# Understanding pressure with barometer as example

• dhruv96patel
In summary, the mercury in a tube filled with mercury is suspended/rises to 760 mm mark due to pressure of weight of mercury being equal to pressure of atmosphere.
dhruv96patel
In a barometer where a tube filled with mercury is inverted in dish filled with mercury, the mercury in the tube is suspended/rises to 760 mm mark. I read this is due to pressure of weight of mercury being equal to pressure of atmosphere.

My question is that, if i were to take the tube out of dish, will the mercury still stay in as atmosphere pressure has not changed.
Whats the physics behind pipettes in which the liquid stays suspended and does not flow out even when its removed from the dish?

For this, my view of pressure is that in barometer, mercury molecules exposed to surface are under force of atmosphere and mercury molecules exert same force back and consequently have same pressure through out mercury in dish. When the tube containing mercury is put in dish, the mercury in tube applies force to mercury in dish and force of mercury in tube is greater. Thus, mercury flows out until its mass decreases enough so that force of mercury in tube equals that of mercury in dish. And this seems to happen when mercury is 760 mm high in tube at atmospheric pressure.

If my microscopic view is flawed, please correct it. And answer my question according to correct microscopic view of pressure.

Try the experiment yourself.
BUT use something less volatile than mercury ... say water in a tube, in a dish of water

My question is that, if i were to take the tube out of dish, will the mercury still stay in as atmosphere pressure has not changed.
Whats the physics behind pipettes in which the liquid stays suspended and does not flow out even when its removed from the dish?

Ahhh but will it really ?
pull the tube up out of the dish of water and see what happens
Consider the airpressure on the surface of the dish of liquid, and what effect it has on the column of liquid in the tube :)

Dave

You need to consider what the pressure is at the top surface of the mercury column in the barometer and on the top surface of the water in the pipette.

k so pressure on surface of dish would be same as atmospheric pressure 1 atm. pressure in tube at top would be 0 as its vacuum. So mercury stays suspended. But if i take the tube filled with mercury out of dish, pressure in tube at top will still be 0 and pressure at opening of tube (since its in contact with air) will be 1 atm. so shouldn't mercury still stay in and not flow out when i take the tube out of dish?

Theoretically yes. But it may be unstable if you tilt the tube slightly off vertical. The pipette works better in this regard, even though there isn't a perfect vacuum above the liquid.

There are two other forces you need to consider: surface tension (which is what keeps the liquid in the pipette) and gravity (which is what will make the mecury fall out of the tube).

Thanks a Lot every one!

## 1. What is pressure?

Pressure is defined as the amount of force applied to a certain area. In scientific terms, it is measured as force per unit area.

## 2. How does a barometer measure pressure?

A barometer measures pressure by using a column of mercury or air to balance the weight of the atmosphere. As the pressure changes, the height of the column changes, allowing for the measurement of pressure.

## 3. What is the relationship between pressure and altitude?

The higher the altitude, the lower the air pressure. This is because as altitude increases, there is less air above pushing down on the surface, resulting in a decrease in pressure.

## 4. Can pressure affect the weather?

Yes, pressure can affect the weather. Changes in pressure can indicate the movement of air masses and can result in changes in temperature, humidity, and wind patterns, which ultimately affect the weather.

## 5. How is pressure used in everyday life?

Pressure is used in many everyday applications, such as checking tire pressure, using a pressure cooker, and measuring blood pressure. It is also used in many industrial and scientific processes, such as in weather forecasting and studying atmospheric conditions.

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