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| (Source: Joey Rozier, http://mrjoro.org/blog/2005/01/photographic-frustration.html) |
Have you ever wondered how stoplights sometimes seem to change in your favor just as you drive up to them? The answer lies in a particularly useful electromagnetic device called an inductor.
Cut to the Chase
Underneath the asphalt roadbed lies a wire; it may be shaped in a rectangle, parallelogram, octagon, or any other shape with opposing parallel sides. The ends of this wire connect through pipe channels in the ground to the grey box that can be found next to any stop light; this forms a closed-loop circuit. When your car's steel chassis is over the loop in the road, the metal in the chassis disturbs the 'status quo' of the magnetic field surrounding the wire loop in the road.
In other words, before your car approached the light there was a magnetic field of a certain value, and the sensor detecting the current flowing through the wire in the road was accustomed to that state. However, in driving up to the light you're changing the environment of the loop. The current within the wire reacts to that change through the fundamental law of physics known as Faraday's Law. This altered current that was spawned as a reaction to your car driving up travels through the wires to the sensor in the grey box, which then prompts the light to change and allows you to go on your way.
A Longer Explanation
Introduction to Inductors
An inductor is a coil of wire centered around a core. The core can be either magnetic (ferrous) or non-magnetic (nonferrous)--this includes air! Inductors are commonly either cylindrical or toroidal in shape--imagine a donut with wires wrapped around it (in this case, the edible part of the donut is the core). The wire may have space in between each revolution or be wound tightly adjacent to each other. The inductor must be wired into a circuit in order for electrical current can flow through an entire loop.What do inductors do?
The chief behavior of an inductor is to oppose any change to the magnetic field that surrounds it by generating a 'voltage' like a battery. The field can be quantified in terms of the magnetic flux, which is a product of the strength of the magnetic field in the core region and the area of the wire loop encircling it. This concept is called Faraday's Law, after its creator, Michael Faraday, the British chemist and physicist. Magnetic flux can be described as a density of field lines inside the wire loop; a low number of field lines inside an area corresponds to a low magnetic flux, while conversely a tight packing of field lines within the same area is a high magnetic flux._________________________________________________________________________________
Let's deconstruct the effects of Faraday's Law. Think of each magnetic field line as a pool noodle, and imagine that you want to fit as many noodles as you can through a hula hoop. The easiest way to accomplish this task would be to hold the hula hoop perpendicular to the pool noodles so that the biggest possible area can be filled with pool noodles.
Now imagine holding the hula hoop slanted, at an angle from vertical, but trying to still fit horizontal pool noodles through it. You won't be able to fit as many through because the effective cross-sectional area of the loop has shrunk! This example represents a decrease in magnetic flux; you still have the same number of pool noodles as before (i.e. the strength of the magnetic field is the same) but your hula hoop has effectively decreased in size and therefore can't fit as many noodles through it (i.e. the cross-sectional area of the loop has decreased).
There's one more aspect of inductance that is extremely important to consider: inductors depend on a change in magnetic flux to operate. In other words: if you had the same number of pool noodles within your hula hoop the entire time, to an inductor it would be equivalent to having no pool noodles inside the hoop the entire time. In order for inductance to work, the number of field lines within a loop has to change over time. In the first example, we kept the location of the field lines/pool noodles the same and changed the dimensions of the area we used to contain them. In the second example below, we will keep the size and location of the area/hula hoop the same and change the position of the field lines to acheive the same effect: a change in magnetic flux.
Here's a similar example, using field line location as the independent variable: If you place the hula hoop on the ground, hug a bunch of pool noodles, and run over the hula hoop while recording the whole thing with a video camera, you could pick out the frame where all the pool noodles you were holding were completely inside the loop. That instant in time corresponds to the greatest magnetic flux because the biggest number of pool noodles were inside the hula hoop. If you watched your video in slow motion, you could see increasingly more of your armful of pool noodles fitting into the loop until the moment that they were all inside.
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They say good things come in threes....here, have a third example! Now, we are going to keep the position of our loop (the hula hoop) and our field lines (the pool noodles) the same the whole time. The element of change here is going to come from an increase or decrease in the amount of pool noodles you're holding in your arms. You stand inside the hula hoop on the ground and hold a couple of pool noodles vertically. Your trusty buddy whom you've recruited to help you explore physics has the rest of the pool noodles from before, and hands them to you one by one.
By the time they hand your the last noodle, your arms are stretched from holding all the pool noodles! As you have increased the number of pool noodles that you're holding, you've been increasing the magnetic flux through our hula hoop loop (like the analogy to magnetic flux being a 'density' of magnetic field lines). If you were to start the test initially holding all the pool noodles and giving them to your friend, you would be decreasing the magnetic flux through the loop.
These three examples, although somewhat silly, illustrate the necessity for some quantity to be changing in order for Faraday's Law--the principle governing the prompt response of stop lights--to act.
That's all fine, but how does this relate to cars?
When your car pulls up to a red light, it passes over a wire loop embedded within the road's surface, which is the equivalent of our hula hoop, to continue the analogy. There may be 1 or more turns (revolutions) in the wire loop which increases the strength of response by an amount proportional to how many turns exist in the loop. In other words, if you double the number of turns in the loop, you will double the change in magnetic flux as a result of any given magnetic field/area combination. |
| Here's how you know there's a loop in the road underneath you! (Source: http://modernvespa.com/forum/wiki-trafficsignals) |
Now, your car has a metal chassis, which makes it a good conductor of electricity. The ability of this electrical conductor to affect the magnetic field around the loop is equivalent to the number of pool noodles you can hug as you run across your backyard over your hula hoop. When there is no car over the loop in the road, the inductance is a baseline value. The gray box (often visible next to many stoplights) contains a sensor that reads the current through the loop in the road; it becomes accustomed to the current resulting from that baseline level of inductance. However, the sensor in the gray box is always testing the loop to see if the inductance changes. When your car pulls up to the light, the amount of field lines in the loop changes--it increases by a lot! The sensor no longer reads a baseline level of inductance; the addition of field lines caused by the addition of the car means that the inductance is now much higher. Once the sensor detects that difference, the computer in the grey box tells the light to change the signal, and off you go on your way.
Thanks for reading! If you have any questions or comments about what I've covered here (or any ideas as to what I should cover next), feel free to post a comment below! Alternatively, send me an email at nghagler@ucdavis.edu and include 'Blog' in the subject line to tell me what you think!
References
http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/question234.htm
https://www.youtube.com/watch?v=KvzJn09DqaM#t=376
http://modernvespa.com/forum/wiki-trafficsignals
http://www.ehow.com/about_5113181_metal-magnetic.html
Sears and Zemansky's University Physics, 13th edition
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