# Force from fluctuations in atmospheric pressure

1. Dec 7, 2011

### HarryH

Say you have a giant mercury barometer. Let's suppose that the area exposed to atmospheric pressure is 1 square meter

Normal pressure at sea level is 101325 Pa
Lets say the variation in 24 hours in atmospheric pressure goes from a low of 100,000 Pa to a high of 102,000 Pa

Using

Pressure (Pa) = Force (N) / Area (m2)

I find that the change in pressure results in a Force of 2000N upon the mercury in the barometer.

Is this correct?

2. Dec 7, 2011

### technician

Yes.... the pressure changes by 2000Pa which is a change of 2000Newtons on every square metre of surface.... it sounds a lot but... 2000N is the weight of roughly 200kg
200kg spread on 1 square metre is 0.02kg on 1 square cm....20 grammes on a square cm

3. Dec 7, 2011

### technician

Another way to look at it... a change of 100,000 to 102,000 Pa is ONLY a change of 2%....not much.
Big numbers but small changes

4. Dec 7, 2011

### HarryH

Right.

So say if instead of using mercury, it was simply an instrument that stored then released this 2000N of Force over a given time and distance.

We can calculate work and power from this.

Potentially, could small amounts of energy be extracted from this?

(ie, extracting the energy that would normally be making the mercury move up and down a vial.)

5. Dec 7, 2011

### HarryH

true, but the starting and end points of pressure are surely irrelevant? wouldnt it be the relative change in pressure that matters for the equation?

6. Dec 7, 2011

### technician

I can see what you are getting at !!! But a word of caution!!.. do some calculations to determine the energy involved. A pressure change of 2000Pa will produce a change in height of a mercury column (or any liquid column for that matter)
The height change is given by :
hρg = Pressure so 2000 = h x 13,600 x 9.81 (the density of mercury is 13,600kg/m^3)
this gives a change in height of 0.015m (1.5cm)
If you do an energy calculation to find how much energy is needed to lift 1 square metre of mercury through a height of 0.15m you get (use mgh) 2000Joules (check my maths !!!!)
Is it worth it ???? who knows, who decides.
If you compare this with the energy that can be obtained from changes in water level due to the tides (nothing to do with atmospheric pressure) I think you would go for the tides.

7. Dec 7, 2011

### Curl

Don't some really pro wrist watches make use of this for energy, thus never need batteries or solar cells or winding?

8. Dec 7, 2011

### HarryH

Seems to be small changes in pressure due to atmospheric temperature though, not weight?