Kerala Syllabus Class 8 Basic Science: Chapter 03 Pressure - Questions and Answers
Std 8: Physics: Chapter 03: Pressure - Questions and Answers
♦ Pressure
When a wooden plank is put on the mud, the surface area that comes in contact increases, and pressure decreases. So, the weight of the lorry gets distributed, and the lorry would not get stuck in the mud.
♦ Inclusive sports is the inclusion of students with special needs along with others under a common sports law or regulation. Through this, the right to equal participation can be ensured.
• A brick is placed vertically and then horizontally on a sponge. In both cases, does the weight of the brick remain the same? Are the compressions on the sponge equal?
The compressions on the sponge are not equal.
• The weight of the child remains the same whether they stand or lie on the mattress. Yet, the mattress is compressed more while standing. What could be the reason for this? Let's do an activity.
♦ Fix a single nail on the cardboard as shown in the figure and place a balloon on top of it. Place a small weight on the balloon. The balloon burst. Now, nail multiple pins very close to each other, as shown in the second figure. Place the same weight on top of the balloon.
♦ Observe the result. Record the observations in the table below.
Table analysis: Even when the same force is applied in each case, the results vary depending on the change in surface area in contact. This variation is due to the difference in pressure.
♦ What is pressure?
Pressure is the force acting normally per unit area.
♦ Observe the picture.
♦ Who will win the race? Write your opinion. What could be the reason?
Ducks can walk faster in muddy places than hens because ducks have webbed feet. These provide a larger surface area and reduce pressure. So the weight of the duck is distributed, reducing the chance of sinking. Hen has no webbed feet, so they will sink.
♦ Measurement of Pressure
See the figure of two concrete slabs placed in sand. Each of them is made of cubes weighing 18 N fixed together. The area of one side of the cube is 0.36 m².
♦ Referring to the figures, complete the following table.
| Measurements | Slab 1 | Slab 2 |
|---|---|---|
| Force experienced on the sand | 180 N | 180 N (10 X 18 N) |
| Surface area of contact | 0.36 X 4m² | 0.36m² |
| Force experienced per unit area | 180/0.36 X 4 = 125 N | 180/0.36 = 500 N |
♦ What is thrust?
The force acting normally on a surface is called thrust.
Pressure: Force acting normally per unit area.
| The SI unit of pressure is pascal 1 pascal = 1 N/m² |
|---|
♦ Now, find out the pressure exerted by the above slabs on the sand.
• Pressure exerted by slab 1 = 125 N/m²
• Pressure exerted by slab 2 = 500 N/m²
♦ Let us analyse a different situation.
A concrete block weighing 100 N is placed on sand in three different ways as shown in the figure.
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When it is placed vertically (Fig.1)
• Which position will cause the concrete block to sink deeper into the sand? Why?
The block will sink deeper when it is placed vertically, because the surface is less and so the pressure is greater.
| When thrust remains constant, the pressure is inversely proportional to the surface area. This means, when the surface area increases, the pressure decreases and vice versa. |
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♦ Try to lift your school bag by tying it with a thin twine. How do you feel? What if you lift it using a wide strap as shown in the figure? Can you explain the difference by relating its area to pressure?
When we use a thin twine, its surface area is very less and the pressure will be greater. When the strap is wide, its surface area is greater. So, the pressure becomes low. The weights get distributed.
♦ Explain the following situations in the figure. Suggest more such situations.
• The area of the wheel that comes in contact with mud increases, and pressure decreases. So the weight of the bulldozer gets distributed, and it would not get stuck in mud.
• A man lying on a nail bed would not get injured, because while lying, the area of the body that comes in contact with nails increases, and so pressure decreases. That is, the weight of the body gets distributed to all nails.
• Basements of the buildings have more width than the walls.
• Tips of pins and safety pins are pointed.
• Cutting edges of the spade and axe are made thinner.
♦ Liquid Pressure
♦ Let us try the activity given below.
Tie a polythene cover tightly around your hand as shown in the figure.
Just like solids, liquids can also exert pressure.
Liquids exert pressure on all sides of the container. It is due to the irregular movement of the molecules.
| The normal force exerted by a liquid is called thrust. The thrust that a liquid exerts per unit area is called liquid pressure. |
|---|
♦ Activity
♦ Make a hole at the bottom of a plastic bottle. Close the hole and fill the bottle with water. Open the hole and observe the water jetting out.
