Peoples Physics Concepts

http://www.ck12.org Chapter 18. Thermodynamics and Heat Engines

18.3 Gas Pressure and Force

• Explain the basics of pressure and force, how pressure varies with altitude in our atmosphere, and the ideal
  gas law.

Students will learn the basics of pressure and force, how pressure varies with altitude in our atmosphere, and the

ideal gas law, which connects the concepts of volume, pressure, temperature, and the number of particles that

compose a gas.

Key Equations

P=F/A pressure equals the force divided by the area of application

P=P 0 e

−ah

exponential model for atmospheric pressure

PV=NkT Ideal Gas Law

An ideal gas is a gas where the atoms are treated as point-particles and assumed to never collide or interact with

each other. If you haveNmolecules of such a gas at temperatureTand volumeV, the pressure can be calculated

from this formula. Note thatk= 1. 38 × 10 −^23 J/K

Guidance

• The pressure of a gas is the force the gas exerts on a certain area. For a gas in a container, the amount of
  pressure is directly related to the number and intensity of atomic collisions on a container wall.


• Anidealgas is a gas for which interactions between molecules are negligible, and for which the gas atoms or
  molecules themselves store no potential energy. For an “ideal” gas, the pressure, temperature, and volume are
  simply related by the ideal gas law.


• Atmospheric pressure (1 atm= 101 ,000 Pascals) is the pressure we feel at sea level due to the weight of the
  atmosphere above us. As we rise in elevation, there is less of an atmosphere to push down on us and thus less
  pressure.

Example 1

How many molecules of gas does it take to equalize the pressure inside a 1 liter box, that originally starts with no

gas inside, with the atmospheric pressure at room temperature (300 K)?

Solution

To solve this problem, we just plug in the given values into the ideal gas law.