I would like to know what viscosity, density, and >>
releative density is
in words that I would be able to understand because the infoarmation
that I found in the library is not clear enough for me to understand.
(This information will be used fo my science fair project)
Oh well, I am not sure if I can do better than the library but I try.
Let's see. What is viscosity? Viscosity is a concept. It was created and
used for telling how easy or difficult it is for a fluid to flow. As such,
it is a measure of, or indicative of, a fluid's resistance to flow.
Here is a simple experiment you can do. Dip a spoon in a glass of water,
and another identical spoon in a jar of honey. Try to pull the spoons out
AT THE SAME SPEED. Which one is harder to pull out? The honey, of
course. So, we say that the viscosity of honey is higher than that of
water, or that honey is more viscous than water.
Next, heat up the honey jar slightly and do the same experiment. You will
see that upon heating, honey becomes more water-like, that is, less
viscous. You need less force to pull out the spoon at the same
speed. From this experiment we learn that viscosity of a fluid is affected
by its temperature.
Technically, viscosity is defined as the ratio of shear stress to shear
rate. It unit is Poise. You'd say huh?
To illustrate this complicated sentence, you can do this experiment. Take
a pencil eraser (or a piece of rubber) and two pieces of wood, each twice
the length of the eraser.
Glue one piece of the wood to one side of the rubber and glue the other
piece to the opposite face of the rubber. The pieces of wood must extend
in opposite directions.
Now, hold the ends of the wooden pieces and try the move them in the
opposite directions but in line with the length of the rubber. The
resistance of the rubber to be "sheared" (that is deformed in opposite
directions) as you pull the wooden handles is a measure of its
viscosity. How much force (shear) you need to apply to deform the rubber
by a given amount in a given time (shear rate) tells you what the viscosity
of rubber is.
You also note that the concept of viscosity applies not only to fluids but
also to plastics, rubber, and other things that can "flow" or deform.
Density is far simpler to describe. Suppose you have a carton of milk
whose volume is one liter (1000 cc).
If you fill it up with water and weigh it, it will be about 1 kg. So, the
density of water is about 1 1kg/liter or 1000 kg/cubic meter. Now, if you
empty the carton, and fill it up with honey and weigh it, it may be 1.5 kg
(as an example). Thus, density, which is a measure of how "dens" something
is, is 1.5 kg/liter or 1500 kilogram per cubic meter. Note that this has
nothing to do with how viscous honey or milk is. You can fill the carton
with sand and measure its density the same way.
Do viscosity and density ever mix up? No, never. They are unrelated to one
Why are people sometimes confused about these two? Well, there is a
reason. In some engineering problems you have to consider both viscosity
and density of a material. In those cases, a new term (concept) is coined
called "kinematics viscosity." This is simply the viscosity of the
substance (as discussed above) divided by its density. Why bother with
this? Well, because you do not have to carry two numbers around in your
calculations (for the same reason we say 2 instead of constantly saying 1
plus 1). Also, if you need to compare two fluids in your work, instead of
comparing their viscosity and then their density, you can just use their
kinematics viscosity and thus compare two instead of four numbers.
Finally, relative density... that is real easy. It indicates how dense
some material is with respect to another. Say, if you know that the
density of honey is 1500 kg/cubic meter and the density of water is 1000
kg/cubic meter, you can say that honey is denser by a factor of 1.5. Or
the relative density of honey is 1.5. Easy!
Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
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Update: June 2012