# Homework Help: Spark at train's head

1. Mar 3, 2007

### Kolahal Bhattacharya

1. The problem statement, all variables and given/known data

When an electric train runs,we are familiar to the sparks at the end of cable connections.Why does it occur?Is it due to voltage fluctuations?

2. Relevant equations

3. The attempt at a solution

2. Mar 3, 2007

### PhanthomJay

Are you talking about the sparks between the train's pantograph pickup and the overhead energized wire catenary, and the sparks that occur between an energized third rail and the contact pickups on the train? If so, these sparks occur only occasionaly at areas where full contact between the wires or energized rail is momentarily lost. Based on this momentary loss of contact, can you explain why sparks might occur in this situation?

3. Mar 3, 2007

### Kolahal Bhattacharya

I cannot really understood your technical terms...However,I hope you are talking about the same problem.I should say the structure is like a wire over a structure like < whose base is attached to the train.
Well,what I suppose,during the momentary cut of contact,the back emf=-LdI/dt is high which desparately tries to retain the connection.So,a spark is seen.

4. Mar 4, 2007

### PhanthomJay

It's not the induced voltage that causes the spark. There is a large potential difference in emf between ther enegized wire and contact that has been de-energized during the momentary loss of contact. . Air, normally a very good insulator, lies between those 2 points. It is a very thin layer. What happens that causes teh spark (arc)? Note: if this train was riding in a vacuum, there would be no spark.