8.9. Pressure Exerted by Liquids and Gases

Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out.

This chunk introduces the topic of pressure exerted by liquids and gases. It raises questions about whether these substances exert pressure and the relevance of the area over which this pressure acts. Understanding pressure is crucial as it affects how we interact with liquids and gases in our daily lives.

Consider how a soda can is pressurized. When you open it and hear the 'hiss' of gas escaping, it's the pressure inside the can equalizing with the outside atmosphere. This illustrates how gases exert pressure.

Activity 8.8 Take a transparent glass tube or a plastic pipe. The length of the pipe/tube should be about 25 cm and its diameter should be 5-7.5 cm. Also take a piece of thin sheet of a good quality rubber, say, a rubber balloon. Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe at the middle, keeping it in a vertical position. Ask one of your friends to pour some water in the pipe. Does the rubber sheet bulge out? Note also the height of the water column in the pipe. Pour some more water. Observe again the bulge in the rubber sheet and the height of the water column in the pipe. Repeat this process a few more times. Can you see any relation between the amount of the bulge in the rubber sheet and the height of the water column in the pipe?

This activity helps students visualize how pressure from water acts on surfaces. As they pour water into the tube, the rubber sheet bulges. The more water added, the higher the water column, and consequently, the more the rubber sheet bulges. This signifies the relationship between liquid height and the pressure it exerts.

Think of how a large balloon inflates when water is poured into it. The more water you add, the more the balloon expands, similar to the rubber sheet bulging out as the water level rises.

Activity 8.9 Take a plastic bottle. You can take a discarded water or soft drink bottle. Fix a cylindrical glass tube, a few cm long near its bottom as shown. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet as you did in Activity 8.8. Now fill the bottle up to half with water. What do you observe? Why does the rubber sheet fixed to the glass tube bulge this time? Pour some more water in the bottle. Is there any change in the bulge of the rubber sheet?

This activity involves observing the behavior of water within a closed container. When water is poured into the bottle, it exerts pressure, causing the rubber sheet at the glass tube's mouth to bulge. The pressure exerted by the water in the bottle is transmitted to the walls and causes the rubber to exert outward. Hence, adding more water changes the bulge proportionately to the increase in pressure.

Imagine a water balloon. When you fill it up, the pressure from the water makes the balloon stretch. Similarly, in this activity, the water pressure makes the rubber sheet bulge outward.

Activity 8.10 Take an empty plastic bottle or a cylindrical container. You can take a used tin can or a used plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom. Now fill the bottle with water. What do you observe? Do the different streams of water coming out of the holes fall at the same distance from the bottle? What does this indicate?

This activity demonstrates that liquids exert equal pressure at the same depth. As the bottle is filled with water and water flows out of the holes, if all the streams are equal, it shows that pressure at that depth is consistent across these openings. This uniformity indicates that liquid pressure acts equally at the same depth from the surface.

Think about a fountain. The water shoots up from the same height and spreads out evenly. The equal rise from the holes shows how liquid pressure works in real life, just like how the water comes out uniformly from the fountain at equal pressure.

Do gases also exert pressure? Do they also exert pressure on the walls of their containers? Let us find out.

This chunk prompts students to consider whether gases exert pressure like liquids do. Gases exert pressure in all directions, and this pressure is felt wherever the gas comes into contact with a surface, such as the walls of its container. Understanding gas pressure is crucial in many scientific and practical applications.

When you inflate a balloon, the air inside pushes against the walls of the balloon, making it expand. This pushing against the balloon's surface is an example of gas exerting pressure.

So, we find that gases, too, exert pressure on the walls of their container.

This summary confirms that both liquids and gases exert pressure, impacting how they behave in different contexts like containers and environment. The pressure exerted by these substances is a fundamental concept in physics, highlighting their interaction with the world.

Every time you drink from a straw, you're experiencing the pressure of the liquid being pushed up into your mouth. This is the pressure that liquids and gases exert, at work in our daily lives.

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