Winds, Storm and Cyclones
Science Class Seven
Activity Solution 8.4 to 8.6
Activity: 8.4
Can you blow and lift?
Hold a strip of paper, 20 cm long and 3 cm wide, between your thumb and forefinger as shown in the Figure given in the book.
Now blow over the paper.
Paheli thinks that the strip will be lifted up. Boojho thinks that the strip will bend down.
What do you think will happen to the paper?
Solution
Here Paheli is right but Boojho is wrong.
Since the high-speed wind creates a zone of low pressure. So, when the paper strip is blown after holding it between the thumb and forefinger, it will be lifted up because of the low pressure above the strip.
Activity: 8.5
• Take a boiling tube.
• Stretch a balloon tightly over the neck of the tube.
• You can use tape to make it tight.
• Pour some hot water into a beaker.
• Insert the boiling tube with the balloon in the hot water.
• Observe for 2–3 minutes for any change in the shape of the balloon.
• Take the tube out, and let it cool down to room temperature.
• Take some ice-cold water in another beaker and place the tube with the balloon in cold water for 2–3 minutes.
• Observe the change in the shape of the balloon.
Think and try to answer:
What makes the balloon inflate when the boiling tube is placed in hot water?
Answer
When the boiling tube is inserted into the hot water, the air inside expands because of getting heat. The expansion of air inside the boiling tube makes the balloon inflate.
Why is the same balloon deflated when the tube is kept in cold water?
Answer
We know that on cooling the volume of a gas decreases.
When the same boiling tube is put in cold water, the air inside the tube gets colder, i.e. the temperature of the air inside the boiling tube decreases this decreases the volume of air inside the tube. Because of the decrease in the volume of air inside the tube, the pressure reduces resulting in the balloon being deflated and attached to the mouth of the boiling tube.
Can we infer from the first observation that air expands on heating?
Answer
Yes, we can infer from the first observation that air expands on heating.
Can you now state what happens to the air in the boiling tube when it cools down?
Answer
When the boiling tube is kept in cold water, the air inside the tube also cools down due to this the volume of the air inside the boiling tube decreases.
Activity: 8.6
Take two paper bags or empty paper cups of the same size.
Hang the two bags in the inverted position on the two ends of a metal or wooden stick.
Tie a piece of thread in the middle of the stick.
Hold the stick by the thread (Fig. 8.7) as in a balance.
Put a burning candle below one of the bags as shown in the figure.
Observe what happens.
Why is the balance of the bags disturbed?
Does this activity indicate that warm air rises up?
As the warm air rises up, it pushes the bag above the candle.
Does the disturbance of the balance suggest that the warm air is lighter than the cold air?
Can you now explain why smoke always rises up?
Solution
Question: Why is the balance of the bags disturbed?
Answer
When a burning candle is placed just under one of the paper bags, the air in the bag gets heated and warmed up. On heating, the air inside the bag expands and takes up more space. This makes the air lighter and the bag rises up in the air.
The rising up of the bag because of the warm air inside it disturbs the balance of the bags.
Question: Does this activity indicate that warm air rises up?
Answer
Yes, this activity indicates that warm air rises up.
Question: Does the disturbance of the balance suggest that the warm air is lighter than the cold air?
Yes, the disturbance of the balance suggests that the warm air is lighter than the cold air.
Question:Can you now explain why smoke always rises up?
Answer
Smoke contains warm air, on heating the air expands and takes more space, which makes the smoke lighter than the air surrounding it. The lighter air rises up in the air. This is the cause why smoke always rises up.
Paheli's Question (1) Page number 85
Question: I wonder why the winds shown in the figure are not in the exact north-south direction.
Answer
The wind flows from either north to south or south to north direction. This happens because of uneven heating of land between different longitudes.
But the rotation of the earth changes the direction of the winds slightly.
QuestionBijhoo's Question (1) Page number 85
Question: I want to know what these winds do for us.
Answer
The winds flow because of uneven heating of different places on earth.
The uneven heating of land and sea or ocean brings rain which is beneficial for agriculture. For example, 'Monsoon'. Monsson brings rain which facilitates agriculture. So, the monsoon is very much important for us.
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