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enter image description here

I have a basic question on a closed manometer:

I see that in this case the pressure of gas is lower than that of the liquid, which is why the liquid is higher on the left side. Now, what I don't understand is why do we say that the gas is at a negative pressure? Wouldn't that mean there's no gas at all? Also, how do we know that it's negative? Usually, I only take the difference between the highest point and the lowest.

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The manometer shown doesn't measure absolute pressure, just relative pressure. In this car the pressure of the gas in the closed tube versus the pressure of the gas in the bulb.

As you have drawn it, the pressure of the fas in the tube is higher. If you measure pressure in the bulb relative to the pressure in the tube the answer is negative. If you measured absolute pressure (which this manometer can't do because of the lack of any external reference in the closed system) both would be positive but one would be higher than the other.

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If the manometer were open at both ends, the two fluid levels would be identical. If the bulbed gas were at ambient pressure, ditto. That the fluid is higher at the bulb end says the pressure in the bulb was lower than ambient pressure, and lower still for the weight of the differential fluid head, height times density.

Why is the diameter of the tube, short of capillary tubing, irrelevant? You can then tell us the "negative" pressure in the bulb.

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