Class 7 Chapter 7 Notes

Heat Transfer in Nature

Class 7 Science Chapter 7 Notes | Complete Reference Document

1. Introduction

Pema and her brother Palden live in Gangtok. One cold winter evening, they sit near a fireplace. Palden talks about his trip to Kerala during winter and says that Kerala is warmer and more humid than Gangtok.

Grandfather explains, "Kerala is closer to the equator and has a long coastline, so it is warmer and more humid."

Palden adds, "We learned that the Sun is the main source of heat and light, and places near the equator are usually hot."

While they talk, Pema watches her grandmother cooking thukpa in a large metal pan. She asks, "Why are cooking utensils usually made of metal?" Palden answers, "Because metals are good conductors of heat."

Let us now study the nature of heat and the ways in which heat is transferred in nature.

2. Conduction of Heat

Conduction is the process by which heat is transferred from the hotter part of an object to its colder part through direct contact. In solids, especially metals, particles vibrate more when heated and pass energy to neighbouring particles without changing their positions significantly.

When one end of a metal object is heated, the particles at that end gain energy and vibrate more strongly. These vibrations pass from one particle to the next, transferring heat along the object. For example, when a metal pan is placed on a flame, heat travels through the pan and makes the pan hot.

Heat flows from a region of higher temperature to a region of lower temperature until the temperatures become more nearly equal.

Activity: Demonstrating Conduction in a Metal Strip

Materials needed: A metal strip made of aluminium or iron, four pins, a candle or spirit lamp, a stand or two bricks, wax and a heat source.

Steps:

• Take a metal strip about 15 cm long and attach four pins to it using small drops of wax. Keep the pins roughly 2 cm apart.

• Secure the strip across a stand or between two bricks so that the pins hang freely.

• Heat the end of the strip that is away from the stand using a candle or spirit lamp.

• Observe which pin falls first and note the order in which the pins fall.

Prediction: Predict the order in which the pins will fall as the strip is heated.

Observation: The pin closest to the flame, Pin I, falls first. It is followed by Pins II, III and IV. The wax holding each pin melts in sequence because heat travels along the metal strip by conduction.

Conclusion:

• Conduction is the transfer of heat from the hotter part of a material to its cooler part through direct contact between particles.

• In this activity, heat travels from the hot end of the metal strip towards the cooler end. The drops of wax melt one after another, causing the pins to fall in sequence.

• Metals are good conductors of heat. This is why metal utensils are commonly used for cooking.

Conductors and Insulators of Heat

Good conductors of heat allow heat to pass through them easily.

• Metals such as aluminium and iron are good conductors of heat.

• This property makes metal cooking utensils and pans useful for transferring heat efficiently from a flame to food.

• The handles of many cooking utensils are made of wood or plastic because these materials are poor conductors of heat and reduce the risk of burns.

Poor conductors of heat, or insulators, do not allow heat to pass through them easily.

• Materials such as wood, glass, clay and porcelain are poor conductors of heat.

• Tea or coffee cups made of clay or porcelain retain heat for a longer time.

• Woollen fabrics trap air and help to keep us warm.

• The presence of air between layers of clothing reduces heat flow and helps to keep the body warm.

How Air Acts as an Insulator and Its Applications

1. Woollen fabric and heat retention:
• Woollen fabric traps air in the tiny pores between its fibres.
• Since air is a poor conductor of heat, it reduces the flow of heat from our bodies to the colder surroundings.
• Trapped air acts as insulation. Wearing several layers traps more air and keeps us warmer in cold weather.

2. Blankets and trapped air:
The air between two thin blankets makes them warmer than a single thin blanket because the trapped air slows down the loss of heat from the body.

3. Insulated houses: Using heat transfer principles:
• Houses in very hot or very cold regions use insulation so that indoor temperatures remain comfortable.
• Materials and building methods are chosen to reduce unwanted heat gain in summer and heat loss in winter.

4. Hollow bricks and heat insulation:
• Some houses use hollow bricks for their outer walls.
• Air trapped inside the hollow parts acts as a poor conductor of heat and reduces heat transfer through the wall.
• This helps keep houses warmer in winter and cooler in summer by slowing the flow of heat.

📌 Fascinating Facts: Traditional Himalayan Insulation The upper regions of the Himalayas, such as the Mori block of Uttarkashi in Uttarakhand, experience extremely cold climates and heavy snowfall in winter. To keep houses warm in such conditions, people build walls with two wooden layers and fill the space between them with materials such as cow dung and mud. Both wood and mud are poor conductors of heat and act as natural insulation, reducing heat loss and helping to keep houses warm.

3. Convection

Convection is the process of heat transfer in liquids and gases in which heated particles move and carry heat with them. This movement produces currents that redistribute heat throughout the fluid. Convection is involved in many natural and everyday phenomena, such as sea breezes, land breezes, boiling water and rising smoke.

How Convection Works

• When a liquid or gas is heated, its particles gain energy and spread farther apart, making the heated fluid less dense so that it rises.

• Cooler, denser fluid moves down to take its place. This produces a continuous cycle called a convection current.

