pressure at the bottom of a lake - HELP PLEASE

samfisher2013

Hi everyone,

Could anyone help me with this question.

A lake has an average temperature of 10degrees celcius and a maximum depth of 40 m . if the atmospheric pressure is 598mmHg , what is the absolute pressure at the bottom of the lake? what is the gauge pressure?

(mercury density = 13600kg/m.cubed)
water density @ 10degrees celcius = 999.7 kg/m.cubed

I got 49200m for the first part, not sure how to do the second part.

Reply 1

Stonebridge

The "gauge pressure" is what would be measured by a depth gauge and just involves the depth of water. (p=hρg) The "absolute" pressure is the gauge pressure plus the atmospheric pressure acting on the surface.
http://en.wikipedia.org/wiki/Pressure_measurement#Absolute.2C_gauge_and_differential_pressures_-_zero_reference

its apporox 5 atm

Any answers anyone? I just need to double check my answers.

Reply 4

Stonebridge

Original post by samfisher2013

Any answers anyone? I just need to double check my answers.

If you want us to check your answers, post your working and your answers here.
What unit ("m"?) is the answer you posted originally?
You need to use either N/m²(Pascal), mm of Hg or Atm

Reply 5

samfisher2013

For the first part:

Pressure at bottom of lake = 10^5Pa + 1000kg/m^3(9.8m/s^2)(40m) =

492000kg/m.s^2

(edited 9 years ago)

Reply 6

samfisher2013

Original post by Stonebridge

If you want us to check your answers, post your working and your answers here.
What unit ("m"?) is the answer you posted originally?
You need to use either N/m²(Pascal), mm of Hg or Atm

For the first part:

Pressure at bottom of lake = 10^5Pa + 1000kg/m^3(9.8m/s^2)(40m) =

492000kg/m.s^2

Reply 7

Stonebridge

Original post by samfisher2013

For the first part:

Pressure at bottom of lake = 10^5Pa + 1000kg/m^3(9.8m/s^2)(40m) =

492000kg/m.s^2

The gauge pressure is, as I said in my 1st post, just hdg
where h is depth of lake, d is density of water, g is acceleration due to gravity. This is found from the three values you multiplied together above. (Highlighted.)

The absolute pressure is that value added to atmospheric pressure.
You have used a value of 10⁵ Pa but this is not the value given in the question, which is 598mmHg.
Do you know how to convert mmHg to Pa? You are specifically given the density of mercury in order to do this.