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Controlling Traffic In Real Time
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Controlling Traffic In Real Time

License: CC BY-NC 3.0

At many stop lights throughout the United States circular loops have been cut into the pavement to allow detector loops to be placed near the surface. These loops sense when a metallic object above a certain critical mass passes overhead. When this happens an electrical signal is sent to a controller unit letting it know the light needs to be changed.

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Credit: Petey21
Source: http://commons.wikimedia.org/wiki/File:Led_traffic_lights.jpg
License: CC BY-NC 3.0

Traffic lights help control traffic and prevent car accidents [Figure2]

  • Stop lights are one method of controlling the flow of traffic at major intersections. These lights work through a combination of either fixed time control of through various dynamic control methods. One of the dynamic methods employs the use of induction loops. These loops operate by sensing when a large metal objects passes by it. When a large enough metallic object passes by the loop a current is induced in a nearby wire. This current sends a signal to the traffic control unit, letting it know as object has passed by that is most likely an automobile.
  • Inductance is the principle that when a there is a change in current, there is an electromotive force that is created in both the system (usually a conductor) and any nearby conductors. This is a result of the relationship between the electromotive force and a changing magnetic flux as well as the fact that any steady state current creates a steady magnetic field.
  • The first of these is represented by an equation which signifies that an electromotive force is equal to the time rate of change of the magnetic flux.

\varepsilon = \oint \overrightarrow{E} \cdot \overrightarrow{dl} = -\frac{d}{dt}\phi _{m}

Where the magnetic flux, \phi _{m}, is the magnetic field passing through a given area and \frac{d}{dt} is the time rate of change..

Explore More

Using the information provided above, answer the following questions.

  1. Will a current only be induced in a closed circuit when a magnetic is moving through the loop or will a stationary magnet suffice?
  2. If instead of moving a magnet THROUGH the induction loop, could you instead increase or decrease the strength of the magnetic field to induce a current?
  3. If you were able to increase the diameter of the loop seen in the video, would a current be induced?

Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ Credit: Petey21; Source: http://commons.wikimedia.org/wiki/File:Led_traffic_lights.jpg; License: CC BY-NC 3.0


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