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Q & A: Water temperature

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Most recent answer: 10/22/2007
Q:
What affects water temperature
- alexander (age 9)
miami beach,usa
A:
Hi Alexander,

The temperature of a substance is a measurement of the average thermal energy per molecue of that substance. When something is hot, its molecules move around more rapidly, bouncing into each other in random paths. When it is cold, the molecules move more slowly. Anything that makes water molecules jiggle more rapidly heats the water up, and anything that takes this energy away cools the water down.

Thermal energy usually goes from one place to another by one or more of the big three mechanisms: conduction, convection, and radiation. I'll give examples with water to illustrate what each of these are:

Conduction:

You can put the water in a pot on an electric stove or hotplate. The jiggling atoms of the hotplate bounce on those of the pot, making them jiggle and these bounce on the water molecules making them jiggle. The hotplate, pot, and water have to touch each other to make this work.

Convection:

In the pot of water on the stove, the water on the bottom will get hot first. Hot water is less dense (lighter for the same volume) than cold water, and it will rise up, just as light solid objects will float. Cold water will fall to take its place, and this cold water will get warm by conduction in contact with the bottom of the pot. The net effect is to heat up all the water in the pot even though only the bottom is heated by conduction.

Radiation:

I see you're writing from Miami Beach! Radiation heating of water is just what the sun does -- sunlight shining on the ocean heats it up because sunlight has energy and when the water absorbs the sunlight it gets that energy. Water near the North Pole gets less sunlight and more darkness, and so it radiates its heat away in the form of infrared rays and cools down (and freezes).

There are lots of additional ways to change water temperature, which are varients on these:

Put an ice cube in the water (conduction, convection)

Put it in the refrigerator or freezer (conduction, convection).

Allow it to evaporate -- it takes energy (540 calories/gram -- these are calories with a little "c" - the calories on food are Calories: 1000 of the little ones make a big one) to evaporate water. This takes heat energy away from the water and cools it down (and convection evens out the temperature through the water).

Stir it up: Mechanical motion gets converted to random thermal motion by viscous drag.

Put it in the microwave: energy from the microwave fields shakes the water molecues around either by electrical conduction through a resistive material or by making the electric dipoles of the water molecules constantly move back and forth or both of these effects. (This is a form of radiative heating, but with much lower-frequency radiation than light.)

Can you think of more ways to affect water temperature?

Tom

(published on 10/22/2007)

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