Questions About Pressure

1. How the pressure matters - put a small rod on your palm and abobe it put a 100 gm weight. repeat the same experiment replacing the rod with a sharp pin. Force is same, the pressure aren't.

2. How does the pressure matter 2 - did you ever use old fashined cycle pump to inflate the tyre ? With a forvce on it , not even equal to your weight, you fill the tyre to carry a couple of person's weight. Again you are filling to pressure and then the same is carrying load due to increased area. We do it day in and out with hydraulic jacks, pressurising to 100T or more, by simple hand pumps.

3. The vapour pressure

As per Wiki,

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed container. All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.

The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

The vapor pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation. The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature where the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

What does it physically mean ?

If you keep a liquid (or even solids) - they tend to spontaneously evaporate into their vapour form (gaseous phase) - _{of course above a certain critical temperature, but let us forget that aspect..} Now these are alerady evaporating but then why don't they simply boil and become gaseous? It is because the atmospheric pressure holds it back. Once the vapour pressure crosses the atmospheric pressure, they win and the liquid boils. Of course it is clear that the vapour pressure is temperature dependent. The vapour pressure usually is the pressure in a closed container, in which you put the liquid, and wait till the system comes in equillibrium, ie the air above the liquid becomes saturated. If the pressure increases above this, the vapours will have a tendency to convert back to liquid form and if reduces, more vapours will emanate till a new balance is reached.

Pressure is very easy to understand at an intuitive level. Find a 100 kg man to help. Have him put on carpet slippers (soft shoes) and step on your toes. Now have him put on golf shoes (with spikes) and step on your toes. Got it?

Another example of pressure vs. force: bulletproof vests.

emc_c

Thanks a lot for your help