If you put a straw into a small cup of water, why is the water level in the straw higher than in the cup???

Question

Straw sticking straight up...

Just noticed this, this morning and thought it was odd.

The pressure should equalize even if the opening of the straw is at the bottom of the cup...I thought

Answer 1

Capillary action. The smaller the diameter of the tube, the higher the liquid will rise.

Answer 2

Hi Surface tension. The water forms a meniscus in the straw. Make the straw small enough and capillary action will draw the water up even higher.

Answer 3

Sorry to say but it is not because of surface tension. It is because of atmospheric (barometric) pressure. The straw is acting like a barometer with water instead of mercury. The demonstration that you have described utilizes Pascal's Principle which is "any change in the pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls".
h is the height of the water in the straw is determined by the density of the water and acceleration due to gravity.
h = P atm/ρ h2o x g
h = 1.013 x 10⁵ Pa / 1.00 x 10³ kg x 9.8 m/s² = 10.33
Normally mercury is used for barometers because of its density and liquidity. A large change in atmospheric pressure results in a rise or fall of millimeters of mercury. With water, which is far less dense, the barometer would have to be 10.33 meters tall to allow for changes in barometric pressure.

Answer 4

Surface tension. It exceeds atmospheric pressure inside the straw.

Answer 5

the molecular force of attraction between the water molecules and that of straw molecules on the surface is greater than that of attraction between water molecules.This force of no so heavy.The water moves up until the attraction force is dominated by weight of water.
This phenomenon is called as surface tension.

Answer 6

It is so, because of the upthrust which exist between the liquid and the straw surface