An inductor is
about as simple as an electronic component can get -- it is
simply a coil of wire. It turns out, however, that a coil of
wire can do some very interesting things because of the magnetic
properties of a coil.
In this edition of HowStuffWorks,
we'll learn all about inductors and what they're used for.
In a circuit diagram, an inductor
is shown like this:
To understand how an inductor can work in a circuit, this
figure is helpful:
What you see here is a battery, a
bulb, a coil of wire around a piece of iron (yellow)
and a switch. The coil of wire is an inductor. If you
have read How
Electromagnets Work, you might recognize that the inductor
is an electromagnet.
you were to take the inductor out of this circuit, what you
would have is a normal flashlight. You close the switch and
the bulb lights up. With the inductor in the circuit as shown,
the behavior is completely different.
water...One way to
visualize the action of an inductor is to imagine a
narrow channel with water flowing through it, and a
heavy water wheel that has its paddles dipping into the
channel. Imagine that the water in the channel is not
Now you try to start the water flowing. The paddle
wheel will tend to prevent the water from flowing until
it has come up to speed with the water. If you then try
to stop the flow of water in the channel, the spinning
water wheel will try to keep the water moving until its
speed of rotation slows back down to the speed of the
water. An inductor is doing the same thing with the flow
of electrons in a wire -- an inductor resists a
change in the flow of
The light bulb is a resistor (the resistance creates
heat to make the filament in the bulb glow -- see How Light
Bulbs Work for details). The wire in the coil has much
lower resistance (it's just wire), so what you would expect
when you turn on the switch is for the bulb to glow very
dimly. Most of the current should follow the low-resistance
path through the loop. What happens instead is that when you
close the switch, the bulb burns brightly and then gets
dimmer. When you open the switch, the bulb burns very brightly
and then quickly goes out.
The reason for this strange behavior is the inductor. When
current first starts flowing in the coil, the coil wants to
build up a magnetic field. While the field is building,
the coil inhibits the flow of current. Once the field is
built, current can flow normally through the wire. When the
switch gets opened, the magnetic field around the coil keeps
current flowing in the coil until the field collapses. This
current keeps the bulb lit for a period of time even though
the switch is open. In other words, an inductor can store
energy in its magnetic field, and an inductor tends to
resist any change in the amount of current flowing through it.
The capacity of an inductor
is controlled by four factors:
Putting iron in the core of an inductor gives
it much more inductance than air or any non-magnetic core
- The number of coils - More coils means more inductance.
- The material that the coils are wrapped around (the
- The cross-sectional area of the coil - More area means
- The length of the coil - A short coil means narrower (or
overlapping) coils, which means more inductance.
The standard unit of inductance is the henry. The
equation for calculating the number of henries in an inductor
H = (4 * Pi * #Turns * #Turns * coil
Area * mu) / (coil Length * 10,000,000)
The area and length of the coil are in meters. The term
mu is the permeability of the core. Air has a
permeability of 1, while steel might have a permeability of
Let's say you take a coil of
wire perhaps 6 feet (2 meters) in diameter, containing five or
six loops of wire. You cut some grooves in a road and place
the coil in the grooves. You attach an inductance meter to the
coil and see what the inductance of the coil is.
Now you park a car over the coil and check the inductance
again. The inductance will be much larger because of the large
steel object positioned in the loop's magnetic field. The car
parked over the coil is acting like the core of the inductor,
and its presence changes the inductance of the coil. Most traffic
light sensors use the loop in this way. The sensor
constantly tests the inductance of the loop in the road, and
when the inductance rises it knows there is a car waiting!
Usually you use a much smaller coil. One big use of
inductors is to team them up with capacitors
to create oscillators. See How
Oscillators Work for details.
For more information on inductors and related topics, check
out the links on the next page.
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