Heat Transfer in Nature - Class 7 Science Chapter 2 (Short Notes)

This chapter explains how heat moves from one place to another in nature and how this heat transfer affects our daily life, weather, and the water cycle.

There are three main modes of heat transfer:

1. Conduction
2. Convection
3. Radiation

Conduction

Activity: Heat Transfer Through a Metal Strip

• A metal strip is taken and four pins are attached using wax at equal distances.
• One end of the strip is heated using a candle flame.

Observation

• The pins fall one by one.
• Order of falling: Pin I → Pin II → Pin III → Pin IV

Reason

• The end near the flame becomes hot first.
• Heat travels through the metal strip from the hotter end towards the colder end.
• As heat reaches a pin, the wax holding it melts and the pin falls.

Conclusion: 

Heat is transferred from the hotter region to the colder region of a material. This process is called Conduction.

Definition

Conduction is the process of heat transfer from the hotter part of an object to the colder part without actual movement of particles. In the process of conduction:

• Particles vibrate and transfer heat to neighbouring particles.
• Particles do not move from their fixed positions.
• Heat transfer mainly occurs in solids.

Good Conductors of Heat

Materials that allow heat to pass through them easily are called good conductors of heat.

• Examples: Iron, Steel, Copper, Aluminium
• Uses: Cooking utensils: Pressure cookers: Frying pans

Because metals conduct heat quickly, food gets heated efficiently.

Poor Conductors of Heat (Insulators)

Materials that do not allow heat to pass through them easily are called poor conductors or insulators.

• Examples: Wood, Glass, Plastic, Clay, Porcelain, Air
• Uses: Handles of cooking utensils, Tea cups, Thermos flasks, Electrical insulation

Why Do Woollen Clothes Keep Us Warm? 🧥

Wool contains many tiny pores. These pores trap air. Air is a poor conductor of heat. Therefore, body heat cannot escape easily into the surroundings. As a result, we feel warm during winter

Why Are Two Thin Blankets Better Than One Thick Blanket?

A layer of air gets trapped between the two blankets. Since air is a poor conductor of heat, it reduces heat loss from our body. Therefore, two thin blankets keep us warmer than a single thick blanket.

Heat Transfer and Houses 🏠

In very hot or cold regions, houses are designed to reduce heat transfer. 
Example - Houses in Himalayan regions often have: Wooden walls, Mud filling, Cow dung layersWood and mud are poor conductors of heat. They prevent heat loss and keep houses warm.

Hollow Bricks

• Many modern houses use hollow bricks.
• The hollow spaces trap air.
• Since air is a poor conductor, houses remain:
• Cool in summer
• Warm in winter

Convection 

Pema notices smoke rising from burning firewood and wonders why it always moves upward. To understand this, we study convection. 

Activity: Rising Hot Air 

• Two paper cups are suspended from a wooden stick. 
• A candle is placed below one cup. 

Observation: The cup above the candle rises. 

Reason: Air near the candle gets heated. 

• When air is heated, it expands and becomes lighter. It rises upward. 
• The cooler air is heavier and moves downward. 

Conclusion: Heat transfer in gases occurs through the movement of particles. This process is called convection. 

Definition of Convection:  

Transfer of heat through the actual movement of particles of liquids and gases is called as Convection. 

Activity: Expansion of Air 

1. Fill a baloon partially inflated inside sunlight. 
2. Do you see a change in its volume after some time?

• Observation: A partially inflated balloon becomes larger when kept in sunlight. 
• Reason: The air inside gets heated. The air expands and occupies more space. Hence the balloon swells. 

Why Does Smoke Rise? 💨 

Smoke contains hot gases and tiny solid particles. Since, the smoke is hotter and lighter than surrounding air, it rises upward.

Convection in Liquids 💧

It can be understood by observing diffusion of Potassium Permanganate in Water

1. A beaker half-filled with water is taken. 
2. A grain of potassium permanganate is placed at the bottom centre. 
3. Heat is supplied from below. 

• Observation: A coloured streak rises from the centre and comes down from the sides. 
• Reason: Water at the bottom gets heated. It expands and becomes lighter. The hot water rises. Cooler water from the sides moves down to replace it. This movement continues until all water gets heated. 
• Conclusion: Heat transfer in liquids occurs through convection currents. 

Land and Sea Breeze 

Land and water heat and cool at different rates. It is because of more conduction ability of sand as compared to water. 

• In the day time, sand absorbs heat quickly. So, air on sand becomes hot and move upward. Cold air from sea moves towards sand to fill the empty space. To an observer standing on the coast, it feels like the cold air is coming from sea to land. The movement of cold air from sea to land in the day time is called as Sea breeze.

• In the night time, sand looses heat quickly and becomes colder than the sea. So, air on sea becomes hotter and move upward. Cold air from land moves towards sand to fill the empty space. To an observer standing on the coast, it feels like the cold air is coming from land to sea. The movement of cold air from land to sea in the night time is called as Land breeze.

