Answer this please

[nishant]

while measuring th thermal conductivity of water the upper part is kept hot,why is this so?
is this so because heat conduction is easier downwards.

 

[SGT]

Cold water is denser than hot water, so it goes down. Since water is a bad heat conductor it takes a long time to equalize the temperature.
When you heat water, the fire is in the down position, so the heated, less dense water, goes up and is replaced by cold water. This phenomenon is called convection.

 

[nishant]

So what is the answer:To stop convection or it is easier for the heat to be conducted downward by keeping the upper layer hot?

 

[jdavel]

nishant,

Good question!

To measure any method of heat flow, you have to eliminate other possible methods. Heat flow by conduction is not affected by gravity. Heat flow by convection is affected by gravity, in fact, it's caused by gravity. So convection can only happen when there is warm water below cold water. Heating from the top eliminates that possibility, so it eliminates convection as a method of heat flow. All that's left is conduction.

 

[nishant]

I am still not able to get the answer totally correctly.BY maintaing the upper layer hot our main aim is to stop convection or is it the fact that if we maintain the upper layer hot we will be able to conduct heat downwards quite easily?

 

[OlderDan]

I am still not able to get the answer totally correctly.BY maintaing the upper layer hot our main aim is to stop convection or is it the fact that if we maintain the upper layer hot we will be able to conduct heat downwards quite easily?

It is to prevent convection from happening. If convection happens, you will not be able to measure the conduction. You cannot heat from the bottom without causing convection. Gravity will not make it easier for conduction to happen. If you could prevent convection with some other orientation, the conduction would be the same. But you can't eliminate convection with another orientation.

There is however an exception to this hot on top requirement. Water is most dense at 4C. If you wanted to measure the heat conduction in the range from 0C to 4C, you would want the colder water on top of the warmer, denser water.

 

[nishant]

but isn't it the same thing.by keeping the upper layer warm we are actually helping to increase the heat conduction downwards and make it easier?

 

[jdavel]

nishant,

You're not reading carefully.

Conduction of heat DOESN'T DEPEND ON GRAVITY! Conduction works just as well going up as down, or sideways, or any other direction.