Gas Laws

What is pressure?

Force per unit area.

SI unit of pressure?

Pascal (Pa) = 1 N/m².

Example: A 100 kg block on the ground—what is asked?

Calculate pressure exerted using area and weight.

Example: Pressure on swimmer’s hand 1.2×10⁵ Pa, area 8.4×10⁻³ m²—what is asked?

Calculate force acting on the hand.

How does gas pressure behave inside a small volume?

Equal in all points and in all directions.

Atmospheric pressure at sea level?

1.013×10⁵ Pa = 1 atmosphere.

What causes hydrostatic pressure?

Weight of the fluid above a given depth.

Example: Points 5.50 m beneath the surface—what is asked?

Find pressure at both locations (equal due to same depth).

What is a Fortin barometer used for?

Measuring atmospheric pressure.

What is a U-tube manometer used for?

Measuring absolute pressure or pressure difference.

What instrument measures blood pressure?

A sphygmomanometer.

What are Korotkoff sounds?

Sounds used to determine blood pressures when releasing cuff pressure.

Typical systolic/diastolic values?

120/80 mmHg.

Why do gases expand to fill their container?

Their particles move freely and spread out.

Under what conditions do gases behave ideally?

Low pressure & high temperature.

Why do real gases approximate ideal gases at low density?

Particles are far apart with negligible interactions.

State the ideal gas law relationship.

P∝nT/V. (Or PV = nRT)

What does the ideal gas law say about pressure?

Pressure ∝ number of moles & absolute temperature; pressure ∝ 1/volume.

Example: Alveoli (r = 0.125 mm, 14% oxygen, 310K)—what is asked?

Find number of oxygen molecules in a sac.

Conceptual example: Why do bubbles in beer grow as they rise?

Pressure decreases → volume increases (Boyle’s law).

Boyle’s Law (constant T, constant n)?

PV=constant.

Charles’s Law (constant P, constant n)?

V/T = constant.

Gay–Lussac’s Law (constant V, constant n)?

P/t = constant.

What thermometric property does a constant-volume gas thermometer use?

Absolute gas pressure.

What is absolute zero in Celsius?

–273.15°C.

Example: Helium in a sealed sphere at 1.013×10⁵ Pa, 20°C cooled to –196°C—what is asked?

New pressure if volume stays constant; discussion on effect of shrinking volume.

Example: Gas from 1 L compressed to 0.5 L at constant temperature—what is asked?

New pressure via Boyle’s law; discussion on slow vs. fast compression (slow = temperature constant).

Example: Why does aerosol spray feel cold?

Gas expands rapidly → temperature drops (adiabatic cooling).

What major topics are included in Topic 8?

• Pressure
• Pressure in fluids (hydrostatic pressure)
• Ideal Gas Law
• Boyle’s Law
• Charles’s Law
• Gay-Lussac Law
• Constant Volume Gas Thermometer