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Circuit Components: The 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?



The magnetic field around a current-carrying wire.

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.

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.



Magnetic fields around two wires with opposite currents.

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 around the pair of wires will actually be weaker than the magnetic field around a single wire, because they will partially cancel each other out. This phenomenon can be useful in cases where we want to minimize the magnetic field, but it doesn't help here, where we want to increase and control it.



Magnetic fields around two wires with matching currents.

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.

Is there an easy way to accomplish this?



The magnetic field around a coil of wire.

The figure to the left 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.



Some typical coils.

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.



Schematic symbols for inductors.

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.


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