and gases. In radiation, heat travels without any medium. Many daily events involve more than one process. For heating water on a stove, the flame heats the base of the pan by conduction, the water heats by convection currents, and the warmth we feel around the flame and pan is by radiation. Human beings use these ideas to design clothes, houses, cooking vessels, flasks and many devices. To decide which process is acting, ask: Is there contact? Do particles move? Is a medium absent?

The Sun's heat makes water from oceans, lakes, rivers, soil and plants change to water vapour by evaporation. The vapour rises, cools and condenses into tiny droplets that form clouds. These fall as rain, snow or hail, called precipitation. The continuous movement of water between the surface, the sky and back is the water cycle. Some of the rainwater flows over the ground into rivers and the sea, and some seeps into the soil. The seeping of water through soil and rocks is called infiltration. Water moves fastest through gravel, slower through sand and slowest through clay. Water that gets stored in the spaces between soil particles and in rocks is groundwater. The underground layers that store and give out water are aquifers, from which we get water through wells, hand pumps and bore wells. Overuse of groundwater and concrete surfaces that stop infiltration lower the groundwater level, so we use rainwater harvesting and recharge pits to refill it.