Measuring Pressure
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Transcript of 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 DetailWhy 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 = h
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 = h
So Pgas = Pair + h
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 = hSo Pair = Pgas + h
Or Pgas = Pair- h
PHgPair
Sample Problems
PAIR = 765 mm
h = 27 mmh = 13 mm
PAIR = 753 mm
h = 20 mm
Pair = 790 mm
Find the pressure of the gas in each manometer.Pay attention to whether the manometer is open or closed!
1. 2. 3.
Sample Problem Answers
1. Pgas+ h = Pair
Pgas= 790 mm - 20 mm = 770 mm Hg
2. Pgas = vacuum + 13 mm = 13 mm Hg
3. Pgas = Pair + h
Pgas = 765 + 27 = 792 mm Hg