Year+13+Inductance

__**The Inductor**__

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 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 light bulb, a coil of wire around a piece of iron (yellow) and a switch. The coil of wire is an **inductor**. You'll hopefully recognize that the inductor is an electromagnet. If 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 behaviour is completely different. ­The light bulb is a **resistor** (the resistance creates heat to make the filament in the bulb glow). 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 behaviour 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**.

 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 flowing initially.  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 electrons**. ||
 * **Think About Water...[[image:water_boat.jpg align="right"]] **

=Henries = The **capacity** of an inductor is controlled by four factors:
 * The number of coils - More coils means more inductance.
 * The material that the coils are wrapped around (the core)
 * <span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">The cross-sectional area of the coil - More area means more inductance.
 * <span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">The length of the coil - A short coil means narrower (or overlapping) coils, which means more inductance.

<span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">Putting **iron** in the core of an inductor gives it much more inductance than air or any non-magnetic core would. The standard unit of inductance is the **henry**.

<span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;"> ­ =<span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;"> = =<span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">Inductor Application: Traffic Light Sensors = <span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">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. <span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif; font-size: 130%;">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!