Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Paramjit Kaur on 08/05/2010 23:30:02
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Paramjit Kaur asked the Naked Scientists:
Hi Chris
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I will be sitting for my science exams and would like to know if you could help me with a science question.
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The questions is :-
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A can of milk is taken and a small hole is pierced to the can.
Why does the milk flows slowly when the hole is small but when the hole is enlarged the milk flows faster?
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Greatly appreciate your assistance.
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Regards
Pam
What do you think?
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I think you should study viscosity.
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To a first approximation there is no need to study viscosity it is simple dynamics.
Let us assume the hole is in the base of the can and there us a particular depth of liquid in the can this means that there is a particular pressure (force per unit area) on the base of the can. This force will be used to accelerate a particular volume of milk as it flows through the hole. you can think of this as a small column of liquid. if the hole is made bigger the force remains the same and to a first approximation so will the length of the column of liquid but the area of the hole will increase therefore increasing the flow in proportion to the area of the hole. viscosity will just cause deviations in this simplistic dynamic model.
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Paramjit Kaur asked the Naked Scientists:
Hi Chris
I will be sitting for my science exams and would like to know if you could help me with a science question.
The questions is :-
A can of milk is taken and a small hole is pierced to the can.
Why does the milk flows slowly when the hole is small but when the hole is enlarged the milk flows faster?
Greatly appreciate your assistance.
Regards
Pam
What do you think?
WAWAWait are you saying the fully sealed can of milk is poked to make a single small hole?
The fluid viscosity may have something to do with it with respect to the fluid's membrain at the opening but, Atmosphereic pressure has a major affect here. In a single vented container, tipping the container so gravity will do some of the work.
The fluid membrain in the orafice pathway cross section will need to be pushed aside to permit the exchange of external to internal atmospheric pressure. Coin the phrase (vapor lock) when this process is prohibited. Coin the phrase (bottle neck), when this process is enhibited!
During the draining process, the fluid and air flows need to share this orafice creating a pulsing (plop, plop) action caused by this exchange. This is one of the senarios that slows fluid flow. Fluid Viscous can play a certain percentage part by not moving out of away fast enough to permit the air to exchange the same measured volume ratio of fluid to air with each pulse(in with air)/(out with the fuid).
The bigger the hole or larger orafice cross section the easier the exchange the faster the flow.
ie... same force applies here too...
Fill the sink with water and submerge a sturdy plastic see thru cup so it is 3/4 full of water use different level it will always work.
Flip it under the water not letting it drain. Holding the cup by its bottom and lift it straight up from the water not letting its mouth exposed to the surface air. You see you have lifted the water out of the sink due to the vapor lock in the cup. Now take a paper plate and place it so it seals the mouth and hold it there while finish lifting it out of the water holding the bottom, (opposite end of the mouth of the cup), firmly.
Release your other hand holding the plate. What happens?
Some of the oldest things can always enlighten the mind.
Since we have some idea, we can solve this by using a clean can of your non-carbonated drink with a flip top opener.
Hold the can sideways and at the side bottom edge poke the small hole there. Now use a clean bowl to avoid a mess, orientate the can upright so that the hole at the bottom of the can faces the inside center of the bowl. Like playing targetting the battleship on the porcelain not to wet the walls and extreme areas.
Note: a slow fluid flow or non at all depend on the size of the cross section of the hole.
Here we see there is an attempt of air pressure moving its way into the can.
We are not satisfied with this action? So, Haaaaa relief is on the way. Let us flip the top off the can watch the flow. Now the inside and outside pressure are not opposing each other relative to the fluid. c.i.p.
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My first reply assumed that the top of the can was open at the top like a bucket. The question is not clear on this matter. If the top is closed like a baked bean tin you must remember that liquid will initially flow out until the air pressure inside gets low enough to prevent it and then whether more flows out depends on whether air is sucked back into the opening like a bottle glugging as you empty it.