Science (EVS K-5) · Class 7 · Winds, Storms and Cyclones · Term 3

Air Exerts Pressure

Understand the fundamental concept that air has weight and exerts pressure on everything around us, and see how this pressure affects our daily lives.

TL;DR:Have you ever wondered what invisible force holds the water in an inverted glass or helps you sip your favourite drink through a straw? Let's explore the powerful, unseen push of the air all around us.

• CBSE Learning Outcomes
• NCERT Class 7 Science: Chapter 8 - Winds, Storms and Cyclones

About This Topic

This topic, 'Air Exerts Pressure', is a cornerstone of the middle school physics curriculum as outlined by the NCERT framework for Class 7 Science. It moves students from a qualitative understanding of air as just 'something that is all around us' to a quantitative, scientific concept of air having weight and exerting force. The concept of atmospheric pressure is fundamental for understanding a vast range of phenomena, from everyday occurrences like drinking with a straw and the working of a syringe, to larger concepts in meteorology such as wind patterns and weather forecasting, which are crucial in the Indian context.

The pedagogical approach should be heavily inquiry-based, relying on simple, low-cost experiments that provide dramatic and undeniable evidence of this invisible force. These hands-on activities are not just for engagement; they are critical for confronting and correcting common misconceptions, such as the idea that air is weightless or that pressure acts only downwards. By connecting these scientific principles to local and familiar examples, like why a packet of chips puffs up during a trip to the hills or how a suction pump works, teachers can make the learning more relevant and memorable for students.

Key Questions

1. Explain how simple activities demonstrate that air exerts pressure.
2. Analyse why a balloon inflates when you blow air into it.
3. Compare the air pressure inside and outside a sealed bottle.

Learning Objectives

• Demonstrate through at least two simple activities that air exerts pressure.
• Explain that air pressure is exerted in all directions, not just downwards.
• Define atmospheric pressure as the pressure exerted by the weight of the air in the atmosphere.
• Relate differences in air pressure to everyday phenomena like the working of a dropper, a syringe, or a straw.
• Analyse why a balloon inflates when air is blown into it, using the concept of internal and external pressure.

Key Vocabulary

Pressure	The force applied on a surface, divided by the area over which the force is distributed.
Atmosphere	The thick layer of air that surrounds the Earth.
Atmospheric Pressure	The pressure exerted by the weight of the atmosphere.
Force	A push or a pull upon an object resulting from the object's interaction with another object.
Vacuum	A space where there is no matter, or a space with very low pressure compared to the surrounding air.

Watch Out for These Misconceptions

Common Misconception

Air is weightless and does not take up space.

What to Teach Instead

Air is made of molecules which have mass. Therefore, air has weight and is pulled down by gravity. An inflated balloon is heavier than a deflated one, and air fills the container it is in.

Common Misconception

Pressure is only a downward force.

What to Teach Instead

Air pressure is exerted equally in all directions: up, down, and sideways. This is because air molecules are in constant, random motion, colliding with surfaces from every angle.

Common Misconception

An 'empty' bottle or glass contains nothing.

What to Teach Instead

An object that we call 'empty' is actually full of air. This air exerts pressure on the inside surfaces of the container, balancing the pressure from the outside.

Active Learning Ideas

Experiential Learning

The Crushing Can Experiment

Pour a small amount of water into an empty aluminium can and heat it until steam comes out. Using tongs, quickly invert the can into a trough of cold water. The can will be instantly crushed by the outside air pressure.

15 min·Whole Class

Experiential Learning

The Upward Push: Glass and Card Trick

Fill a glass to the brim with water and slide a piece of stiff cardboard over the top. While holding the card in place, carefully turn the glass upside down. The card will stay in place, held up by the air pressure from below.

10 min·Pairs

Experiential Learning

The Syringe Pull

Give students a plastic syringe (without a needle). Ask them to block the tip with a finger and try to pull the plunger. They will feel a strong resistance, demonstrating that the outside air pressure is pushing the plunger in.

10 min·Individual

Real-World Connections

• Drinking liquids using a straw.
• The working of vacuum cleaners to suck up dust.
• Using a syringe to draw medicine or a dropper to pick up liquids.
• Sticking a rubber sucker or hook onto a smooth wall.
• Inflating the tyres of bicycles and cars to the correct pressure.

Assessment Ideas

Quick Check

Ask students to draw a diagram of the 'Glass and Card Trick' and use arrows to show the forces at play (water weight pushing down, air pressure pushing up).

Quick Check

Present a scenario-based question: 'A partially inflated balloon is taken from Chennai to Shimla. What will happen to the balloon and why?' This assesses their understanding of pressure and altitude.

Quick Check

Provide a simple checklist with 'I can explain...', 'I can demonstrate...', 'I am still confused about...' statements related to the learning objectives for students to reflect on their own understanding.

Frequently Asked Questions

If air pressure is so strong, why don't we get crushed by it?

We are not crushed because the fluids inside our bodies, like our blood, exert an outward pressure that balances the atmospheric pressure pushing in on us.

How does a simple drinking straw work?

When you suck on a straw, you remove the air from inside it, creating an area of low pressure. The higher atmospheric pressure outside pushes down on the surface of the drink, forcing the liquid up the straw into your mouth.

Is the air pressure the same on a mountain and at the seaside?

No, air pressure is lower on a mountain. This is because there is less air above you pushing down. This is why it is harder to breathe at high altitudes and why sealed packets of snacks puff up.