Gases

Section 1: Gasses and Pressure

Collisions of air molecules generate pressure

Pressure- The force per unit area on a surface.
Pressure increase is based on the increase of the number of collisions amongst particles with the inside of the walls of its container.
The collisions cause an outward push or force against the walls.
The amount of pressure that is exerted by a gas depends on its volume, temperature, and number of molecules.
Ex: Increasing the temperature of a gas causes an increase in the temperature, which causes the KE(kinetic energy) of the particles increases, which increases the pressure because of the number of collisions that will occur.
The equation to calculate pressure: Pressure = Force/Area
Note: The larger the area of contact, the lower the pressure. The smaller the area of contact, the higher the pressure.
Summary: If there are more gas particles, then more collisions will occur, therefore the pressure will be increased.

### Pressure depends on force and area

The SI unit for force is a newton(N).
Newton- A force that will increase the speed of a one-kilogram mass by one meter per second each second that the force is applied.
At Earth’s surface gravity has an acceleration rate of 9.8m/s^2.
Ex: A ballet dancer with a mass of 51 kg, what amount of force would the dancer exert?
(51kg * 9.8m/s^2) = 500N
No matter where the dancer stands on Earth she will exert the same amount of force, however the pressure that is exerted against the floor depends on the area of contact that occurs.

Atmospheric Pressure

The atmosphere that surrounds the earth acts as a shell that exerts pressure.
The pressure of the atmosphere can be thought of as being caused by the weight of the gases that are found within the atmosphere.
The atmosphere contains 78% nitrogen, 21% oxygen, and 1% of other gases such as argon and carbon dioxide.

Atmospheric pressure is the sum of pressures of these individual gasses. (A concept that will revisit us again)

Barometers

Barometer- A device that is used to measure atmospheric pressure.
The first type of barometer was introduced during the early 1600s by Evangelista Torricelli in the early 1600s.
Toricelli sealed a long glass tube at one end and filled it with mercury while holding the open end with his thumb. He inverted the tube into a dish of mercury without allowing any air to enter the tube.
Once he removed his thumb the mercury column in the tube dropped to a height of 30 inches.
The space that is above the mercury in the tube acts like a vacuum. The mercury in the tube pushes downward because of the gravitational force.
The column of mercury in the tube only falls to a certain point because the atmosphere exerts pressure on the surface of the mercury outside the tube.
The pressure is transmitted through the fluid mercury and is exerted upward on the column of mercury.
The height of the mercury in the tube is contingent on the atmospheric pressure, or force per unit area. The amount of pressure measured in the tube is in terms of the mercury column’s height within the barometer.
The atmospheric pressure at any given place depends on the elevation and weather conditions.

If the atmospheric pressure is greater than the average at sea level, the height of the mercury column in a barometer will be greater than 760mm. If the atmospheric pressure is less, the height of the mercury column will be less than 760 mm.

Manometers

All gases exert pressure.
Manometers can be used to measure the pressure of an enclosed gas sample.
The difference in the height of the mercury in the 2 arms of the u shaped tube of a manometer is used to measure the oxygen gas pressure in the container.
Ex: Take a can filled with air. The atmosphere exerts pressure on the can. The air inside the can push outward and balances the atmosphere’s inward pressure.

Units of Pressure

Many different units are used to measure pressure.
Atmospheric pressure is often measured using a mercury barometer, pressure can be expressed in terms of the height of a mercury column.