Radiation 

People feel warm while sitting near a fireplace. The heat reaches them even though they are not touching the fire. It happens even in absence of particles or vacuum. 

Definition: 

Radiation is the transfer of heat from one place to another without requiring any material medium. 

Characteristics of Radiation: 

• No medium is required. 
• Heat travels directly. 

Examples: 

• Heat from the Sun reaches Earth through radiation. 
• A hot utensil kept away from the flame gradually cools. It continuously radiates heat into the surroundings. 
• During Summer Light-coloured clothes reflect most of the heat. Therefore, they keep us cooler.
• During Winter Dark-coloured clothes absorb more heat. Therefore, they keep us warmer. 

Heat transfer during Cooking 🍳

When water is heated in a pan, it gives back heat in all three modes. 

• Conduction: Heat moves from flame to pan. 
• Convection: Heat spreads throughout the water. 
• Radiation: Warmth is felt around the flame and pan. 

Thus, all three modes of heat transfer operate together. 

Summary of Heat Transfer 

Conduction: 

• Heat transfer without movement of particles. 
• Mainly occurs in solids. 

Convection: 

• Heat transfer through movement of particles. 
• Occurs in liquids and gases. 

Radiation: 

• Heat transfer without any medium. 
• Heat travels directly. 

More to Know: Traditional Bukhari Heater 🔥  

A Bukhari is a traditional heater used in Himalayan regions to keep rooms warm during winter. It is made of iron and burns wood or charcoal as fuel. It has a long chimney to carry smoke outside and a flat top that can be used for cooking. 

A Bukhari shows all three modes of heat transfer—conduction through its metal body, convection through the circulation of warm air, and radiation from its hot surface.

Water Cycle 💧☁️🌧️ 

Movement of water on, above and below the surface of earth is called as water cycle.

Water exists in three forms: 

1. Solid: Snow, Ice, Glaciers 
2. Liquid: Rivers, Lakes, Oceans 
3. Gas: Water vapour 

Steps of Water Cycle: 

The Sun is the main source of energy that drives the water cycle. It provides the energy required for the conversion and movement of water through various processes which are involved in the water cycle.

1. Evaporation ☀️ Water from rivers, lakes, and oceans gets heated by the Sun and changes into water vapour. 
2. Transpiration 🌳 Plants release water vapour into the atmosphere through leaves. 
3. Condensation ☁️ Water vapour rises and cools. Tiny droplets form clouds. 
4. Precipitation 🌧️❄️ Clouds release water as:, Rain, Snow, Hail 

Definition: 

The continuous movement of water between the Earth's surface and atmosphere is called the Water Cycle. 

Importance of Water Cycle 

• Replenishes rivers and lakes 
• Maintains groundwater 
• Conserves total water on Earth 
• Redistributes water across regions  

Activuty: Seepage of Water Beneath the Earth 🌎💧

1. Clay, Sand and Gravel are filled inside three different bottles.  
2. Water is poured through bottles. 
3. Speed of seepage of water through each bottle is noticed.

• Observation: Water passes through all bottles with different speeds. 
• Gravel → Fastest 
• Sand → Slower 
• Clay → Slowest 
• Reason: The speed of flow of water though the medium depends upon the empty space among the particles. 
• Gravel has larger spaces between the particles. Water can move through these spaces more easily. 
• Clay has smaller spaces between the particles so water moves very slow through it. 
• Sand has moderate spaces between the particles so speed of flow of water is also medium. 

Infiltration

The process by which surface water seeps through soil and rocks is called infiltration. 

• Groundwater: Water that infiltrates and gets stored below the Earth's surface is called groundwater. 
• Aquifers: Underground layers of rocks and sediments that store groundwater are called aquifers.

Importance of Aquifers 

• They supply water through wells, hand pumps, bore wells 

Groundwater Depletion

The decrease in the amount of water present in aquifers is called as Groundwater depletion. Groundwater is decreasing because of excessive extraction, increasing population, urbanisation, more concrete surfaces, less vegetation 

Groundwater Conservation 

Ground water can be saved by several methods. 

• Rainwater harvesting: Rainwater is collected and allowed to infiltrate into the ground. 
• Recharge Pits: These structures help refill underground aquifers. 

Varahamihira : An Indian Scientist 👨‍🔬

Varahamihira was a famous astronomer and mathematician of the 6th century CE. 

In his work Brihatsamhita, he predicted seasonal rainfall using: Cloud patterns, Wind patterns, Position of stars, Position of the Moon, Natural phenomena 

Ice Stupa ❄️ 

Ice Stupas are artificial ice structures used in Ladakh. 

Formation: 

1. Water from mountain streams is sprayed into freezing air. 
2. The water freezes layer by layer.
3. A cone-shaped ice structure is formed. 

Importance: It melts slowly during spring and rovides water for farming. It helps overcome water scarcity when needed.