# A weekend question

Discussion in 'General Electronics Chat' started by studiot, Sep 17, 2010.

1. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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There have been a number of questions lately about the polarity of current carriers.

So suppose you were asked to provide a practical demonstration to prove, and preferably measure, that the carriers of current in a metal are negatively charged how would you do this?

2. ### beenthere Retired Moderator

Apr 20, 2004
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I might place a vacuum tube rectifier (no PN junction) in a circuit and measure the polarity of voltage across it when it conducted. Reversing the polarity of the applied voltage would check for other charge carriers. I might even have a 5Y3 around somewhere.

I would have to look into the details of the Millikan oil drop experiments before doing the measurements. Post docs are handy for such things. Or one can apply lots of voltage and demonstrate cathode rays. Moving the rays with a magnet demonstrates the polarity of the charge being moved.

3. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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If you are providing a source of current carriers to your metal sample, how do you prove that the carriers (which may be negative from your source) are not exchanged for ones of the opposite polarity when they enter the metal and re-exchanged again when they leave?

Incidentally this question is really aimed at bright young things who may be dazzled by all the modern paraphanelia, now available. The question was first, and most elegantly, answered in 1917.

4. ### beenthere Retired Moderator

Apr 20, 2004
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I am not. The critters carry charge. Moving the charge makes current.

Why? An underlying principle of science is to always take the simplest explanation. We already know metals have loosely bound electrons, that they carry negative charge, and that an applied EMF will cause them to move. Where would the other carriers come from?

5. ### Wendy Moderator

Mar 24, 2008
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The vacuum diode tube was my first thought.

Another example would require something much beefier, sputtering machines and such carry ions of the material with electrons. A sputtering machine actually coats the target material (usually aluminum ceramics) with the cathode material (IE, gold, tantalum, etc.). Add a little gas for chemistry and you can have things like tantalum nitride coated drill bits or resistors.

Last edited: Sep 18, 2010
6. ### Ghar Active Member

Mar 8, 2010
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I would think the Hall effect is the standard way but even that assumes you accept a lot things as fact.

7. ### Markd77 Senior Member

Sep 7, 2009
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Maybe electrolysis? But it still requires knowing how the ions are charged.

8. ### Nik Well-Known Member

May 20, 2006
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Photo-electric effect ??

9. ### debjit625 Well-Known Member

Apr 17, 2010
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Can anybody prove that electrons are negatively charged and protons are positively charged ?

As per me positive and negative are just relative quantities and assumed by some short of reference point, so to answer the question we need some sort of reference to compare with the stuff flowing through the metal.

10. ### retched AAC Fanatic!

Dec 5, 2009
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I just used my multi-meter and checked. Yup! Electrons are negative.

Boy is it hard to keep that atom from jumping around.

11. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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You can take your tongue out of your cheek, it's not quite so stupid as ......

I am referring to the Tolman- Stewart experiments where there was direct conversion of mechanical energy to electronic current flow.

12. ### debjit625 Well-Known Member

Apr 17, 2010
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Yes Tolman- Stewart experiment proves that in a conducting body the free charges are electrons,but I think the prove to be done was for the flow of current due to negatively charged particals,so how do you prove electrons are negative? Does it is given that we can assume that electrons are negatively charged w.r.t netural (null charge).

13. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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The direction of the galvanometer deflection shows the polarity of the charge carriers.

14. ### debjit625 Well-Known Member

Apr 17, 2010
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Galvanometers just shows the direction of the current i.e which way the particals are flowing,not the charge of the particals flowing in a circuit.

I think it is not possiable to prove unless we assume electrons as negatively charged atleast till the date 2010/Sept/19.

15. ### Chrisoborski Member

Oct 12, 2009
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What exactly is a current carrier?

I'm assuming your not talking about a conductor. Or Metal.

Oct 12, 2009
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17. ### studiot Thread Starter AAC Fanatic!

Nov 9, 2007
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It may be already 19 sept 2010 in India, but us backward folk further west calibrate our trusty ballistic galvanometers by the simple expedient of testing against a Leyden Jar. Since we have already decided which terminal of the Leyden Jar to call positive all we have to do is note the direction of deflection of the galvanometer and compare with the direction yielded from the slip rings on the ST apparatus.

But please don't let this argument detract from the magic of the achievement of Tolman and Stewart.

Edit
I am perfectly happy to provide a more detailed reasoning & explanation, but you did say it can't be done, so I would be interested to learn your reasons as to why not?

Last edited: Sep 18, 2010
18. ### beenthere Retired Moderator

Apr 20, 2004
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The oddest thing is that a real consensus does not exist. We - officially - at AAC take the position that the charge carriers in wires are the one most concerned with observed current, and so focus on electrons which carry negative charge - http://forum.allaboutcircuits.com/showthread.php?t=32158

People like to point out ionic transport in batteries must involve positive charges getting moved. Others point out that the numbers work out as well with conventional flow. I would say that hole flow in a PN junction is entirely a fiction, but then I would get jumped on over that, so I won't.

At the end of the day, it's all those electrons that have done the majority of the useful work, so we say they are the majority charge carriers, meaning electron flow is king.

19. ### Wendy Moderator

Mar 24, 2008
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Actually that was done in the first posts. The devices in question ( such as vacuum tube diodes and sputtering machines) only work one way, and that is negative to positive flow. These simple devices nail it down pretty thoroughly. As to referencing polarities, that can be done with meters, they reliably and dependably point which charge is which. The theory hangs together very well.

The examples in chemistry referring to ionic flow are valid, but they are also referring to materials other than electrons. In other words, it is a chemical reaction that is occurring using electricity as a driving force.

I'll go ahead and say it, hole movement is an illusion. The holes are fixed, with electrons moving through the material it makes it appear the holes move. Like many concepts in electronics, there is the simple concept, the more advanced concept, and the correct concept. The first two allow beginners to wrap their heads around the concept and move on, but as the beginners become expert they can understand more later, and make the mental jumps necessary. This would be the point for some members to start arguing I suspect, but the truth still comes through.

Last edited: Sep 18, 2010
20. ### debjit625 Well-Known Member

Apr 17, 2010
790
186
That seems we have a reference point.....,i.e.. you assume that the terminal is positive not proved it....

I said it cant be proved without referring a point,in your case you reffered the positive and with respect to that you proved that electrons are negative.
Leyden Jar proves that their are two types of charges opposite to each other but not proves which one is negative and which one is positive,its us who named the charges.

What I said was it cant be proved that electrons are negative unless you reffer positive charge and null charge.

Offcourse,its a big achievement.