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CBSE Questions for Class 7 Physics Winds Storms And Cyclones Quiz 6 - MCQExams.com
CBSE
Class 7 Physics
Winds Storms And Cyclones
Quiz 6
What can you say about the density of air inside a hot air balloon?
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Density of air inside a hot air balloon is more than the density of air outside it
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Density of air inside a hot air balloon is less than the density of air outside it
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Density of air inside a hot air balloon is the same as the density of air outside it
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Either A or B
Explanation
Hot air inside baloon is lighter than colder air outside. The reason for this is when air gets heated up it expands and becomes less dense than the air surrounding it ,also the distance between the molecules increases. So hot air occupies more volume and density decreases because less mass is present in same volume.
When warm air rises at a place, what happens to the air pressure there?
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Air pressure remains unaffected
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Air pressure increases
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Air pressure decreases
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Cannot be determined
Explanation
When hot air rises from an area, a region of space is left with no air hence air pressure decreases there due to less air molecules.So a region of low pressure is created.And cooler air from horizontal side flow here creating again high pressure region due to lot of air molecules.
Where should an exhaust fan in the kitchen window be set up?
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On top of the window
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At the base of the window
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Anywhere on the window
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Exhaust fan need not be installed on the window
Explanation
Exhaust fans are used to expel excess smoke and heat being generated while cooking in the kitchen. As smoke and hot air rise up in the air fast, exhaust fan should be set up on the top part of the window.
On heating the air it _________(expands/contracts) and becomes _______(less/more) dense.
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expands; less
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expands; more
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contracts; less
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contracts; more
Explanation
Air expands on heating because the particles of air on becoming warm, move away from each other and take up more space. So we can say that air on becoming hot occupies more space. Since the particles move away from each other, the air becomes less dense and lighter in weight.
A balloon is fixed on the mouth of a bottle as shown in the image. What will be observed when the bottle is left in the sun for an hour?
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The balloon will stay unaffected
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The balloon will deflate
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The balloon will inflate
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Anything can happen.
Explanation
When the bottle is kept in sunlight for an hour it heats up and the air inside the bottle expands. Thus the balloon will inflate.
Hot air balloon rises up in the air because:
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hot air is heavier than cold air
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Hot air is lighter than cold air
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Hot air contains water vapors which push the balloon up
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None of the above
Explanation
Hot air rises up because it is lighter than cold air. As the air inside the balloon is hotter than the air surrounding the balloon it rises up.
Where should you place the air-conditioning unit to cool a room faster?
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High on the wall
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Near the floor
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Midway between the floor and cieling
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AC height does not affect cooling
Explanation
Hot air rises up and colder air settles down in a room. If AC is placed high on the wall, the cold air blown by AC will settle on the floor and relatively hotter air will rise up. This motion of air current cools the room faster.
Hence Option A
Hot air rises because:
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It is denser than cold air
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It is lighter than cold air
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It occupies more space than cold air
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Both B and C
Explanation
Hot air rises because it is less dense than cold air. Hot is lighter, and occupies more space than cold air.
The emissions from a burning incense stick are seen to rise up always. Why is it so?
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Hot air rises up
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Hot air settles down
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Air exerts pressure
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Air has the capacity to do work
Explanation
Smoke from incense sticks consists of hot air. As hot air always rises up, the smoke is seen to rise up.
A balloon is attached to the neck of a boiling tube and placed in a cold bath. Which of the following would be observed in the setup after a while?
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The balloon would inflate
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The balloon would deflate
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The balloon remains unaffected
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Any of the above can happen
Explanation
Air expands on heating and contracts on cooling. When the boiling tube is kept inside a cold bath, the air inside the boiling tube will contract and the balloon will defllate.
Paheli fixes two balloons on the mouth of two bottles, say A and B. A is kept in the sun and B is kept in the refrigerator for an hour. Pick the correct observation for Paheli's experiment
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Balloons in both A and B expand
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Balloons in both A and B contract
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Balloon in A contracts and in B expands
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Balloon in A expands and in B contracts
Explanation
Since A is kept in sun the air inside the bottle will get heated and the molecules of air will gain energy and will move farther from each other and occupies more space now.In other words hot air expands.So the baloon attached to A will expand.
Since B is kept in refrigerator the air inside the bottle will get cool and the molecules of air will lose heat energy to environment and will move closer to each other because of reduced motion.And occupies less space now.In other words cold air contracts.So the baloon attached to B will contract.
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