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