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?

If you pull two conductors with current flowing between/thru them away from each other, there will be a very small gap at first and the inductance of the conductors will resist the current change by creating a voltage rise and this will usually be large enough to exceed 74 volts/mil.

An arc [spark] occurs whenever the voltage between two conductors exceeds 74 volts per mil [mil=1/1000 th inch].
In air [or some other gas] it is determined by something called Paschen's Curve which says , in part, that the voltage varies so as to increase with pressure. This is how we who work in nuclear physics with high voltages keep the arcs from occurring and causing unwanted losses. We pressurize the equipment.

Stated another way, all you need is enough field strength created by a voltage gradient to pull the electrons from the outer shell of the gas between the two conductors.

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The voltage rise between the conductors as they separate can be understood more clearly as the current attempting to continue [due to the persistence of the magnetic field around the conductors] and resulting charging of the capacitance formed by the two conductors and the space/gap between them.

What is the need to being so much roundabout.It's due to inductance as a little thinking would suggest.

Welcome to Sciforums, Paul. It sounds like you know your stuff.

What do you do?

Pete
I presently teach Mathematics at a private High School in Thailand, but I spent 35 years working as an EE in nuclear physics [particle beam accelerators] and in Silicon Valley [CA] working on internet, satellite, and handheld device hardware.

Neelakash
Your 'voltage fluctuations' remark made it clear that a response for you needed to begin on page one.
It should be an embarrassment to you that I took the time to give a teacher's clear and complete explanation .......... and you replied with an arrogant "can't you expalin it succinctly".

Ignore the ungrateful. I found your explanation to be both interesting and informative.

In air [or some other gas] it is determined by something called Paschen's Curve which says , in part, that the voltage varies so as to increase with pressure. This is how we who work in nuclear physics with high voltages keep the arcs from occurring and causing unwanted losses. We pressurize the equipment.

That or immerse the equipment in oil :)

One part of the Paschen curve that I've always wondered about is the low-pressure regime, where the arc voltage decreases with pressure. The increase with pressure in the high-pressure regime makes sense, as there are more atoms between the electrodes that you have to ionize. But can anyone explain why the reverse is true when pressure gets very low?

Yes, oil works too.
Maintaining equipment immersed in oil at a working accelerator is a nightmare usually.
Gas however can be pumped out leaving clean devices/hardware ready for inspection, calibration or repair.


When the pressure gets very low, there are fewer molecules of gas with which to establish and sustain the arc and overcome the voltage gradient at the conductor surface. Thus the required voltage increases.
See the curve here .....
www.duniway.com/images/pdf/pg/Paschen-Curve.pdf

OK,I am sorry.I should have appreciated.But I was searching for siple qualitative arguments and later found that the "brakes" in current are due to machanical vibration when the structure slides over the overhead wire.So,it makes things more clear to me.We need to visualise the things,after all.
If you were hurt,I am regretting again.But,I was not "ungrateful",I simply told you there is really no need to be eleborate with that.