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
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
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.
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?
(republished on 07/27/06)