• When the bottle is full, then the force of water flowing out is greater.
• When the water level decreases, the force also decreases.
| Pressure increases as the depth in a liquid increases. |
|---|
Liquid pressure depends upon the height of the liquid column.
♦ Activity
Attach a syringe on a board as shown in the figure.
Raise the bottle. The force applied by the piston will change according to the pressure of the water. The thin stick deflects according to the force exerted on the piston.
♦ Repeat the experiment using salt solution instead of water.
Observation: While using salt solution, the stick deflects more because the saline water is more than denser than water. So it can exert more pressure.
| The density of a liquid influences its pressure. If the density of a liquid increases, the pressure also increases. |
|---|
Liquid pressure depends on:
• Height of the liquid column (h)
• Density of the liquid (d)
♦ Manometer
Fix a plastic tube on a board in a U shape. Fill it with water. Connect a funnel to one end of the tube as shown in the figure.
♦ Now take different solutions in the beaker and measure the pressure at different levels. Write your observations in the table below.
Suppose the water level is at 5 cm.
• In which position is the pressure highest?
The pressure is highest at the bottom
• Which liquid exerts the highest pressure?
Saline water exerts the highest pressure
♦ Fill a deep vessel with water. Dip a straw into the water and blow gently. Do bubbles come up?
• What happens to their size when they reach the surface?
The size of air bubbles increases as they move upwards.
• Explain their change in size.
When they move upwards, the liquid pressure decreases, and the volume of the bubbles increases.
♦ Explain why dams are built with a wider base.
At the bottom of the dam, the liquid pressure is very high. To withstand this high pressure, dams are constructed with a wider base.
♦ Gas Pressure
Gas Pressure is the force exerted by the particles of a gas on a unit area of a surface. This is due to the collision of gas particles.
| The force exerted by a gas normally on a unit area is gas pressure. |
|---|
♦ What are the factors influencing gas pressure?
• Number of particles
As the number of particles increases, the number of collisions also increases. Then the walls of the container experience more pressure.
| Gas pressure depends on the number of particles. As the number of particles increases, gas pressure also increases |
|---|
• Volume of the container
When volume decreases, the distance between particles decreases, and the number of collisions increases and thus gas pressure increases.
| Gas pressure depends on its volume. |
|---|
• Temperature
When the temperature increases, the speed of particles increases and the collision rate will also increase. This causes an increase in pressure.
| Gas pressure depends on its temperature. |
|---|
♦ What is atmospheric pressure?
• The weight of air column per unit area on the earth's surface is atmospheric pressure.
• The unit of atmospheric pressure is 'bar.
♦ What instrument is used to measure atmospheric pressure?
Barometer.
♦ The atmospheric pressure is the weight of air column experienced per unit area. So, what is the change that occurs in atmospheric pressure as altitude increases?
• As altitude increases, atmospheric pressure decreases.
• Atmospheric pressure depends on the density of air and the height of the air column above that surface.
• At sea level, the height of air column in unit area is maximum. So the atmospheric pressure is greater.
• At high altitude (above the surface of a mountain), the height of air column is less. So there, the atmospheric pressure is less.
| Atmospheric pressure decreases as height from the Earth's surface increases. |
|---|
♦ What is standard atmospheric pressure?
• Atmospheric pressure at sea level is known as standard atmospheric pressure.
• It is defined as the weight of a mercury column that is 0.76 m high, with a unit cross-sectional area, exerting a pressure of 1 atm.
Standard atmospheric pressure = 1 atm = weight of 0.76 m mercury column with unit area.
| Unit of atmospheric pressure is 'base'. 1 bar = 100,000 Pascal (Pa). SI unit of pressure is N/m² or Pascal 1 atm = 101325 Pa. or 101325 N/m² |
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• Calculate the force exerted by atmospheric pressure on a table surface of area 1 m². Atmospheric pressure is 101325 Pascal.
Force = Pressure × Area
= 1 Pa × 1 m²
= 101325 N
This is equivalent to the weight of an object with a mass of 10339 kg. We can say the force exerted by the atmosphere on 1 cm² is 10.1325 N
• If such a large force is exerted on the table, why doesn't it collapse?
