# Electric Current

## Electrons move through circuits to produce electric current.

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Practice 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 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

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 (\begin{align*}\varepsilon\end{align*}) and changing magnetic flux (\begin{align*}\phi_m\end{align*}) is represented by an equation which signifies that an electromotive force is equal to the opposite of time rate of change (\begin{align*}\frac{d}{dt}\ \end{align*}) of the magnetic flux.

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

The magnetic flux, \begin{align*}\phi _{m}\end{align*}, 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.

#### Explore More

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?

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