Chapter 30, Problem Q30.1DQ

In an electric trolley or bus system, the vehicle’s motor draws current from an overhead wire by means of a long arm with an attachment at the end that slides along the overhead wire. A brilliant electric spark is often seen when the attachment crosses a junction in the wires where contact is momentarily lost. Explain this phenomenon.

To determine

the phenomenon used in electric trolley system.

Explanation of Solution

The key terms explaining the electric trolley system is generation of standing waves and the back emf of the trolley motor.

In the above mentioned electric bus system the vehicle’s motor draws current from a single wire connection. The return current runs through the steel rails of the track.

As the train moves, the contact shoes (long arm) slides along the wire and this leads to generation of standing waves in the over head wires.

The standing waves occur because the medium (bus) is moving in the opposite direction to the wave. It can also rise due to interference between two waves travelling in opposite directions. These waves are reflected back and forth in the electric line.

With respect to Sparking, it occurs when there is loss of contact or improper contact between the long arm of the bus and the contact wire of the system. The under-lying mechanism is that it is due to the "back EMF" (reverse voltage) from the trolley motor, when contact is interrupted. That is this emf opposes the change in current which had earlier induced it. This concept is resourced from Lenz law.

Conclusion:

Thus, the mechanism of electric trolley system is explained.