Elenco Electronic Playground 50-in-1 Experiments - Manual (Page 40)

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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 24 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
Parallel
=
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.
Plunger
Water Pipe
Large Hose Filled
with Water
L
1
x L
2
L
1
+ L
2
Inductor
Symbol for
Inductor