• For example, when water is heated in a pan, warmer water rises while cooler water sinks, distributing heat throughout the pan.

Convection in Gases and Liquids

Convection in gases: In air, heated air expands, becomes less dense and rises. Smoke rises because it contains hot gases and tiny solid particles released when something burns. These hot products are less dense than the surrounding air and therefore rise.

Convection in liquids: In liquids such as water, heated regions become less dense and rise, while cooler regions sink. This creates circular flow patterns that mix the liquid and distribute heat.

Activity: Why Smoke Rises (Paper Cup Experiment)

Materials needed: Paper cups, a wooden stick, threads and a burning candle.

Procedure:

• Hang the paper cups in an inverted position on the wooden stick using threads so that they can move freely.

• Suspend the stick freely from its midpoint and balance it horizontally with one inverted cup hanging from each end.

• Light a candle and place it under one of the cups.

• Observe the behaviour of the cups.

Observations and explanation:

• The cup with the candle under it rises because the air inside and below the cup is heated.

• Heated air expands and becomes less dense, so the cup rises. This demonstrates how warm air moves upwards during convection.

Example of air expansion: A partially inflated balloon placed in the Sun expands because the air inside it becomes warm and expands, making the balloon larger.

Smoke rising: When an incense stick burns, the smoke, which contains hot gases and tiny solid particles, is warmer and less dense than the surrounding air. It therefore rises by convection.

Activity: Convection in Water (Potassium Permanganate)

Materials needed: A 500 mL heat-resistant beaker, water, a straw, a small grain of potassium permanganate as a coloured tracer, a candle, a tripod stand and wire gauze.

Procedure:

• Fill the beaker halfway with water.

• Use the straw to place a tiny grain of potassium permanganate at the centre of the base of the beaker.

• Place a candle under the centre of the beaker's base and heat it gently.

• Observe the movement of the coloured streak in the water.

Observations: A streak of colour moves upwards from the centre and comes down along the sides of the beaker as heating continues.

Explanation and conclusion:

• The water at the bottom near the heat source becomes warmer, expands, becomes less dense and rises.

• Cooler and denser water from the sides moves down to replace it, producing a circular motion.

• This continuous motion is an example of convection in liquids. It shows how heat is transferred by the bulk movement of a fluid.

4. Land and Sea Breeze

• During the day, land near the coast heats up faster than seawater because different materials heat at different rates.

• Warm air over the land rises, creating a low-pressure area above the land.

• Cooler, denser air from the sea moves in to fill this low-pressure area, producing a sea breeze that is felt on the coast during the day.

• At night, land cools faster than the sea. Cooler air over the land flows towards the warmer sea, producing a land breeze.

Activity: Comparing Heating of Land and Water

Materials required: Two identical bowls, soil, water and two laboratory thermometers.

Procedure:

1. On a sunny day, place soil in one bowl and water in the other, filling both to the same level.
2. Place one thermometer bulb in the soil and the other in the water. Ensure that the bulbs do not touch the sides or the bottoms of the bowls.
3. Place both bowls in sunlight and observe the rise in temperature for 20 minutes.

Observations: After 20 minutes, the temperature of the soil rises more than the temperature of the water. Thus, soil heats up faster than water.

Cooling experiment:

1. Bring the set-up indoors after heating and allow both the soil and water to cool for 20 minutes.
2. Observe their rates of cooling.

Conclusion:

• Soil heats up and cools down faster than water.

• These differences in heating and cooling cause land and sea breezes.

5. Radiation

• Radiation is the transfer of heat through electromagnetic waves. It does not require a medium.

• Heat from a hot object can reach us directly by radiation. For example, we feel the warmth of a fire even when we are not touching it.

• The Sun's heat reaches the Earth by radiation. The Sun's surface emits energy that travels through space as waves and warms the Earth.

• Because radiation does not need a medium, we feel the Sun's warmth even though space is nearly a vacuum.

Examples of Heat Transfer in Daily Life (All Three Processes)

Many everyday situations involve conduction, convection and radiation together. For example, when water is heated in a pan:

• Heat travels through the metal base and sides of the pan by conduction.

• Water inside the pan is heated and circulates by convection.

• The warmth felt around the flame and pan is partly due to radiation.

📌 Fascinating Fact: The Himalayan Bukhari In the upper Himalayan region, a traditional room heater called a bukhari is used to keep rooms warm in winter. A bukhari is an iron stove in which wood or charcoal is burned. A long pipe acts as a chimney and releases smoke. The flat top of the bukhari can be used for cooking by placing utensils on it. When a bukhari is used, conduction, convection and radiation work together to warm the room and cook food.

6. Water Cycle

Water exists in three states in nature:

• Liquid: Water in oceans, rivers and lakes.

• Solid: Snow, glaciers and ice sheets in mountains and polar regions.

• Gas: Water vapour in the atmosphere.

