NCERT Class 7 Science Curiosity Chapter 07 Heat Transfer in Nature PDF Download

Chapter 7: Heat Transfer in Nature

Pema and her brother Palden reside in Gangtok. On a cold winter evening, they are sitting around a fireplace. Palden shares his experiences of visiting Kerala during the winter vacation. He says that compared to Gangtok, winter in Kerala is comparatively warm and humid. Both Pema and Palden are curious about why some places are so cold and others quite hot.

Hearing them express their curiosity, their grandfather, a retired science teacher, says, "Kerala is closer to the equator than Sikkim and it also has a long coastline, which results in warmer and more humid weather conditions". Palden replies, "Yes, we learnt in Grade 6 Science and Social Science that for us on the Earth, the Sun is the main source of heat and light, and around the equator, the climate is generally hot".

As they are talking, Pema is keenly observing her grandmother cooking thukpa (a traditional Sikkimese dish) in a large metal pan. Pema asks, "Why are cooking utensils generally made of metals?" Palden immediately responds that they had studied in the chapter 'The World of Metals and Non-metals' that such materials are good conductors of heat.

[Figure: Pema, Palden, grandmother and grandfather around a fireplace in Gangtok, See in your textbook]

How does heat get transferred in these materials?

Let us perform an activity to learn why certain materials are good conductors of heat.

7.1 Conduction of Heat

Activity 7.1: Let us experiment

Caution - This activity should be carried out under the supervision of a teacher or an adult.

• Take a strip of a metal, such as aluminium or iron, about 15 cm long.
• Attach four pins to the strip with the help of wax such that they are arranged at nearly equal distances (about 2 cm apart), as shown in Fig. 7.1.
• Secure the strip to a stand and label the pins as I, II, III, and IV, as shown in Fig. 7.1. (If a stand is not available, place the strip between two bricks for support.)
• Heat the end of the strip that is away from the stand with a candle or a spirit lamp.
• What will happen to the pins? Will they remain attached to the strip or will they fall?
• Predict the order in which the pins will fall from the strip.
• Record your observations in Table 7.1.

[Figure 7.1: Heat transfer in a metal strip, See in your textbook]

Teacher's Note

When you do this experiment, pin I always falls first because heat travels along the metal from where the candle heats it. The pins fall in order (I, II, III, IV) because each one is farther from the heat source. If all pins fell at the same time, heat would not be travelling through the metal - but it always does, so they cannot fall together.

You observed that the pin closest to the candle flame (pin I) falls first, followed by pins II, III, and IV. Why does pin I fall before pin II? Why did all the pins not fall together?

From your observations, what can you infer? Do you think that heat is being transferred along the metal strip from the end that is being heated? As the heat travels along the strip and approaches a pin, the wax holding it melts and the pin falls. Here, the transfer of heat takes place from the hot end of the strip to the colder end. The process of heat transfer from the hotter part of an object to the colder part is called conduction. In this process, the particle that gets heated, passes the heat on to its neighbour, and so on. However, the particles themselves do not move from their positions.

Materials like metals that allow heat to pass through them easily are called good conductors of heat. Because metals are good conductors of heat, we use utensils made of metals for cooking. In solids, heat transfer takes place mainly through the process of conduction.

If we use a strip made of a material like wood or glass in place of a metal strip to perform Activity 7.1, the pins will not fall. Can you think of the reason for this based on our learning from the chapter 'The World of Metals and Non-Metals'?

Materials such as glass and wood do not allow heat to pass through them easily and are poor conductors (insulators) of heat. Clay and porcelain are also poor conductors of heat - that is why tea or coffee kept in such cups stays hot longer.

Teacher's Note

Air is a poor conductor of heat - this is why woollen clothes keep you warm in winter. The wool traps air between its fibres, and this trapped air acts as an insulator, not letting your body heat escape easily. Remember: the material itself does not produce heat; it just stops heat from moving away.

List some materials around you and classify them as good or poor conductors of heat in Table 7.2.

Does your list include air? If it is there on the list, where have you placed it?

You must have experienced that during winters, we prefer wearing woollen clothes to keep ourselves warm.

Key Points

• Heat flows from a hotter part of an object to a colder part through a process called conduction, where particles pass heat to their neighbours without moving themselves.
• Good conductors of heat like metals allow heat to pass through them easily, which is why cooking utensils are made from metal.
• Poor conductors of heat, called insulators, such as wood, glass, clay and wool, do not allow heat to pass through easily and are used to keep things warm or cold.
• Air is a poor conductor of heat, which is why woollen clothes trap air and keep you warm in winter.

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