Atmospheric Pressure

• Air has mass and weight — it exerts pressure on every surface.
• Atmospheric pressure ≈ 1.01 × 10⁵ Pa at sea level.
• How drinking straws, syringes, rubber suckers, and ink fillers use air pressure.
• Pressure decreases with altitude; links to weather and breathing on mountains.

1. Cause — weight of air column above a point.
2. Standard value — about 101325 Pa (1 atm); supports ~76 cm mercury in barometer.
3. Instruments — mercury barometer (Torricelli), aneroid barometer (no liquid).
4. Applications — straw, syringe, rubber sucker, inverted glass of water with cardboard.
5. Altitude — less air above → lower pressure → lower boiling point of water.
6. Real world — aircraft cabin pressure, mountain sickness, weather forecasting.

Why water stays in an inverted glass covered with cardboard (NCERT activity)

Step 1 — Fill glass completely; slide cardboard on rim (no air gap).
Step 2 — Invert quickly over sink.
Step 3 — Atmospheric pressure on cardboard (upward) ≥ weight of water column.
Step 4 — Remove cardboard carefully — water stays until seal breaks.
Conclusion: Air pressure holds water up.

• Thinking sucking pulls liquid up by itself (outside air pressure pushes liquid up).
• Misconception: vacuum in straw has no force (low pressure inside; higher outside).
• Confusing barometer (measures pressure) with thermometer.
• Assuming no air pressure in closed room (pressure still ~1 atm).

• Define atmospheric pressure.
• Why is it harder to breathe on high mountains?
• Name two devices that use atmospheric pressure.

Open the Practice tab for graded questions on Atmospheric Pressure.