During summer, snow and ice melt because of the Sun's radiation and feed rivers that flow into oceans. Water from oceans, rivers and lakes evaporates because of the Sun's heat. Plants release water vapour through transpiration. Water vapour rises, cools and condenses to form clouds. Clouds cause precipitation in the form of rain, snow or hail. The continuous movement of water from the Earth's surface to the atmosphere and back is called the water cycle.

Importance of the Water Cycle

The water cycle redistributes and replenishes water on Earth. It ensures that water moves from oceans and other water bodies to the atmosphere and returns as precipitation. This cycle is essential for maintaining water supplies for ecosystems, agriculture and human use.

📌 Know a Scientist: Varahamihira Varahamihira was a famous astronomer and mathematician of the 6th century CE from Ujjaini, now called Ujjain, in Madhya Pradesh. In his work Brihatsamhita, he described methods of predicting seasonal rainfall based on observations of cloud formation, wind patterns, the positions of stars and the Moon, and other natural phenomena.

7. Seepage of Water Beneath the Earth's Surface

Activity: How Water Seeps Through the Ground

• Take three transparent 1 L plastic bottles and cut them roughly in the middle.

• Make a small hole in the cap of each bottle and keep the bottles inverted.

• Fill one bottle with clay, another with sand and the third with gravel.

• Place identical beakers below the bottles and add 200 mL of water to each bottle.

• Collect and measure the water that flows out of each bottle for 10 minutes and compare the amounts.

Water flows fastest through gravel, more slowly through sand and slowest through clay. Differences in particle size and in the spaces between particles explain this behaviour:

• Gravel: Large spaces between particles allow water to flow quickly.

• Sand: Smaller spaces produce slower flow.

• Clay: Very small spaces restrict water flow the most.

Theory and Definitions

• Infiltration is the process by which rainwater or surface water soaks down through soil and rocks beneath the Earth's surface.

• The ease of infiltration depends on the size and connectivity of the pores, or spaces, between soil and rock particles. Wider, well-connected pores allow faster seepage.

• When water seeps into the ground, it collects in the pores of sediments and in fractures of rocks. This stored water is called groundwater.

• The underground layers of rock and sediment that store groundwater are called aquifers.

• We access groundwater by digging wells or drilling bore wells into aquifers. The depth of groundwater varies from a few metres to hundreds of metres depending on the location.

• Groundwater is a vital resource but it is not unlimited. Excessive extraction for agriculture and domestic use can lead to depletion.

Human Impacts and Conservation

• Urbanisation reduces natural areas for infiltration. Less vegetation and more concrete surfaces decrease the rate at which rainwater soaks into the ground.

• As a result, groundwater recharge is reduced and groundwater levels may fall.

• Methods to conserve and recharge groundwater include rainwater harvesting, which involves collecting and storing rainwater, and building recharge pits to allow water to soak into the ground.

Water scarcity makes life difficult, so people use methods to save and store water for dry periods.

📌 Science and Society: Ice Stupa In Ladakh, a practical solution called an ice stupa is used to store winter water as ice. The ice melts slowly in spring and supplies water for farming and daily use. • In Ladakh, spring streams often dry up because the Sun's heat is not sufficient to melt mountain snow early in the year. • During winter, water from mountain streams is sent through pipes and sprayed into the very cold air. • This sprayed water freezes layer by layer to form a tall, cone-shaped ice structure called an ice stupa. • The ice stupa melts slowly in spring, releasing water gradually to help meet farming and daily needs during the spring water shortage.

8. Terms to Remember

Term	Definition
Conduction	Heat transfer through direct contact, in which particles pass energy to neighbouring particles without large-scale movement.
Convection	Heat transfer in liquids and gases by the movement of heated, less dense fluid that carries heat with it.
Radiation	Heat transfer through electromagnetic waves. It does not require a medium.
Insulator	A material, such as wood or glass, that does not allow heat to pass through easily.
Evaporation	The process by which liquid water changes into water vapour because of heat.
Condensation	The change of water vapour into liquid water on cooling, as in cloud formation.
Precipitation	The release of water from clouds as rain, snow or hail.
Transpiration	The release of water vapour by plants.
Aquifer	An underground layer of rock or sediment that holds groundwater.
Seepage	The movement of water through soil or rock into the ground.
Bukhari	A traditional Himalayan heater that uses wood or charcoal for warmth and cooking.
Ice Stupa	A cone-shaped artificial ice structure used in Ladakh to store water in winter and release it in spring.

9. Summary

Heat is transferred in nature by three processes: conduction, convection and radiation. Conduction occurs mainly in solids through the transfer of energy between neighbouring particles and makes metals suitable for cooking utensils. Convection occurs in liquids and gases when heated, less dense regions rise and cooler material sinks, creating currents such as sea breezes and land breezes. Radiation transfers heat by electromagnetic waves and does not require a medium; the Sun warms the Earth by radiation.

The water cycle includes evaporation, condensation, precipitation, transpiration and the movement of water through the ground. It is driven mainly by the Sun's heat and redistributes Earth's water. Groundwater is replenished by infiltration into aquifers, but human activities can reduce groundwater recharge. Therefore, methods such as rainwater harvesting, recharge pits and innovations such as ice stupas are important for water conservation.