Answer 5a X2018-01 EE106_Electro... https://www.thecn.com ffice documents Downlo

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Answer 5a

X2018-01 EE106_Electro... https://www.thecn.com ffice documents Download Note: if you cannot preview the file, you can direct download it from this blue "Download" button. May-August 2017 Q5(a) 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. Discuss this phenomenon. (5 marks) (b) An electric toothbrush has a base designed to hold the toothbrush handle when not in use, the handle has a cylindrical hole that fits loosely over a matching cylinder on the base. When the handle is placed on the base, a changing current in a solenoid inside the base cylinder induces a current in a coil inside the handle, this induced current charges the battery in the handle. We can model the base as a solenoid of length, with NB turns as shown in Figure Q5b. It carries a source current, I and has a cross-sectional area, A. The handle coil contains NH turns. Coil i (Base) Coil 2 (Handle) NR Saved to Favourite pages CHANGE

Explanation / Answer

The vehicle's motor is drawing current from the overhead wire.The attachment which slides on the power line is called a pantograph.As the attachment passes through a junction and losses contact momentarily,this basically 'Breaks' or 'stops' the continuous current that was already being drawn from the wire.Now generally the traction systems draw large amount of current.

And large amount of current basically means that large amount of energy is flowing through the wires and the sliding contact.By conservation of energy we know that this energy can't suddenly disappear.And we also know that air is not a conductor.So where does all this energy goes??It actually goes into buildup of a huge voltage across the air gap,which is so high that it causes the breakdown of the air that is in between the wire and the sliding contact.

So here we have it,we had a current that was flowing,it suddenly finda that there is no conduction path because of air.So there is a Voltage buildup that breaks down the air,it gets ionised,and conduction continues via flash or spark through the air gap.

These sparks are the primary reason that the contact has to be changed from time to rime.

Hope this helps...

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