Master Chapter 7 Class 7 - Heat Transfer in Nature (Curiosity) with comprehensive NCERT Solutions, Practice Questions, MCQs, Sample Papers, Case Based Questions, and Video lessons.
Start Learning NowContent is being added. Please visit again soon.
Welcome to Chapter 7. Our journey begins on a cold winter evening in Gangtok. Pema and her brother, Palden, are sitting around a fireplace. Palden, who just returned from a vacation in Kerala, remarks how different the two places are. Even in winter, Kerala was warm and humid, while Gangtok is cold.
This simple, everyday observation sparks their curiosity: Why are some places so cold and others quite hot?
Their grandfather, a retired science teacher, offers a clue. He explains that Kerala's warmth is due to its long coastline and its closeness to the equator, where the Sun's heat is more intense . This is a good start, but it only opens the door to a much bigger question.
As the family talks, Pema is watching her grandmother cook thukpa in a large metal pan over the fire. She asks another question: "Why are cooking utensils generally made of metals?" . Palden, remembering his lessons from Chapter 4, correctly answers that metals are good conductors of heat.
But this simple answer leads to the real scientific mystery that this chapter is all about: "How does heat get transferred in these materials?". How does the heat from the fire even get to the thukpa? How does the heat from the Sun, millions of miles away, get to the beaches in Kerala?
This chapter is a complete investigation into this invisible, powerful force. We are going to explore the three distinct ways that heat travels through the universe and in our everyday lives: Conduction, Convection, and Radiation.
We begin our first investigation to see exactly how heat moves through a solid metal pan. In Activity 7.1, we will set up a long metal strip and attach several small pins to it with drops of wax, spaced an equal distance apart .
Then, we will heat only one end of the strip with a candle. What happens? The pins do not all fall at the same time. The pin closest to the flame falls first. A few moments later, the next pin falls, then the next, and then the last one.
This simple experiment proves that the heat is traveling along the strip, from the hot end to the cold end. This process is called conduction.
How it works: In solids, heat is transferred when the particles get hot and "pass" the heat energy to their neighbours, who pass it to their neighbours, and so on down the line. The particles themselves do not move from their positions; they just hand off the energy.=
Conductors vs. Insulators: Materials like metals that allow heat to pass through them easily are called good conductors of heat. This is why we make cooking pots out of metal.
Materials like wood, glass, clay, and air do not allow heat to pass through them easily. They are called poor conductors, or insulators. This is why your pot has a wooden or plastic handle—to stop the heat from reaching your hand. This is also why we wear woollen clothes in winter. The wool traps a lot of air, and this layer of insulating air stops our body heat from escaping.
Conduction explains how heat moves through a solid pan. But as Pema observes, the smoke from the fire isn't solid, and it is moving. It's rising up . Why?
This leads to our second investigation.
Convection in Gases (Air): In Activity 7.2, we will hang two inverted paper cups on a stick, like a balance. When we place a burning candle under one cup, that cup rises. Why? The candle heats the air inside the cup. When air (or any gas) gets hot, it expands and becomes lighter than the surrounding cool air. Because it is lighter, it rises. This is why smoke (which is just a mix of tiny particles and hot air) always rises .
Convection in Liquids (Water): Does this also happen in liquids? Yes. In Activity 7.3, we will heat a beaker of water from the bottom. By placing a tiny grain of potassium permanganate (for colour) in the water, we can see what happens. A "coloured streak" of water from the bottom gets hot, rises to the top, then moves to the sides, cools down, becomes heavier, and sinks back to the bottom to be heated again .
This continuous, circular movement of heat through liquids and gases is called convection. It is not the heat being passed along; it is the actual hot particles themselves moving from one place to another.
The most powerful example of convection is the weather we feel in coastal areas. We will learn why the land and sea feel different at different times of day.
The Rule: Land heats up faster than water, and it also cools down faster than water.
During the Day (Sea Breeze): The land gets hot very quickly. The air above the land gets hot and rises. The air over the sea is now cooler and heavier, so it rushes in toward the land to fill the space. This is the cool, refreshing sea breeze .
At Night (Land Breeze): The process reverses. The land cools down quickly, but the sea stays warm. Now, the air above the sea is hotter, so it rises. The cooler air from the land rushes out toward the sea to take its place. This is the land breeze .
We have now solved two puzzles. Heat gets through the pan by conduction, and the water inside the pan (and the smoke outside) moves by convection.
But this leads to a final, more difficult question. When Pema and Palden sit around the fire, they feel warm. How does the heat reach their faces?
It cannot be conduction, because the air between them and the fire is an insulator, not a good conductor.
It cannot be convection, because the hot air is rising up to the ceiling, not sideways to their faces.
There must be a third way. This third way is radiation.
Definition: Radiation is the transfer of heat in the form of waves. It does not require any medium (any particles) to travel .
The Ultimate Example: This is the most important method of all. It is how the heat from the Sun travels through the complete emptiness of space to reach and warm the Earth.
Everyday Examples: All objects give off (radiate) heat. A hot utensil cools down by radiating heat to its surroundings .
Application: This is why we wear different coloured clothes. Light-coloured clothes reflect most radiation, which is why we wear them in the summer to stay cool. Dark-coloured clothes absorb more radiation, which is why we wear them in winter to stay warm .
These three concepts—conduction, convection, and radiation—are the fundamental pillars of how heat and energy move. They explain everything from a simple cooking pan to the entire planet's weather. But they are also distinct, abstract, and easy to confuse.
This is exactly where Teachoo comes in. We are here to be your guide, breaking down these three big ideas into simple, clear, and logical pieces. We will help you see the key difference: conduction is heat passed through solids, convection is heat moved by liquids and gases, and radiation is heat sent through empty space. We connect these concepts to the real-world examples in this chapter—the metal rod, the boiling water, and the Sun—to make sure you never get them mixed up.
This chapter shows us how all three forms of heat transfer work together to power the most important system on our planet: the water cycle.
The Sun's energy travels to Earth via RADIATION.
This energy heats the oceans and lakes, causing water to evaporate (turn into water vapour).
This hot, moist air is lighter than the surrounding air, so it rises high into the atmosphere by CONVECTION.
High up, the air cools, and the water vapour condenses to form clouds.
When the droplets become too heavy, they fall as precipitation (rain or snow).
This rain then seeps into the ground. We will investigate how it seeps by testing clay, sand, and gravel. We will find that water seeps fastest through gravel and slowest through clay, because of the open spaces between the particles .
This process of seeping is called infiltration. The water that seeps down gets stored in vast underground layers of rock and sediment called aquifers. This groundwater is the precious resource we draw up from wells .
From a simple question about why Kerala is warm and why a thukpa pan is made of metal, we have gone on a complete journey to understand the invisible engines that control our planet's climate and water.
To get started on this journey, click on any topic link to begin your exploration.