Wednesday February 27, 2013 (Measuring Pressure and Units of Pressure)

To describe a gas fully, what four measurable quantities must be stated? How are these four quantities interrelated? volume temperature number of molecules pressure Any one is dependent upon the other three. If you know three, you can determine the fourth.

Announcements Happy Inconvenience Yourself Day! Inconvenience Yourself Day is an opportunity to focus on inconveniencing ourselves instead of inconveniencing others. It is also a day to recognize and acknowledge those who inconvenience themselves for others. Acknowledgment can be verbal, a note, or some small token of appreciation.

Announcements I will inconvenience myself and be available after school today until 5:15.

Assignment Currently Open Summative or Formative? Date Issued Date Due Date Into GradeSpeed Final Day QUIZ 19S42/15 FRIDAY WS – The Nature of Gases F192/202/21 FRIDAY QUIZ 20S52/22 3/8 TELPAS Writing Sample F12/253/1 FRIDAY WS – Gas Pressure and Force F22/262/28 FRIDAY

Measuring Pressure A barometer is a device used to measure atmospheric pressure. A barometer is a long glass tube filled with mercury. The space above the mercury in such a tube is nearly a vacuum. The mercury in the tube pushes downward because of gravitational force.

Measuring Pressure The column of mercury in the tube is stopped from falling beyond a certain point because the atmosphere exerts a pressure on the surface of the mercury outside the tube. This pressure is transmitted through the fluid mercury and is exerted upward on the column of mercury.

Measuring Pressure The mercury in the tube falls only until the pressure exerted by its weight is equal to the pressure exerted by the atmosphere. The exact height of the mercury in the tube depends on the atmospheric pressure, or force per unit area.

Measuring Pressure The pressure is measured directly in terms of the height of the mercury column supported in the barometer tube. At sea level at 0°C, the average pressure of the atmosphere can support a 760 mm column of mercury.

Measuring Pressure At any given place on Earth, the specific atmospheric pressure depends on the temperature, the elevation and the weather conditions at the time. 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 760 mm. If the atmospheric pressure is less, the height of the mercury column will be less than 760 mm.

Units of Pressure A number of different units are used to measure pressure. Because atmospheric pressure is often measured by a mercury barometer, pressure can be expressed in terms of the height of a mercury column. Thus, a common unit of pressure is millimeters of mercury, symbolized mm Hg. A pressure of 1 mm Hg is now called 1 torr.

Units of Pressure The average atmospheric pressure at sea level at 0°C is 760 mm Hg. Pressures are often measured in units of atmospheres. One atmosphere of pressure (atm) is defined as being exactly equivalent to 760 mm Hg. In SI, pressure is expressed in derived units called pascals. One pascal (Pa) is defined as the pressure exerted by a force of one newton (1 N) acting on an area of one square meter.

Units of Pressure In many cases, it is more convenient to express pressure in kilopascals (kPa). The standard atmosphere (1 atm) is equal to × 10 5 Pa, or kPa. The pressure units are summarized in the Table on the next slide.

Units of Pressure

Standard Temperature and Pressure Pressure may also be expressed in units of inches of mercury, millibars, and pounds per square inch. The equation formula for the different units of pressure is as follows: 760 mmHg = 760 torr = 1 atm = x 10 5 Pa = kPa = in Hg = millibars = 14.7 psi.

Standard Temperature and Pressure To compare volumes of gases, it is necessary to know the temperature and pressure at which the volumes are measured. For purposes of comparison, scientists have agreed on standard conditions of exactly 1 atm pressure and 0°C. These conditions are called standard temperature and pressure and are commonly abbreviated STP.

Worksheet Measuring Pressure Units of Pressure