Pressure: Relative scale of the units
Before we go too much further it seems very important to me to help you develop a personal sense of what the units we just talked about mean in the world. Are they big units or small units? How do you interpret different values of pressure that you might see in the world? Do you have a personal sense of what that means?
Let's start with the notion that your fingertip (last joint of your index finger) has an area of about 3 cm2 (1 cm2 if you're under 12:)) Two fingers together is about 1 inch2 . Your cell phone has a mass of roughly 200 grams (for a newish one) which is a little less than 1/2 a pound. There are some fussy conversion factors but as a reference if you support your cell phone with 2 fingers what you feel is a pressure of roughly 3500 Pa or 0.5 psi. These are very rough but serve to give you a clear sense that a Pascal is a very small unit of pressure compared to 1 psi and neither one is terrifying. We would usually refer to 3500 Pa as 3.5 kPa for ease of discussion.
1 atm can be expressed as 100 kPa or 14.7 psi which is about what you would feel if you supported 29 cell phones on your fingertips. Put another way 1 atm is like holding 2 gallons of milk in a bag looped over two fingertips IF the bag handle is wide (why is this?)
Given that there are 760 torr in 1 atm how does it relate to Pa and psi?
