Electric Potential: Questions & Answers

[jeekeshen]

Electric Potential !

Am getting confuse with the definition, why does the definition says "workdone is bringing unit POSITIVE charge from infinity to that point" and not NEGATIVE charge? can it be negative also in the definition or is it because we always use positive test charge...

 

[johng23]

If you defined it the other way around, a collection of negative charges would produce a positive potential. That just seems less intuitive. That said, at the end of the day, it's just a definition.

 

[jeekeshen]

Ya its just a definition... but everything is base on it. Just wanted to know if it was the positive test charge that was causing that.

 

[jtbell]

Causing what?

 

[Naty1]

Positive charge is used only because when electricity was first discovered it was thought it was positive, scientists got used to that convention, then electrons were discovered as the actual component of current flow...so for example, the standard current flow arrow in a circuit is still usually referenced as from plus to minus...arrow head toward the minus terminal.

How far is the moon from the earth??

everybody says about 384,000 km...Hardly anybody says "The Earth is about 384,000 km from the moon."...so it's just a convention .

 

[sophiecentaur]

In an attractive situation (electric or gravity - or even a spring under tension), work can be 'got out' as the distance decreases and is 'put in' as the distance increases. The choice of positive charge in the definition of the electric potential is based on that - for consistency, I think.

 

[jeekeshen]

Equations of motion...

What are the conditions that enable the equations of motion to be used except from the fact that acceleration must be uniform ?

 

[AJ Bentley]

it was thought it was positive

Well, they weren't wrong.

Electricity is the flow of charge, which can be positive or negative. Sometimes it's one (metals and n-type semiconductors) sometimes the other (p-types).

Just because we discovered later that the electron carries negative charge doesn't mean we have to change our minds about the polarity of charge.

It's the textbook that's not terribly good - It should say it's the work done in bringing a 'like' charge from infinity - then there's no problem.

 

[Born2bwire]

Well, they weren't wrong.

Electricity is the flow of charge, which can be positive or negative. Sometimes it's one (metals and n-type semiconductors) sometimes the other (p-types).

Just because we discovered later that the electron carries negative charge doesn't mean we have to change our minds about the polarity of charge.

It's the textbook that's not terribly good - It should say it's the work done in bringing a 'like' charge from infinity - then there's no problem.

It would certainly be convenient given that it results in the current being defined as flowing opposite to the majority carriers in most devices. The textbook is probably giving the definition for work done in an electric field. This is defined as being referencing a positive charge with respect to the applied electric field.

 

[AJ Bentley]

Maxwell defined potential in his treatise.
He made no distinction between positive and negative in his definition.

http://www.archive.org/stream/treatiseonelectr01maxwrich#page/14/mode/2up"

Since that date, a (large) number of muddy thinkers have elected to specify everything with relation to positive charge.
Every time someone defends it, the confusion perpetuates.

 

[sophiecentaur]

It's all self-referenced, though. Nothing to worry about. In an attractive field the potential is negative and in a repulsive field, the potential is positive.

It would certainly have simplified the lives of millions of students if we hadn't had to tell them "it just happens to be the other way round" (edit - when talking about electron and charge flow).

 

[sophiecentaur]

btw, it's electrons that flow through n and p type material 'really'. After all, it's electrons that are fed in one end and come out of the other! 'Holes' are only a way of thinking (allbeit a useful one). Now, if it were Ions flowing, that would be a different thing but that could only happen in a fluid.

 

[AJ Bentley]

"it just happens to be the other way round"

When I was a student, the subject was taught classically - there was no mention of electrons.

Somehow the discovery of the electron as the dominant charge carrier in metals has come to dominate thinking on the subject as though that particular polarity were somehow 'more important' than the other.

Aside from that, there's a good reason not to equate charge with electrons. When you study it at the quantum level, you start to talk about 'charge' as particles - and the particle in question is generally the coulomb - not the electron.

I prefer to think as I was taught - that the coulomb is the primary particle of EM theory and the electron is just a fancy modern gadget that can safely be ignored.

The point is (and you mentioned it yourself as did others) - doing it that way makes it utterly independent of the carrier. Why make life difficult?

 

[Dadface]

Am getting confuse with the definition, why does the definition says "workdone is bringing unit POSITIVE charge from infinity to that point" and not NEGATIVE charge? can it be negative also in the definition or is it because we always use positive test charge...

There may be some historical reason why positive charge was chosen but that aside the choice is arbitary.Since positive charge was chosen originally and is used in the definitions then it makes sense to stick with it.In defining electrical potential it is useful to define the nature of the charge because unlike gravity the electrical force can be attractive or repulsive.