Water manometer to measure change in pressure

[BlueSkyy]

Homework Statement

An experiment to determine the specific heat of a gas makes use of a water manometer attached to a flask (the figure below ). Initially the two columns of water are even. Atmospheric pressure is 1.4 x 10^5 Pa. After heating the gas, the water levels change to those shown. Find the change in pressure of the gas in Pa.

https://chip.physics.purdue.edu/protected/GiambattistaMimg/chapter-09/fig-035.gif

Homework Equations

P = pgd (gauge pressure)
Pgauge = Pabs - Patm

The Attempt at a Solution

well, i took (1000 kg/m3)x(9.81m/s2)x(.04m) and got 392.4 kPa...that's 392400 Pa, right?
but it's not correct...where did i go wrong? i know that it is displaced .02m up and .02m down for a total of .04m, and the density of water is 1000kg/m3 (for this problem)

any help would be appreciated!

 

[rl.bhat]

1000x9.81x.04 = 392.1Pa., not kPa

 

[ogb p]

I would first commend you on your attempt at solving the problem using the given equations and information. However, it seems that you may have forgotten to take into account the atmospheric pressure in your calculation.

To find the change in pressure of the gas, we need to use the equation Pgauge = Pabs - Patm, where Pabs is the absolute pressure and Patm is the atmospheric pressure. In this case, the given atmospheric pressure is 1.4 x 10^5 Pa. Therefore, the change in pressure of the gas can be calculated as:

Pgauge = (1000 kg/m3)x(9.81m/s2)x(.04m) - (1.4 x 10^5 Pa) = 392.4 kPa - (1.4 x 10^5 Pa) = -1.2 x 10^3 Pa

This means that the pressure of the gas decreased by 1.2 x 10^3 Pa after heating.

In addition, it is important to note that the given figure is in gauge pressure, which means that it is already taking into account the atmospheric pressure. So, you can simply use the given value of 392.4 kPa as the change in pressure of the gas.

I hope this helps clarify your confusion. Keep up the good work in your scientific pursuits!