Heat Transfer in Nature — Question 16

The water cycle helps in the continuous redistribution and replenishment of water on Earth through the movement of water in different stages, such as evaporation, condensation and precipitation. Water from oceans, rivers and lakes evaporates due to the Sun’s heat, forms clouds through condensation, and falls back to Earth as rain or snow through a process called precipitation. The rain refills lakes, rivers and groundwater, ensuring the continuous availability of water on Earth.
Class 7 Heat Transfer in Nature Question Answer (Activities)
Activity 7.1: Let Us Experiment (Page 90)
Table 7.1: Falling of pins
Pin falling first
Reasons for what you observed
Prediction
Observation
Pin I
Pin I
Pin I fell first as it was closest to the burning candle, causing the wax to melt quickly
Pin II
Pin II
As heat transferred from Pin I to Pin II. As expected, Pin II fell after Pin I, due to heat conduction.
Pin III
Pin III
Pin III fell after Pin II, also due to heat conduction.
Pin IV
Pin IV
At last Pin IV fell as it was farthest from the burning candle. So it took the longest time to heat up and melt the wax.
Conclusion: Transfer of heat takes place from the hot end of the strip to the colder end.
Activity 7.2: Let Us Investigate (Page 92)
Fig. (a): Initial set-up (b): Hot air rising up
Observations about the cups
Probable reasons for the observation
The cup under which the burning candle is placed rises up.
It would have rised because the air near the candle and inside the cup heats up, expands and rises.
Activity 7.3: Let Us Find Out (Pages 93-94)
Fig. (a): Initial set-up for demonstration (b): Demonstration of convection in heated water
Observation: When heat is supplied, a streak of colour starts moving up and then coming down from the sides.
Conclusion:
• The streak of coloured water moves upward in the middle and then comes down along the sides. This happens because when the water at the bottom of the beaker gets heated, it expands, becomes lighter, and rises. Cooler water from the sides then moves down to take its place. This process of convection continues until the entire volume of the water becomes heated. Because of this convection, we see the movement of coloured streaks in the beaker.
• Water, like air, is heated through the process of convection.
Activity 7.4: Let Us Investigate (Pages 94-95)
Fig. Measurement of the temperature of soil and water
Observation: When the temperature of the soil and water is measured by placing them in sunlight then the rise in the temperature of soil was more as compared to water after a particular period of time. Conclusion: Soil heats up faster than water.
Activity 7.5: Let Us Investigate (Page 99)
Fig. An activity to compare the flow of water through clay, sand and gravel
Table 7.5: Seepage of water
Bottles filled with
Prediction
Observation
Seepage of water (very slow/slow/ fast)
Seepage of water (very slow/slow/fast)
Bottle 1 (Clay)
slow
very slow
Bottle 2 (Sand)
fast
slow
Bottle 3 (Gravel)
very slow
fast
Conclusion: The spaces or pores between the particles of gravel are much larger than those in sand and clay. This allows water to pass through the gravel much faster. Clay has the smallest particles and smallest pores, so water seeps through it the slowest. Sand falls in between. Thus, the flow of water depends on the size of the soil particles and the space between them. Gravel, with its large gaps, allows the fastest seepage of water among the three materials.