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## Arvind Narayanan's journal

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Puzzle [Sep. 17th, 2008|03:33 am]
Arvind Narayanan
 [ Tags | physics, puzzle ]

If you microwave a glass of tomato juice for a minute, the top gets warmer than the bottom. Why?

With water, not so much. Why not?

This puzzle brought to you by my inability to sleep the night before a trip and the fact that I get hungry every two hours.

 From: (Anonymous)2008-09-17 10:50 am (UTC) (Link)
Warm tomato juice is lighter than cold tomato juice.
And tomato juice takes longer to get hot than water, so the phenomena is more visible on tomato juice.

Good trip.
 From: 2008-09-19 02:04 am (UTC) (Link)
no, i don't think that explains it, see my answers below.
From: (Anonymous)
2008-09-17 03:56 pm (UTC)

### Viscosity?

Are convection currents in the juice slower on account of greater viscosity?

Ambarish
From:
2008-09-19 02:01 am (UTC)

### Re: Viscosity?

yes, i believe that's the reason why TJ behaves differently than water.

but why should either of them have a temperature gradient?
From: (Anonymous)
2008-09-28 10:06 pm (UTC)

### Re: Viscosity?

Microwaves penetrate less deep into TJ than into water on account of TJ's greater viscosity. The bottom central portion of the container is expected to be colder. Hotter liquid goes up, colder comes down. I don't think convection currents have any time to change something here.
From:
2008-09-30 02:30 am (UTC)

### Re: Viscosity?

you seem confused :)

convection is the name of the mechanism that causes hotter liquid to go up.
From: (Anonymous)
2008-10-07 09:24 pm (UTC)

### Re: Viscosity?

Density of most substances decreases with increase in temperature. Hotter liquid comes to the top on account of lower density.
From: (Anonymous)
2008-09-17 07:44 pm (UTC)

### It's how microwaves workd

Because microwave ovens only heat the water molecules, the water in the tomato juice gets heated and rises to the top whereas the rest of the liquid in the juice is not heated and sinks to the bottom. With water, there is more even heating since it is all water molecules.

UH2L
http://www.thingsivenoticed.com
From:
2008-09-19 02:03 am (UTC)

### Re: It's how microwaves workd

no. convection currents act to neutralize temperature gradients, not to create them.

even if they did, that would make the effect more pronounced in water, whereas in reality it's the opposite.
From: (Anonymous)
2008-10-31 10:54 am (UTC)

### Re: It's how microwaves workd

How consistent is the tomato juice? What if the top is water heavy and the bottom tomato heavy.