Heat Transfer in NatureClass 7 Science Notes

Heat Transfer in Nature

Pema and her brother Palden live in Gangtok. Palden noticed that Kerala is warmer and more humid than Gangtok during winter. This made them curious about why different places have different temperatures. Their grandfather explained that Kerala's location closer to the equator and its long coastline contribute to its warmer and more humid climate. The Sun is the primary source of heat and light for the Earth, and regions near the equator generally experience hotter climates.

Pema observed her grandmother cooking thukpa in a metal pan and wondered why cooking utensils are usually made of metals. Palden recalled learning that metals are good conductors of heat.

Conduction of Heat

Conduction is the process of heat transfer from the hotter part of an object to the colder part. In this process, heat is transferred through a material without the movement of the material's particles themselves. The particles that get heated pass the heat on to their neighbors.

• Take a strip of metal (aluminum or iron), about 15 cm long.
• Attach four pins to the strip with wax at equal distances (about 2 cm apart).
• Secure the strip to a stand.
• Heat one end of the strip with a candle or spirit lamp.
• Observe what happens to the pins.

You'll observe that the pin closest to the heat source falls first, followed by the others in order. This happens because the heat travels along the metal strip, melting the wax and causing the pins to fall.

Good conductors of heat are materials that allow heat to pass through them easily, like metals. This is why cooking utensils are made of metals. In solids, heat transfer mainly occurs through conduction.

Materials like wood and glass do not allow heat to pass through them easily and are called poor conductors or insulators. This is why tea or coffee stays hot longer in clay or porcelain cups.

Air is a poor conductor of heat.

During winter, we wear woolen clothes to keep warm. Woolen fabric traps air in its pores. Since air is a poor conductor of heat, it reduces the heat flow from our bodies to the surroundings, thus making us feel warm. Similarly, having air trapped between two thin blankets is more effective than using one thick blanket.

Houses in hot or cold climates are designed to use heat transfer principles. Outer walls made of hollow bricks trap air, which acts as an insulator, keeping the house warm in winter and cool in summer.

Pema noticed the rising smoke from the burning firewood, leading to a discussion about convection.

Convection

Convection is another method of heat transfer that occurs primarily in liquids and gases. In convection, heat transfer takes place by the actual movement of heated particles.

• Adjust the cups so the stick is horizontal.
• Place a burning candle below one of the cups.
• Observe what happens.

The cup above the candle rises. This is because the air around the flame heats up, expands, becomes lighter, and rises, causing the cup to move upwards. Heating a partially inflated balloon in the sun will also demonstrate the expansion of air upon heating.

When an incense stick is burned, the smoke rises because it is a mixture of hot gases and solid particles that are warmer than the surrounding air.

• Take a 500 mL beaker and fill it half with water.
• Place a grain of potassium permanganate at the center of the beaker's base using a straw.
• Place a candle right below the center of the beaker's base.
• Observe the movement of the colored streak in the water.

As you heat the beaker, a streak of color starts moving up and then coming down from the sides. The water at the bottom gets heated, expands, becomes lighter, and rises. The cooler, heavier water on the sides sinks to take its place. This cycle continues until the entire volume of water is heated. This process is convection.

Water, like air, gets heated through convection. Heat transfer takes place by the actual movement of particles of liquids and gases.

Land and Sea Breeze

Palden recalled that during his visit to a beach in Kerala, the sand was hotter than the sea during the day, but cooler at night. This is because different objects heat and cool differently.

• Take two identical bowls.
• Fill one bowl halfway with soil and the other halfway with water.
• Place a laboratory thermometer in each bowl, ensuring the bulbs are immersed in the soil and water.
• Place the setup in sunlight.
• Measure the temperature of the soil and water every 5 minutes and record the data.

After 20 minutes, the temperature of the soil rises more than the water, indicating that soil heats up faster than water. Similarly, when brought indoors to cool, the soil cools faster than water.

This differential heating and cooling leads to land and sea breezes in coastal areas.

