Can anyone tell me why, in a level pipe, pressure increases with the radius? It makes no sense to me cause I would think the opposite would be true... That an increase in radius would decrease pressure. Studying for my physics final tomorrow and like everyone got this question wrong on the last test, my professor wouldn't really explain it :S
I'm no expert, but had to do a lil Googling cause I too assumed it'd decrease with a radius increase. Seems it has to do with Bernoulli's principle, states that the sum of the pressure, altitude, and velocity will be equal throughout the pipe. So if your pipe is 1 inch wide and it moves into a 3 inch wide section, the substance will have to spread out to fill the volume and therefore decrease the velocity (speed) of your flow. Altitude won't change, well it might, but just assume altitude is constant. A decrease in your velocity would then make the pressure increase as it'd take more force to move it through the pipe. If it goes back down to a 1 inch pipe, your velocity will increase and therefore decrease the pressure it takes to move it through.
@[member="Mantikore"] is on it for the most part. if you want a full explanation complete with demonstrations and math, check out Prof. Walter Lewin's lecture on the subject at MIT http://www.youtube.com/watch?v=ngABxM7jl0Q
Didn't get to see this before I took my final but thanks for the answers guys! Makes much more sense now