Heat Transfer in NatureClass 7 Science NCERT Solutions
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Q1Let Us Enhance Our Learning
Choose the correct option in each case. (i) Your father bought a saucepan made of two different materials, A and B , as shown in Fig. 7.14. The materials A and B have the following properties-
(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
Solution
The correct option is (d) A is a poor conductor and B is a good conductor of heat. The handle (A) of a saucepan is made of a poor conductor of heat, so it is easy to hold the saucepan, whereas the base (B) is made of a good conductor of heat for faster cooking.
Q1Let Us Enhance Our Learning
(iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose you are fitting a smoke detector in your room. The most suitable place for this device will be:
(a)
Near the floor
(b)
In the middle of a wall
(c)
On the ceiling
(d)
Anywhere in the room
Solution
The correct option is (c) On the ceiling. Smoke is warmer than the surrounding air, so it rises. Therefore, the smoke detector should be placed on the ceiling.
Q1Let Us Enhance Our Learning
(ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the rod, as shown in Fig. 7.15. 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
Solution
The correct option is (b) Pins I and II will fall earlier than pins III and IV. Heat is transferred by conduction. The pins closest to the heat source will fall first.
Q2Let Us Enhance Our Learning
A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small leak. You were given another tumbler by the shopkeeper to put the leaky tumbler in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.
Solution
Yes, this arrangement will help to keep the lassi cold for a longer time. The air trapped between the two tumblers acts as an insulator, reducing the heat transfer from the surroundings to the cold lassi. This is because air is a poor conductor of heat.
Q3Let Us Enhance Our Learning
State with reason(s) whether the following statements are True [T] or False [F].
(i)
Heat transfer takes place in solids through convection. [ ]
(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. [ ]
Solution
(i)
False [F]. Heat transfer in solids mainly takes place through conduction.
(ii)
True [T]. Convection involves the actual movement of particles to transfer heat.
(iii)
False [F]. Sandy materials allow more seepage of water than clay materials because the spaces between sand particles are larger.
(iv)
True [T]. Land breeze is the movement of cooler air from land to sea during the night.
Q4Let Us Enhance Our Learning
Some ice cubes placed in a dish melt into water after sometime. Where do the ice cubes get heat for this transformation?
Solution
The ice cubes get heat from the surroundings. The surrounding air, the dish, and any other objects in contact with the ice cubes transfer heat to the ice, causing it to melt.
Q5Let Us Enhance Our Learning
A burning incense stick is fixed, pointing downwards. In which direction would the smoke from the incense stick move? Show the movement of smoke with a diagram.
Solution
The smoke from the incense stick will initially move downwards for a short distance due to gravity. However, since the smoke is warmer than the surrounding air, it will eventually rise upwards due to convection. (See Figure 7.3 in textbook for a similar example).
Q6Let Us Enhance Our Learning
Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.
Solution
The thermometer in Fig. 7.16a will record a higher temperature. In Fig 7.16a, the thermometer is placed directly above the flame, and the water near the bottom of the test tube will get heated up and becomes hot. It expands, becomes lighter, and rises. The water on the sides of the beaker is comparatively cooler and heavier, and comes down to take the place of the rising water. Then, this water gets heated and in turn also rises. This cycle continues until the entire volume of water gets heated through the actual movement of water particles. This process of heat transfer is known as convection. In Fig 7.16b, the thermometer is placed away from the flame, so it will take longer for the water near the thermometer to heat up significantly.
Q7Let Us Enhance Our Learning
Why are hollow bricks used to construct the outer walls of houses in hot regions?
Solution
Hollow bricks are used to construct the outer walls of houses in hot regions because the air trapped in the hollow spaces acts as an insulator. Air is a poor conductor of heat, so it reduces the transfer of heat from the outside environment into the house, keeping the interior cooler.
Q8Let Us Enhance Our Learning
Explain how large water bodies prevent extreme temperature in areas around them.
Solution
Large water bodies prevent extreme temperatures in areas around them due to water's high specific heat capacity. This means water takes a longer time to heat up and cool down compared to land. During the day, water absorbs heat, keeping the surrounding areas cooler. At night, water releases heat, keeping the surrounding areas warmer. This moderates the temperature, preventing extremes.
Q9Let Us Enhance Our Learning
Explain how water seeps through the surface of the Earth and gets stored as groundwater.
Solution
Water seeps through the surface of the Earth through a process called infiltration. When rainwater falls on the ground, it percolates through the soil and rocks. The spaces between soil and rock particles allow water to seep downwards. Water can infiltrate more readily if the spaces between soil and rock particles are wider, open, and interconnected. This water accumulates in underground layers of sediments and rocks, called aquifers, where it is stored as groundwater.
Q10Let Us Enhance Our Learning
The water cycle helps in the redistribution and replenishment of water on the Earth. Justify the statement.
Solution
The water cycle is the continuous movement of water on, above, and below the surface of the Earth. It involves evaporation, condensation, precipitation, and collection. Water evaporates from oceans, lakes, and rivers, rises into the atmosphere, and condenses to form clouds. These clouds then release water as rain or snow (precipitation). This precipitation replenishes water bodies and seeps into the ground, becoming groundwater. This cycle redistributes water from one place to another and ensures that water sources are replenished, maintaining a balance in the Earth's water resources.