You know that a magnet can attract some objects, but not others.
For instance, a magnet will attract a paper clip, which is made
of metal, but not an apple. That is not all. If you try to attract
an object made of copper or aluminum with a magnet, it will not
work. What this tells you is that there must be a property that
some materials have that makes them feel the attraction of a magnet.
An apple, a US penny (which is made of copper), or a piece of aluminum
do not have this property and they do not feel the attraction of
a magnet. On the other hand, a paper clip does. Something else you
may want to think about is that the shape of an object does not
appear to change this property: if you cut the apple into slices,
still it is not attracted by the magnet; and if you uncoil the paper
clip, it still is attracted. So, shape does not account for this
similar concept holds in the case of electricity. You know there
exists a property of objects, called charge. Charge can only be
positive, negative, or zero. If an object has a positive charge,
you can say it is positively charged. If it has a negative charge,
you can say it is negatively charged. If an object has no charge,
you can say it is uncharged, or neutral. You have learned that,
if two objects have like charge (both positive or both negative),
they repel. On the contrary, if they have opposite charge, they
attract. You may have been told that if both objects are neutral,
they are indifferent to one another - they neither attract nor repel.
The idea that neutral objects neither attract nor repel one another
seems to make a lot of sense. Most of the time, you are neutral
and the walls of your house are neutral, too. And, in fact, you
do not feel attracted by the walls of your house! However, as most
things you learn in physics, this concept is only valid to a point.
It is important for you to understand that, in fact, neutral objects
do act on one another when they are very close. This is very important
in practice in a new field of technology where everything is extremely
small. This field is called microtechnology.
The word ``micro'' refers to the fact that, in this type of technology,
things are as small as one-millionth of a meter, or even smaller.
When you studied the decimal system, you have seen the length of
a meter. You have also seen that, if you divide a meter into one
thousand parts, you get millimeters. A millimeter is quite small,
but you can still see it. Now imagine dividing that one millimeter
into a thousand parts again! What you get is called a micrometer.
You cannot see a micrometer on your meter stick - it's just too
small. You know that the diameter of a circle is its size measured
by a segment going across its center. Well, the diameter of any
human hair is approximately 100 micrometers. Imagine a small machine
made of parts the size of a small fraction of a human hair. Building
such small machines is what microtechnology is all about.
Since in microtechnology every part is so very small, everything
is also very close. For instance, in some micromachines engineers
build something that looks like a microbridge crossing over a microgap
below (can you understand the meaning of all the words that begin
by the word micro- in this sentence?). Well, if the microbridge
is built too low over the material below, it may be attracted by
it so strongly that it sticks to it! And yet, both the bridge and
the material below are completely neutral! Therefore, engineers
must be careful to not build micromachines with parts that are so
close that they may stick together. These forces that cause small
parts to stick together are called surface forces, because they
act between the surfaces of each object. For instance, the lower
surface of the bridge and the upper surface of the material below
attract one another.
Well, you know about electricity and about magnetism. You also know
that electricity and magnetism are used in very many appliances
and machines that you use in every day life. Electric motors, computers,
lights, television sets, telephones - all these have been made possible
by understanding and exploiting electricity and magnetism. That
is, when you understand a force, you can exploit that force to make
a machine out of it, or an appliance that will do some type of work
Nowadays, scientists and engineers are trying to see whether it
is possible to make new machines and new appliances that will use
surface forces instead of electricity and magnetism. Since surface
forces are very strong when the machine parts are small and so very
close, it surely makes sense to try and see if we can use this to