Hello folks,
Some good points already have been mentioned and i was online so i
thought i would add a little to this thread...
Inductance and capacitance are both properties of the universe.
What this means more or less is that inductance is present even
in space itself, even when you dont have any wire at all.
But i think your question was more aimed at finding the difference between
a straight plain ol' wire and a coil of wire (inductor) and it's a good question
because both the wire that makes the straight wire AND the wire that makes
the coil could both be the same length, so why is the coil considered so
different?
The main reason i think has been already given, but i'd like to do a comparison
of the straight wire that is say 3 feet long and a coil of wire wound on
a 5/16 inch diameter wooded dowel but uses the exact same length of wire,
which is also 3 feet exactly.
This means we have two constructions: one is 3 feet of perfectly straight
wire and the other is 3 feet of coiled wire, wound on a 5/16 inch coil form.
We choose 5/16 inch diameter because the circumference is roughly
1 inch which is a nice round number to work with later.
When the straight wire is energized with a current, a magnetic field is created
around the wire and this field exists out away from the wire to some distance.
The field density around the first inch of the wire is about the same as the field
around the second inch of wire too. Thus, whatever field we find within the
first inch we find within the second inch too, and third inch and so on.
Now when the coiled wire is energized, the first inch works almost like
the straight wire, except because the wire is in a circle here it forces
the field to have a different shape around the wire. The new shape
compacts the field inside the center of the loop and so forces the field
to reach farther out the front and back of the loop rather than out
from the sides as with the straight wire. It's like squeezing a round
balloon with both hands...the ends bulge out more.
Now we come to the next inch of the coiled wire. This does the same thing,
and because its field also bulges out the ends it interacts with the
field from the first inch of coiled wire that also has the bulging field.
It just so happens that because the two loops are oriented side by side
with the current flowing in the same direction in both inches of the coil
the two fields reinforce each other which creates a stronger field.
Thus, the two turns of wire at the first two inches of the coil have a
stronger field than the first two inches of the straight wire.
With the coil, this action takes place for each turn of wire.
With the straight wire, there is no reinforcement of fields so the
field strength is less in the straight wire.
Now when the fields in both contructions collapse, a current is
induced into the wires, and this is what induction is. The current is
higher when the initial field intensity is higher, or to put it another way,
the higher the field the more induction takes place.
In this way we have more induced current in the coil then in the straight
wire so the whole idea turns out to be that we wanted to make a
bigger value inductor with the same amount of wire!
This lowers the cost considerably, so inductors are made with coils
rather than straight wires.
As a side effect, it seems easier to house a small coil of wire than a
very long straight piece of wire.
Another way of looking at the coil as compared to the straight wire is this...
The induced current in the wire is subject to a distance as well as the
field itself. The closer the wire is to the source of the field the higher
the current induced. Since a wire that is three feet long is, after all,
three feet long, its field is SPREAD OUT over a fairly great distance.
When the field collapses, each inch of field can only work on that one
inch of wire. But when the field in the coil collapses, all the coils are bunched up
very close to each other so that when the field collapses each turn of
wire is very close to the source of the field, so the induced current
is higher, which again means the coil of wire has a much higher
value of inductance than the straight wire.