# Homework Help: Physics question on absolute pressure?

1. Jan 30, 2010

### delfinofrank

Hey, here's the question:

"If a gauge reads -295mmHg where athmospheric pressure is 101.7 kn/m^2 and the vapor pressure is 2.8 kN/m^2, what is the absolute pressure?"

I converted the gauge pressure to 39.36 kN/m^2 but where do i go from here??!?! I know the equation is p_gauge= p_absolute - atmospheric, but where do I factor in the vapor pressure?!?!?!?!?!

2. Jan 30, 2010

### PhanthomJay

The water vapor pressure in the atmsosphere contributes to its atmospheric pressure. So if the atmospheric and gauge pressures are given, the absolute pressure is__________??

3. Jan 30, 2010

### delfinofrank

Are you sure it's water vapor pressure or could it be the vapor pressure of the mercury?

4. Jan 30, 2010

### delfinofrank

What I did was I added the gauge pressure and the athmospheric pressure and then subtracted the vapor pressure to get 59.34 kN/m^3. is that right?

5. Jan 30, 2010

### PhanthomJay

When a pressure reading is given in mmHg, it is just an alternative unit of pressure which can be (like you did) converted to paschals or atmospehres, etc. In the US, atmospheric pressure is usually expressed in inches of Hg, or millibars, and there is no mercury in the air (or at leasts not much)!

I interpret vapor pressure as water vapor pressure in the atmosphere, although there are other vapors present, like oxygen and nitrogen, the sum total of which contributes to the atmospheric pressure. Thus, I do not agree with your answer.