Elenco Basic Electronic Experiments - Manual (Page 40)

The Inductor: The inductor can best be described as electrical momentum (momentum is the power a moving object
has). In our water pipe analogy the inductor can be thought of as a very long hose wrapped around itself many times as
shown here:
Since the hose is long it contains many gallons of water. When pressure is applied to one end of the hose with a plunger
the water would not start to move instantly, it would take time to get the water moving. After a while the water would start
to move and pick up speed. (This is also similar to a long freight train, which takes more than a mile to get to full speed or
to stop). The speed would increase until limited by the friction (resistance) of the hose as normal. If you try to instantly
stop the water from moving by holding the plunger, the momentum of the water would create a large negative pressure
(suction) that would pull the plunger from your hands.
Inductors are made by coiling a wire, hence they are also called coils. From the above analogy it should be apparent that
a coiled hose will pass DC (a constant or unchanging current) with only the resistance of the hose, which in electronics will
be very low since the hose is a wire. If the pressure on the plunger is alternated (pushed then pulled) fast enough then
the water in the coil will never start moving and the AC (constantly changing current) will be blocked. Coils in electronics
follow these same principles - a coil will pass DC and block AC. Recall from above that a capacitor will block DC but pass
AC. When determining the response of a circuit to DC, inductors are treated as closed switches and capacitors are treated
as open switches. For the AC response, the values of the inductors and capacitors must be considered along with the rate
at which the current alternates (called the frequency). For DC changes to the circuit (called transients), such as closing
the switch to connect a battery to capacitor circuit, the circuit response is initially AC and then reverts to DC.
How do inductors in series and parallel add up? You saw in Experiment 26 that changing the connection point on the
inductor (to reduce the length of the coiled wire) reduced LED brightness. If you think of this in terms of the coiled hose
then it is easy - longer hoses will hold more water, hence more inductance. Two hoses in parallel will result in more water
coming out (less inductance), since the same water pressure applies to each hose. This situation should sound familiar
since inductances in series and parallel add together just like resistors do. For advanced students, the mathematical
relationship is (“L” represents inductance):
L
Series
= L
1
+ L
2
L
1
x L
2
L
Parallel
=
______________
L
1
+ L
2
The inductance is expressed in henrys (H, named after Joseph Henry who developed electromagnetic induction at the
same time as Faraday), or more commonly in millihenrys (mH, thousandths of a henry) or microhenrys (
μH, millionths of
a henry). A typical inductor and its symbol are shown below:
Inductors and Transformers: Our water pipe analogy we have been using all this time is not entirely accurate. Electric
current is not the same as water. It is a flow of sub-atomic particles called electrons that not only have electric properties
but also magnetic properties; in the water pipe analogy you would have to think of the water as containing millions of very
small magnets. Inductance expresses the magnetic effects between electrons flowing in the wire of a coil. The number of
turns (windings), diameter, and length of the coil affect the inductance, the thickness of the wire does not. The material
inside the coil also affects the inductance; if you wrap the coil wire around an iron bar (which has strong magnetic
properties) then the magnetic effects are increased and the inductance is increased. This does not apply to capacitors,
which store electric charge in an electric field, not a magnetic field.
40
PLUNGER
LARGE HOSE FILLED WITH WATER
WATER PIPE
Symbol for INDUCTOR
INDUCTOR