During the day, the land heats up faster than the water, causing the warm air above the land to rise. Cooler air from the sea moves towards the land, creating a sea breeze. Windows in coastal houses often face the sea to take advantage of this cooling breeze.

At night, the process reverses. The land cools down faster than the water in the sea. The warmer air above the sea rises, and cooler air from the land moves towards the sea, creating a land breeze.

The direction of the wind reverses during the day and night near the seashore.

Radiation

Radiation is a method of heat transfer that does not require a medium.

Pema and Palden felt warm sitting around the fireplace because of radiation. The heat transferred directly from the fire to them. Similarly, the Sun's heat reaches Earth through radiation.

All objects radiate heat. A hot utensil cools down over time by radiating heat to its surroundings.

Light-colored clothes are more comfortable during summer because they reflect most of the heat, while dark-colored clothes are more comfortable during winter because they absorb more heat.

Conduction, convection, and radiation often happen together.

• Heat is transferred from the flame to the utensil by conduction.
• Water in the utensil gets heated by convection.
• The warmth felt around the flame and the hot pan is due to radiation.

In summary, heat can be transferred by conduction, convection, and radiation.

• In conduction, heat is passed from one particle to the next without the particles moving from their positions.
• In convection, heat is transferred by the actual movement of particles.
• Conduction and convection require a medium, while radiation does not.

The Sun is the main source of heat for the Earth. The heat from the Sun makes the evaporation of water faster.

Water Cycle

Water exists in three states in nature: liquid (oceans, rivers, lakes), solid (snow, ice sheets, glaciers), and gas (water vapor). During summer, snow and ice melt due to the Sun's radiation, flowing into rivers and oceans. This melted ice is replenished by fresh snow during winters.

Water in oceans, rivers, and lakes evaporates into water vapor due to the Sun's heat. Water also evaporates from trees and plants through transpiration.

When water vapor rises, it cools down and condenses to form clouds. Clouds bring rain, snow, and hail, a process called precipitation.

The water cycle is the continuous movement of water upward as water vapor and downward through precipitation, passing through soil, rocks, and plants, and finally returning to water bodies. The water cycle redistributes and replenishes water in rivers, lakes, and oceans, and conserves the total amount of water on Earth. Rainwater flows into water bodies or seeps into the ground.

Varahamihira, an astronomer and mathematician, predicted seasonal rainfall based on cloud formation, wind patterns, and the position of stars and the moon.

Seepage of Water Beneath the Earth

Seepage occurs when water is drawn from wells or handpumps.

• Take three transparent plastic bottles.
• Cut them in the middle and make a small hole in the cap of each.
• Invert them and put clay in one bottle, sand in the second, and gravel in the third.
• Place identical beakers below each bottle.
• Add 200 mL of water to each bottle.
• Collect the water that flows through each bottle for 10 minutes.
• Compare the amount of water that comes through each bottle.

Water seeps fastest through gravel, slower through sand, and slowest through clay. This is because the spaces between gravel particles are wider than those in sand and clay.

Infiltration is the process of surface water seeping through soil and rocks. Water infiltrates more readily if the spaces between soil and rock particles are wider, open, and interconnected.

The water that seeps through is stored as groundwater in the pore spaces of sediments and openings in rocks beneath the surface. Aquifers are underground layers of sediments and rocks that store water in pore spaces. This is the water we extract by digging wells or drilling bore wells.

Groundwater is not unlimited. Excessive groundwater extraction, decreased vegetation cover, and increased concrete surfaces have limited water infiltration, leading to groundwater depletion. Rainwater harvesting and recharge pits are used to replenish groundwater. The water cycle helps recharge groundwater sources, ensuring a sustainable groundwater supply.

In Ladakh, people conserve water by making ice stupas during the winter. During spring, streams often dry up. Water from mountain streams is channeled down through underground pipes and sprayed into the cold air during winter. As it falls, it freezes, creating a cone-shaped ice stupa. The ice stupa melts slowly during spring, providing water for farming and other needs throughout the summer.