NCERT Solutions for Ch7 Heat Transfer in Nature Class 7 Science Curiosity
Book Solutions1
(a) Both A and B are good conductors of heat
(b) Both A and B are poor conductors of heat
(c) A is a good conductor and B is a poor conductor of heat
(d) A is a poor conductor and B is a good conductor of heat
(ii): Pins are stuck to a metal strip with wax and a burning candle is kept below the strip. Which of the following will happen?
(a) All the pins will fall almost at the same time
(b) Pins I and II will fall earlier than pins III and IV
(c) Pins I and II will fall later than pins III and IV
(d) Pins II and III will fall almost at the same time
(iii): A smoke detector is a device that detects smoke and sounds an alarm. Where should it be placed in a room?
(a) Near the floor
(b) In the middle of a wall
(c) On the ceiling
(d) Anywhere in the room
Answer
(i) (c) A is a good conductor and B is a poor conductor of heatExplanation: Saucepans usually have metal bases (good conductors) for heating and handles made of plastic or wood (poor conductors) to prevent burns.
(ii) (b) Pins I and II will fall earlier than pins III and IV
Explanation: Heat travels from the heated end to the other end via conduction. The pins near the flame fall first as the wax melts.
(iii) (c) On the ceiling
Explanation: Smoke rises up due to convection, so detectors must be placed at the highest point.
2
Answer
Yes, it will help.Explanation: The air between the two tumblers acts as an insulator (a poor conductor of heat) and slows down the heat transfer from outside.
3
(ii): Heat transfer through convection takes place by the actual movement of particles.
(iii): Areas with clay materials allow more seepage of water than those with sandy materials.
(iv): The movement of cooler air from land to sea is called land breeze.
Answer
(i) FalseReason: In solids, heat transfer happens through conduction, not convection.
(ii) True
Reason: In convection, particles of liquids and gases move to transfer heat.
(iii) False
Reason: Clay has smaller pores than sand, so it allows less seepage.
(iv) True
Reason: At night, land cools faster, and cooler air moves towards the sea.
4
Answer
When ice cubes are placed in a dish, they start to melt and turn into water. For this transformation to happen, the ice needs to absorb heat energy.
• The ice cubes get heat from the surrounding environment.
• This includes heat from the air around them, the surface of the dish, and even the table they are placed on.
• The heat causes the ice to melt and change into water.
• This heat makes the ice melt and turn into water.
5
Answer
When an incense stick is burning and fixed pointing downwards, the smoke will move upwards. This happens because smoke rises due to convection.
As the incense stick burns, it heats the air around it, making the air less dense. The cooler, denser air pushes the hot air (and smoke) upward. So, even if the incense stick is pointing downwards, the smoke will still rise.
Diagram
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Answer
In the given experiment, the test tube in Fig. 7.16(b), where the thermometer is closer to the flame, will show a higher temperature.
When water is heated by the candle, it becomes hot at the bottom of the test tube. The hot water rises because it is less dense than the cooler water. This movement of water from the bottom to the top is called convection.
Now, let's look at both test tubes:
• In Fig. 7.16(a): The thermometer is placed higher up, away from the hot water. It does not measure the rising hot water directly. So, it will show a lower temperature.
• In Fig. 7.16(b): The thermometer is placed in the area where the hot water is rising. Since the hot water is directly in contact with the thermometer, it will measure the higher temperature.
Therefore, the thermometer in Fig. 7.16(b) will show a higher temperature.
7
Answer
Hollow bricks are used because they trap air inside the spaces between the bricks. Air is a poor conductor of heat, meaning it does not allow heat to pass through easily. This helps to reduce heat transfer from the outside to the inside of the house. As a result, the house stays cooler in hot regions, as less heat enters through the walls.
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Answer
Large water bodies, such as oceans and seas, help prevent extreme temperatures in nearby areas through the sea and land breeze effect.
Sea Breeze:
• During the day, the land heats up faster than the water.
• The air over the land becomes warmer and rises, while the air over the sea remains cooler.
• The cooler air from the sea then moves towards the land, creating a sea breeze.
• This cool air from the sea helps lower the temperature of the land near the water, preventing it from getting too hot.
Land Breeze:
• During the night, land cools down faster than water.
• The air over the land becomes cooler and denser, while the air over the sea remains warmer.
• As a result, the cooler air from the land moves towards the sea, creating a land breeze.
• Thus at night, the water retains heat longer and warms the surrounding air, preventing it from getting too cold.
9
Answer
Water seep through the surface of the Earth :
• When it rains, water falls to the ground and moves through the soil and rocks. This process is called infiltration.
• The water travels down through the layers of soil and rocks, filling the gaps and spaces between them.Storage in aquifers:
• Eventually, the water reaches underground layers called aquifers, which are large areas of rock or soil that can hold water. These aquifers store the water as groundwater.
Thus, water seeps through the soil and rocks into the ground, where it gets stored as groundwater.
10
Answer
The water cycle helps move and refill water on Earth in the following ways:1. Evaporation and Transpiration: Water from lakes, rivers, and oceans turns into vapor (gas) because of the Sun’s heat. Plants also release water into the air through their leaves. This water vapor rises into the air.
2. Condensation: As the water vapor goes up, it cools down and forms clouds.
3. Precipitation: When the clouds get heavy, water falls back to Earth as rain, snow, or hail, giving water back to rivers, lakes, and the ground.
4. Infiltration and Runoff: Some water soaks into the ground to refill underground water sources. The rest flows over the land into oceans and lakes.
Thus, water cycle helps move water around the planet and refill water sources, keeping water available for plants, animals, and humans.