1.7.Pressure GCSE Physics David Raju Vundi

Pressure Pressure is the amount of force applied to a known area. We can calculate it using the equation Pressure = Force (N) / Area (m2) We can reduce pressure by reducing the size of the force or increasing the size of the area.

Pressure and Density Density = Mass/Volume A property of the material. Pressure = Force/Area Depends on the height of the fluid. Same in all directions. Units are: Force/Area = N/m2. Pascals  1 Pa = 1 N/m2. Atmosphere  1 atm = 1.013 X 105 N/m2.

Pressure is due to the net force of the molecules in a fluid colliding with the walls. A very large number of collisions happen each second. Each collision exerts a tiny net force on the wall.

How to Measure Pressure Barometer measures atmospheric pressure Aneroid Barometer vacuum Patm PHg Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

vacuum air pressure mercury (Hg) barometer: device to measure air pressure

Barometer Empty space (a vacuum) Hg Weight of the mercury in the column atmosphere (atmospheric pressure) Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 401

The barometer measures Water column (34.0 ft. high or 10.4 m) Mercury filled 760 mm = 1 atm Water filled 10400 mm = 1 atm Atmospheric pressure Mercury column (30.0 in. high or 76 cm) The barometer measures air pressure

Barometers Mount Everest Sea level Sea level On top of Mount Everest

Pressure Experiment 100g SAND I think the bigger mass will sink further I think a bigger area will stop the mass sinking SAND

Measuring Pressure

Torricelli Barometer The mercury in the tube pushes down with its weight. The bottom of the tube is open to the atmosphere. The air pushes on the open surface of the mercury. On an average day, the pressure of the air equals the pressure exerted by a column of mercury 760 mm high. Above 760 mm, there is a vacuum in the tube. Weight of mercury

Barometer Detail Why doesn’t the diameter of the column of Hg make a difference? Recall that Pressure = force/area. The “force” is the weight of the mercury, but the pressure that results is that weight divided by the area of the column. So … a bigger column weighs more but also has a proportionally bigger area, and the two factors cancel one another out. The pressure caused by the column of mercury pressing down is independent of the diameter of the column.

Manometer A manometer is comprised of a bulb containing a gas and a U-shaped tube. The U-shaped tube is partially filled with mercury. The weight of the mercury puts pressure on the gas. If the U-tube is OPEN there is also air pressure acting on the gas. The gas molecules put pressure on the mercury. PHg

Closed Manometers There is a balance between the weight of the mercury on the left (PHg) and the pressure of the gas on the right (Pgas). The difference between the heights of the mercury on each side of the tube is a measure of the pressure of the gas. Pgas = Dh vacuum PHg

Open Manometers When gas pressure is greater than atmospheric pressure, the mercury is pushed toward the open end. The balance is between the gas on the right, and the air plus mercury on the left. Pair + PHg = Pgas The weight of the mercury is measured as the height difference: PHg = Dh So Pgas = Pair + Dh Pair PHg

Open Manometers When gas pressure is less than atmospheric pressure, the mercury is pushed toward the gas reservoir. The balance is between the air on the left and the gas plus mercury on the right: Pair = Pgas + PHg The weight of the mercury is measured as the height difference: PHg = Dh So Pair = Pgas + Dh Or Pgas = Pair- Dh PHg Pair

Sample Problems Find the pressure of the gas in each manometer. 1. 2. 20 mm Pair = 790 mm 1. h = 13 mm PAIR = 753 mm 2. PAIR = 765 mm h = 27 mm 3. Find the pressure of the gas in each manometer. Pay attention to whether the manometer is open or closed!

Sample Problem Answers 1. Pgas+ Dh = Pair Pgas= 790 mm - 20 mm = 770 mm Hg 2. Pgas = vacuum + 13 mm = 13 mm Hg 3. Pgas = Pair + Dh Pgas = 765 + 27 = 792 mm Hg