Atmospheric Pressure and Barometric Principles Study Guide
Definition of Atmospheric Pressure: Atmospheric pressure is the weight of the air above us pressing down on us.
The Weight of Air Column: Air has weight, and it pushes down on everything at the Earth's surface due to gravity.
Standard Values at Sea Level: At sea level, the average atmospheric pressure is the pressure from a column of mercury that is 76 cm high.
Derived Units at Sea Level:
In S.I. units: about 1.013 x 10^5 Pa.
In bars: 1.013 bar.
In millibars: 1013.25 mb.
Variation of Atmospheric Pressure with Altitude
Inverse Relatbriionship: As we go higher, atmospheric pressure decreases because there is less air above to push down.
Air Density: Air is thicker near the ground and becomes thinner as we go up.
Height of the Air Column: Higher up, there is less air above you.
Interpretation of Specific Readings: If the pressure on a hill is recorded as 68 cm of mercury, it means the pressure is less than at sea level (76 cm), showing that the hill is at a high altitude.
Construction and Operation of a Simple Barometer
Overview: A barometer measures atmospheric pressure.
Components:
A glass tube, about 1 m long, open at one end and closed at the other.
A container with mercury.
Clean mercury for the barometer.
Step-by-Step Construction:
Filling: Fill the tube with mercury without air bubbles.
Sealing: Close the tube's open end tightly.
Inversion: Turn the tube upside down in the trough of mercury, then remove the seal.
Equilibrium: The mercury level will adjust until it balances the atmospheric pressure.
Suitability of Mercury as a Barometric Liquid
Reason 1: High Density: Mercury is very dense, allowing a shorter height in the barometer (76 cm).
Reason 2: Low Vapor Pressure: Mercury doesn't evaporate easily, helping keep accurate pressure readings.
Reason 3: Visibility: Mercury is shiny and easy to see, making measuring simple.
Additional Advantage: Non-Wetting: Mercury doesn't stick to glass, keeping its height accurate.
Limitations of Water in Barometers
Reason 1: High Column Height (Low Density): Water is much lighter than mercury, needs a tall column (over 10 meters) that is hard to manage.
Reason 2: High Vapor Pressure: Water can evaporate, causing inaccurate measurements.
Reason 3: Adhesion: Water sticks to glass, making readings tricky.
Biological and Practical Effects of Altitude Changes
Leaking Ink Pens at High Altitudes: Ink pens might leak at higher altitudes due to pressure changes.
Nose Bleeding in High Places: When climbing, lower pressure can cause nosebleeds due to blood pressure differences.
Hearing Changes During Elevation: Pressure changes can affect how we hear; our ears need to adjust to changes in pressure.
Review Questions and Key Concepts
Q: What is the average atmospheric pressure at sea level?
A: About 76 cm of mercury.
Q: Why does atmospheric pressure change with altitude?
A: It gets lower because there is less dense air above us.
Q: Why is mercury better than water in barometers?
A: Mercury is dense, needing a shorter column, and has low vapor pressure, ensuring accurate readings.
The statement that the atmospheric pressure on a hill is 68 cm of mercury indicates that the pressure is lower than the standard atmospheric pressure at sea level, which is about 76 cm of mercury. This suggests that the hill is at a higher altitude, as atmospheric pressure decreases with an increase in altitude due to the reduced weight of the air column above.
- Overview: A barometer measures atmospheric pressure.
- Components:
- A glass tube, about 1 m long, open at one end and closed at the other.
- A container with mercury.
- Clean mercury for the barometer.
- Step-by-Step Construction:
- Filling: Fill the tube with mercury without air bubbles.
- Sealing: Close the tube's open end tightly.
- Inversion: Turn the tube upside down in the trough of mercury, then remove the seal.
- Equilibrium: The mercury level will adjust until it balances the atmospheric pressure.