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Author Topic: does electric discharge goes from positive to negative or negative to positive..  (Read 2981 times)

Offline taregg

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« Last Edit: 15/04/2013 19:16:19 by taregg »


 

Offline yor_on

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Take a look here, and see if it gets you more confused :)
http://www.thenakedscientists.com/forum/index.php?topic=28990.0
You will also find some links, that may make sense, or not.
 

Offline evan_au

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Electric Arcs & Sparks
By an "electric discharge", I assume you mean lightning, an electric spark, or the more controlled electric discharge that you get through a fluorescent tube?

The spark travels between one conductor/object which has a slight shortage of electrons (giving it a positive electric charge), and another conductor/object which has a slight excess of electrons (giving it a negative charge).

The process starts with ionisation of the gas between the electrodes, which starts when the voltage gets high enough to rip electrons off the gas - about 1 million volts per meter for air, lower for the mercury vapour inside a fluorescent light tube. This electric field gradient is higher near pointed conductors, like a lightning rod. The ionisation path grows towards the opposite charge, as shown for lightning, here.

So in an electric discharge, the ionisation path initially starts from the end with the highest electrical field gradient, but as it approaches its target, often an ionisation path will start from the far end, to meet in the middle. But this initial ionisation represents a relatively small current, effectively connecting small areas of charge within the gas.

Once an ionised path is established between the excess and deficit of electrons, a much higher current can start to flow. In this flow, the ionised gas, consisting of negative electrons and positive ions (gas atoms with one or more electrons ripped off) are accelerated in opposite directions by the electric field, until they collide with another atom. So in fact the charge moves in both directions simultaneously. Because electrons are lighter, they tend to accelerate faster, and move further, but the total current is the sum of the charge moving in both directions.

These rapidly moving and colliding electrons and ions represent a very high temperature, causing rapid expansion of the gas (faster than the speed of sound), causing the familiar sonic boom of thunder. 

Battery Discharge
On the other hand, if by discharge you mean "discharge a battery", that involves a few different processes...
  • The chemical reactions within the battery results in an excess of electrons at the negative terminal, and a shortage of electrons at the positive terminal.
  • On the outside of the battery, these electrons can move through a copper wire from the negative electrode to the positive electrode, lighting a light globe, or turning a motor.
  • If the battery is used to conduct electrolysis, the electric field can drive positive and negative ions in opposite directions through a liquid.
  • If the battery is used to power an MP3 player or smartphone, conduction in the silicon semiconductor is a mixture of electrons moving from negative to positive, and positive "holes" moving from the positive terminal to the negative terminal (and sometimes they collide and cancel).
  • On the inside of the battery, the results are a little less intuitive, since the energy of the chemical reactions drives negative ions to react at the negative terminal, depositing extra electrons there, and positive ions to react at the positive terminal, absorbing electrons from the positive terminal.
  • Inside the battery, in the electrolyte between the positive and negative terminals, positive and negative ions move in opposite directions. It looks like the negative ions are attracted to the negative terminal, and positive ions are attracted to the positive terminal - the opposite of the mantra we were taught in school that "opposites charges attract and like charges repel"!

Conclusion: In solid metal conductors, conduction of a discharge happens by a flow of electrons from the negative terminal to the positive terminal. In many other materials, conduction is a mixture of positive and negative charges, flowing in opposite directions. 
« Last Edit: 16/04/2013 12:35:23 by evan_au »
 

Offline Pmb

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and why...
If the discharge is of electrons (or other negatively charged particles, which it usually is), then it goes from negative to positive. If the discharge is of positively charged partiles then the opposite is true.

I know of no instance of positive charges being discharged.
 

Offline evan_au

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Probably the most publicly visible case of discharging positive charge is the Large Hadron Collider at CERN.
This accelerates protons (or heavier nuclei) up to almost the speed of light - the energy in this positive current reportedly being equivalent to the energy of a landing Jumbo Jet.
After a few hours of operation, the beam decays a little, so they dump the positive beam into two targets and start again.
 

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