Notes - Inductance
Inductance: Some Useful Analogies
|
Quantity |
Context |
Description |
|
Mass |
Linear Motion |
Measure of the inertia of a system (resistance to motion) |
|
Moment of Inertia |
Rotational Motion |
Rotational equivalent of mass; resistance to rotational motion |
|
Inductance |
Current Flow |
Characteristic of a conductor that opposes a change in current flow |
|
Inductor |
|
One characteristic of electricity is that as current flows it
generates a magnetic field. The greater the current, the stronger
the magnetic field it generates. However, this magnetic field is
generally small and weak, and can't be used for very much. Indeed,
most of the time it doesn't have a noticeable effect on anything
less sensitive than a small compass needle. Is there a way we can
intensify this field so we can experiment with it and study its
properties?
In the figure to the right, electrons are moving through a wire from
left to right, as shown by the blue arrows. This motion of
electrically charged electrons generates a circular magnetic field
around the wire, and extending along the entire length of the wire,
as indicated by the green lines. The direction of the magnetic lines
of force shown here is upwards on the "front" side of the wire, and
downwards behind it. |
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You can always determine the direction of the magnetic field by applying the
Left Hand Rule: Grasp the wire in your left hand, with your thumb
pointing along the wire in the direction of electron flow. Your fingers will
curl around the wire, pointing in the direction of the magnetic field.
Note:
Under the original assumptions of conventional current, this was stated as
the Right Hand Rule, because current carriers were assumed to be
positive. Since we are using the more modern electron current
specifications, we must switch to a Left Hand Rule to correctly describe the
direction of the magnetic field.
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If we have two wires close together, with the same current flowing
through them but in opposite directions as shown to the left, the
magnetic field between the two wires will be the sum of the two
separate fields, and therefore will be stronger than the field
around a single wire. However, this doesn't help much — adding a third wire must reinforce one of these two, but oppose the other. Hmmmm. Maybe we can make use of this phenomenon, but clearly it won't work by itself.
On the other hand, if we put two wires next to each other with each
one carrying the same amount of current in the same direction (see
the figure to the right), an interesting phenomenon occurs. The
magnetic fields between the two wires oppose each other and cancel
out, but the overall field around both wires together is
strengthened. Adding more wires in this manner enhances this effect,
making the overall magnetic field still stronger. |
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The figure to the right shows a wire that has been wrapped into a
spiral structure, forming a coil. This structure combines
both effects of adjacent, current-carrying wires discussed above.
The magnetic field through the middle of the coil is directed from
left to right, and is highly intensified. This magnetic field gives
the coil some interesting and useful properties, which we will cover
in detail when we discuss the behavior of coils in an electrical
circuit.
The property conferred on this component by the concentrated
magnetic field is known as inductance. The effect of
inductance is to oppose any change in current through itself. It
does this by generating an EMF across its terminals which opposes
the applied voltage.
As a result, the current through an inductance can only change
gradually; it cannot change instantaneously as it could with only
resistors in the circuit. The coil will store or release energy in
its magnetic field as rapidly as necessary to oppose any such
change.
Is there an easy way to accomplish this?
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The unit of inductance is the henry (H). By
definition, one henry is that amount of inductance that will
cause a counter EMF of 1 volt to be generated when the current
changes at a rate of 1 ampere/second. Practical values of
inductance range from a few microhenrys (µH) up to tens of
henrys.
The image to the right shows a few typical,
commercially-available coils. The large one to the left is
mounted on an iron core to help concentrate the magnetic field
and thus augment the inductance of the component. It has an
inductance of 1 henry. To its right is a small coil with a
movable core made partly of powdered iron. This allows the core
to be adjusted to set the precise value of inductance, which is
on the order of 30 microhenrys (µH). In the foreground is a 50
millihenry (mH) coil, consisting of multiple layers of wire
wrapped on a non-magnetic core.
Each of these devices can be purchased directly, and each of
them has practical applications in electronics. |
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The schematic symbols to the right represent inductors, or coils.
Symbol A is used for a basic inductor with only air anywhere in the
magnetic field. Symbol B shows an inductor with a core made of
powdered iron (known as ferrite). Such a core helps to
concentrate the magnetic field somewhat, and so increases the
effective inductance of the coil. Symbol C shows a laminated iron
core. This kind of core concentrates the magnetic field greatly, and
therefore increases the effective inductance even more than a
ferrite core.
As you can see, in each case the symbol itself suggests the multiple
turns of wire that form the coil.
For a series connection of inductors, the total inductance, LT is equal to the sum of the individual inductances
For a parallel connection the same formula is found as we used for resistors