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# Electric Current

## Electrons move through circuits to produce electric current.

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Controlling Traffic In Real Time

### Controlling Traffic In Real Time

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

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..

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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?

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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