Heat is a form of energy that makes things hot. We feel heat all around us - from the sun, from burning fire, from our own bodies and even from hot food. In this chapter we learn what heat is, how it is measured, how it moves from one place to another and how we can control its movement. A hot object and a cold object differ in their temperature, and temperature tells us how hot or cold an object is.
The concept of heat and temperature is closely connected but not the same. Temperature is a measure of the degree of hotness or coldness of a body, while heat is the energy that flows from a hotter body to a colder body. When we touch a hot object, heat flows from the object to our hand and we feel it hot; when we touch a cold object, heat flows from our hand to the object and we feel it cold. So our sense of touch is not always reliable, which is why we need instruments to measure temperature accurately.
To measure temperature accurately, we use a thermometer. The clinical thermometer is used to measure the temperature of the human body. It has a bulb at one end containing mercury, a narrow glass tube attached to it, and a scale marked from 35°C to 42°C. When the bulb touches the body, heat from the body makes the mercury expand and rise in the tube, and the level of mercury gives the temperature reading.
The normal temperature of the human body is about 37°C. A clinical thermometer has a kink (a slight bend) in the tube just above the bulb. This kink prevents the mercury from flowing back into the bulb when the thermometer is taken out of the mouth, so the reading stays constant. Before using a clinical thermometer, we must shake it well so that the mercury level falls below the normal mark. It must be washed with an antiseptic before and after use, and handled carefully so that it does not break. The laboratory thermometer, on the other hand, has a wider range of temperatures, from -10°C to 110°C, and is used for measuring temperatures of liquids and other substances in experiments. It does not have a kink and must never be used to measure the temperature of the human body.
Heat always flows from a hotter body to a colder body. There are three ways in which heat is transferred: conduction, convection and radiation. In conduction, heat is transferred from the hotter part to the colder part of a solid without the actual movement of the particles. A metal rod held over a flame gets hot at the other end because heat travels through it by conduction. Metals are good conductors of heat, while wood, plastic, glass and air are poor conductors (insulators). That is why cooking utensils are made of metals but have handles made of wood or plastic.
In convection, heat is transferred in liquids and gases by the actual movement of the heated particles. When water is heated in a vessel, the hot water at the bottom becomes lighter and rises, while the cooler, heavier water from the top sinks down. This sets up convection currents, and this is how the whole liquid gets heated. Sea breezes and land breezes are formed due to convection currents in air, and warm and cold air currents are also the result of convection. Radiation is the transfer of heat that does not require any medium; it can travel even through vacuum. The sun's heat reaches the Earth through radiation, because there is no medium between the sun and the Earth. Dark surfaces absorb more heat and radiate more heat than light-coloured surfaces, which is why people in hot areas often wear white clothes and paint their houses white.
Materials that allow heat to pass through them easily are called conductors of heat. All metals are good conductors; among them, silver is the best, followed by copper, aluminium and iron. Because metals conduct heat well, they are used for making cooking vessels, radiators and the bottoms of utensils. Materials that do not allow heat to pass through them easily are called insulators or poor conductors. Wood, plastic, rubber, wool and air are insulators.
We use insulators in our daily life in many ways. The handles of kettles and frying pans are made of plastic or wood so that heat does not reach our hands. Woollen clothes keep us warm in winter because air trapped in wool is a poor conductor and does not let our body heat escape. We use wooden or plastic mats in the kitchen to keep hot vessels, and insulated cups and lunch boxes are made using materials that do not conduct heat. Understanding conductors and insulators also helps in designing better utensils, buildings and even spacecraft.
Heat is essential for life. We cook our food using heat, we dry our clothes in the sun, and our body maintains a constant temperature by producing and losing heat. Heat is also used in industries, in power generation and in many household appliances. On the other hand, too much heat can be uncomfortable and even dangerous, so we need to control heat transfer.
This is why we design clothes, houses and utensils keeping heat in mind. In winter we wear dark, woollen clothes that absorb heat and trap it, while in summer we wear light-coloured cotton clothes that reflect heat and absorb sweat. The same principles apply to animals and their habitats; animals in cold regions have thick fur and blubber to prevent heat loss. Thus, understanding the science of heat helps us in daily living and in protecting ourselves from extreme weather.
| Mode of Transfer | Medium Required | Example |
|---|---|---|
| Conduction | Solids (no particle movement) | Metal rod heated at one end gets hot at the other |
| Convection | Liquids and gases (particles move) | Sea breeze, boiling water, land breeze |
| Radiation | No medium needed (vacuum possible) | Sun's heat reaching the Earth |
| Thermometer | Range | Kink? | Use |
|---|---|---|---|
| Clinical | 35°C to 42°C | Yes | Human body temperature |
| Laboratory | -10°C to 110°C | No | Experimental liquids |
Heat is an essential form of energy that affects almost everything we do. Temperature, measured by thermometers, tells us how hot or cold an object is, and our sense of touch is not always reliable. Heat is transferred by conduction in solids, by convection in liquids and gases, and by radiation even through a vacuum. Metals are good conductors while wood, plastic and air are insulators, and this knowledge is used in designing utensils, clothing and houses. Understanding heat helps us stay comfortable in different seasons and explains many natural phenomena like breezes and the warmth of the sun.