12 - Gases

Structure of Matter

need to go back to the particle model

• all matter (solids, liquids, gases) is made up of atoms

• means that all matter have some mass

Mass of Air

container w/ a gas in it will weigh more than a container that has been evacuate, air adds weight

Fluids

Substances that can flow and conform to the shape of their containers, which include both liquids and gases.

• gases can be compressed, fluids X & stay at their volume

• need a model to deal w/ gases

What is gauge pressure?

difference n/w the pressure in a container & the atmospheric pressure outside the container

Ideal Gas Model

1. gas particles take up X space on their own

2. gas particles only interact in perfectly elastic collisions

3. gas particles travel in straight-line trajectories b/w collisions

What is needed to describe gases?

pressure, density, mass

• m/volume → how many particles there are in a space

• ignore Fg’s effect, X parabolic motion, X see parabola forming due to speed

Pressure

imagine gas @ atomic level, bunch of particles bouncing around (P = F/A, N/m² or Pa)

• as particles hit walls of container, they undergo a change in direction

• impulse of wall on particle

• atoms exert an = & opposite impulse back on wall

• lots of particles = near constant force, horizontal components cancel e/o out,

• perpendicular net force to surface

• ex → fill up balloon, all Fs add up & spread out over container to keep shape

Measuring Pressure

barometers or manometers

• use differences in air pressure b/w air & enclosed gas to determine pressure

• absolute or gauge pressure measured

Atmospheric Pressure

the pressure exerted by the weight of the atmosphere, typically measured at sea level as 101.3 kPa or 1 atm.

• 1.0 atm = 1.0 × 10⁵ Pa (N/m²) = 14.7 psi

Absolute Pressure

total pressure on a system, all particles in container

Gauge Pressure

is the pressure measured relative to atmospheric pressure, indicating how much pressure is above or below atmospheric pressure.

• P_gauge= P_total - P_atm

Density

measure of how much mass ca fit into a volume, X vary for materials (p = m/V, kg/m³)