# Tea Cup

1. Dec 30, 2005

### physicsman

I was trying to find out why when you mix a cup of tea with loose tea in it, the loost tea is first at the bottom of the cup on the outside, but when you take the spoon out, the loost tea moves towards the middle of the bottom of the cup. couldn't find it anywhere else online, thought i'd try here. thanks.

2. Dec 30, 2005

### Homer Simpson

I dont drink tea, but stirring it is like a centrifuge: loose stuff flies out to the edges. As for the second part, I dont quite get what you are asking. I guess if you lift a spoon out quickly, it will create a low pressure zone in that section of cup, which will pull in the loose stuff in a current. As well likely a convection set up as the hot tea in contact with the cold cup sides sinks, then moves up the middle.

3. Dec 30, 2005

### physicsman

i understand the centrifugal forces that cause the tea to move to the sides when you are mixing, but when you pull the spoon out, the tea keeps on turning and the loose tea slowly moves towards the center. i guess it must be something to do with convection currents and how the heat is escaping from the tea as you mix.

4. Dec 30, 2005

### Homer Simpson

lets assume the spoon removal makes no current at all.

The tea at the sides of the cup will lose temperature as it gives up heat to the outside air through the cup. So since this water is cooler than the rest it will sink right? This will force the tea in the center of the cup to rise from the bottom. There is the current which pulls loose stuff on the bottom towards the center.

This isn't homework, is it?

5. Dec 30, 2005

### physicsman

no, just something i was thinking about after i had some tea. i guess that makes sense. i've been told that it's a really complicated explanation, but it makes sense that the colder tea sinks from all sides, pushing the warm tea into the middle of the cup and up. the loose tea is more dense, so the leaves move in to the middle with the hot tea, but are too dense to rise with the current, so they just remain there. i guess that's a good enough explanation for me. appreciate the help.

6. Dec 30, 2005

### zoobyshoe

"Imagine a flat-bottomed cup full of tea. At the bottom there are some tea leaves, which stay there because they are rather heavier than the liquid thay have displaced. If the liquid is made to rotate by a spoon, the leaves will soon collect in the center of the bottom of the cup. The explanation of this phenomenon is as follows: the rotation of the liquid causes a centrifugal force to act on it. This in itself would give rise to no change in the flow of the liquid if the latter rotated like a solid body. But in the neighborhood of the walls of the cup the liquid is restrained by friction, so that the angular velocity with which it rotates is less there than in the other places nearer the center. In particlular, the angular velocity of rotation, and therefore the centrifugal force, will be smaller near the bottom than higher up. The result of this will be a circular movement of the liquid of the type illustrated in Fig. 1

which goes on increasing untill, under the influence of ground friction, it becomes stationary. The tea leaves are swept into the center by the circular movement and act as proof of its existence."

-Albert Einstein
The Cause Of The Formation Of Meanders In The Courses Of Rivers And Of The So-Called Baer's Law
From Ideas And Opinions Crown Publishers, 1982 edition

7. Dec 31, 2005

### Dr.Brain

While you are stirring with your spoon , the surface if the liquid(tea in this case) takes the shape of parabolic-curvature at the top , a condition something like a whirlpool is created , first the loose particles move with layers of liquid , each particle at a particular distance form the centre as long as you provide ample rotating motion to the layers, thus each layer has a certain 'omega' with which it rotates along with the loose particle work being done by your hand, but when you stop stirring no more centrifugal force is provided , and due to inertia , the liquid still maintains its rotational curvature but now with decreasing effect , and in absence of centrifugal force , the loose particles come under effect of whirlpool . and their distance from centre goes on decreasing ...and they start diverging towards the centre.

BJ

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