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Electric Current

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Controlling Traffic In Real Time

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 road's surface. These loops sense when a metallic object weighing over a certain critical mass passes overhead. When this happens an electrical signal is sent to a controller unit letting it know the light signal 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 flow 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 or through various dynamic control methods. One of the dynamic methods employs the use of induction loops. 
  • Induction loops are used to sense when a large metal object passes by. When a large enough metallic object passes by the loop (most likely an automobile), a current is induced in a nearby wire. The current then sends a signal to the traffic control unit, indicating that a car has passed by. The traffic signal can then change accordingly. 
  • The principle of inductance is based on the idea that when there is a change in current within a system, an electromotive force is created in the system and any nearby conductors. The system is usually a conductor.
  • An electromotive force is created in the system as well as nearby conductors due to the relationship between electromotive force and a changing magnetic flux, as well as the fact that any steady state current creates a steady magnetic field.
  • The relationship between electromotive force (\varepsilon) and changing magnetic flux (\phi_m) is represented by an equation which signifies that an electromotive force is equal to the opposite of time rate of change (\frac{d}{dt}\) of the magnetic flux.

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

The magnetic flux, \phi _{m}, is the magnetic field passing through a given area. This relationship indicates that an electromotive force is created in order to counteract the changing magnetic flux, thereby maintaining a steady magnetic field within a given system.

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Using the information provided above, answer the following questions.

  1. How would the induced current be affected if a stationary magnet rather than a moving magnet was present in a loop?  
  2. Rather than moving a magnet through an induction loop, could you change the strength of the magnetic field to induce a current?
  3. If you were 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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