The table is surrounded by atmosphere. Every part of the table is experiencing the force. The force will balance, and there will be no change in shape, and the table will not collapse.
♦ Take an aluminium can and fill it with hot water. Pour out the hot water and immediately seal the can. Immerse the can in cold water. What do you observe?
Observation: The can get crushed
Inference: • When it is filled with hot water, the can expands and the air go out from the can.
• When the water is poured out immediately, and the can is sealed, the pressure inside the can decreases.
• When the can is immersed in cold water, the pressure inside the can again decreases.
• The atmospheric pressure outside the can is greater than the inside pressure.
• Then the can will collapse.
♦ Fill a glass with water and cover it with a cardboard. Hold the cardboard with your hand and turn the glass upside down. Remove your hands slowly. Does the water fall?
The water does not fall down. The upward force due to atmospheric pressure can balance the weight of water and can hold the entire weight of water in the glass.
♦ Activity
♦ Pour water in a flat container. Place a lit candle in it. Cover the candle with a glass.
Observation
• The candle flame slowly dims and go out.
• The water from the container enters into the glass.
Inference
• Candle burns using the oxygen in the glass and air becomes hot.
• As oxygen is depleted, the flame will go out.
• Air inside the glass cools and contracts, and the pressure decreases.
• The higher atmospheric pressure outside the glass pushes the water into the glass, and the water level rises in the glass until the two pressures balance each other.
♦ Activity - Textbook Page 48
♦ Gently pull back the piston of a syringe and seal the opening with your hand. Then pull the piston all the way back and release. What do you see? Why does the piston go back? Discuss the reason.
When the piston is pulled back and released, the piston will go back.
Reason
After sealing the opening, when the piston is pulled back, the volume inside the syringe increases, and the pressure decreases. The higher atmospheric pressure outside the syringe pulls the piston into the syringe.
♦ Read the following situations and explain them based on atmospheric pressure.
• Rubber suckers stick to smooth surfaces.
When we place the rubber sucker on a surface, the pressure inside the sucker decreases. The higher atmospheric pressure outside the sucker pulls the sucker inwards, and it sticks there.
• Mountain climbers often experience nosebleeds at high altitudes.
At high altitudes, atmospheric pressure is lower. The pressure inside our body is greater than that. Then the two pressures would not be in equilibrium. Then our small blood vessels will break and cause nasal bleeding.
• Holes are made on the injection bottle with a needle during a drip injection.
As the fluid flows, the pressure inside the bottle decreases and prevents further flow because of the higher atmospheric pressure outside. To balance the pressure, allow air to enter the bottle. For this, a hole is made in the bottle with a needle.
• Passengers travelling uphill in vehicles on a ghat road experience ear pain.
Ear pain occurs due to the pressure difference between the middle ear and the outside. The Eustachian tube is responsible for maintaining the equilibrium of pressure. At high altitude, atmospheric pressure decreases. This pressure imbalance can stretch the eardrum and cause pain.
♦ Let’s Assess
1. Give reasons.
• Heavy vehicles have more tyres.
When more tyres are used, the surface area that comes in contact with the road increases, and pressure decreases. Thus, the weight of the vehicle is get dis- tributed. It minimises the damage to the road and tyres.
• Some passengers experience nosebleeds when aeroplanes fly at high altitudes.
At high altitudes, atmospheric pressure is less. Then the pressure inside the body is greater than outside. Then blood vessels will break and cause bleeding.
• The size of the bubbles rising from the bottom of an aquarium increases gradually.
The size of the air bubbles increases as they move upwards. When they move upwards, the liquid pressure decreases, and the volume of the bubbles increases.
• The foundation of buildings is made wider.
As the area increases, pressure decreases. The weight of the building is distributed into more area.
2. Which of the bottles below shows the shape of rising bubbles correctly? Explain why.
Liquid pressure is greater at the bottom. For a bubble coming up, pressure decreases. Hence, the size of the bubbles increases as it moves from bottom to top.
3. Observe the picture below. Which of the following jars containing the same liquid will experience the highest pressure at the bottom? Explain why.
4. Cooking is faster in a pressure cooker than in an open vessel. Explain why.
The cooker is sealed tightly. So steam will be trapped in it, causing an increase in pressure. In this high pressure boiling point of water will increase to 120°C. This high temperature increases the speed of cooking.
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