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My textbook says the following:

Suppose we put $+Q$ and $-Q$ charges at the two ends of a copper wire. The electrons will be accelerated due to an electric field towards $+Q$. They will thus neutralize the charges. There will be current for a very short time and no current thereafter. But if we continuously supply fresh charges, current will flow.

My doubt is: All the electrons have already moved to one side to neutralize the charge, so now how will there be current (flow of electrons) if the electrons have nowhere else to move?

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The point is that $+Q$ and $-Q$ are not new charges being placed there. They just indicate differences in charge.

If you in the wire

  • have a lot of electrons added to one end, then there is a huge total negative charge at that end.
  • There doesn't have to be any positive charges at the other end - simply the fact that there are less negative charges at that other end is enough.

A lot of negative charges gathered at the same spot will strongly repel each other. So any other point nearby with from which the negative charges are repelled less from, makes them want to move there.

All such other places are then defined as points positive charge; not because they have any positively charged particles, but because there is less negative charge here than at the other end. We will symbolise that less negative point as $+Q$, while the negative end is symbolised $-Q$.

  • The negative charge will push each other towards the other end from which they are less repelled. Soon, they are balanced out, and the total charge throughout the wire causing a repulsion force from each point is the same at each point along the wire. No charges flow, no current.
  • If you now add more charges to one end, then we again have a charge imbalance. Then again there is a higher repulsion from this point that from the other end. So, negative charges will as before want to flow to the other end from which they again are repelled less.
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  • $\begingroup$ So are you saying the freshly supplied -Q has some electrons which will move from its point to the other point where all the other electrons are previously placed? $\endgroup$ – Leah May 29 at 4:30
  • $\begingroup$ So basically -Q means having a lot more electrons than the opposite side? $\endgroup$ – Leah May 29 at 4:34
  • $\begingroup$ @leah All the electrons at are not flowing to the other end (although that's how we typically describe it for simplification), they distribute themselves over the wire, and spread out. There are equally many electrons at all points, ahne there is no current. $\endgroup$ – Steeven May 29 at 6:03
  • $\begingroup$ I am still confused, they distribute themselves equally and there is no current? $\endgroup$ – Leah May 29 at 6:55
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    $\begingroup$ Thanks a lot. This was really helpful! My understanding of +Q and -Q was way off . $\endgroup$ – Leah May 29 at 8:58
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My doubt is that all the electrons have already moved to one side to neutralize the charge, so now how will there be current (flow of electrons) if the electrons have nowhere else to move?

You haven't given us the context within which the book statement is made. But if I understand your book correctly, when it says "if we continually supply fresh charges" it means supply both fresh positive and negative charges, not just negative charges (electrons). So there will be a continuous supply of positive charges for the negative charges (electrons) to combine with and neutralize.

Hope this helps.

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  • $\begingroup$ All the electrons are now on one side of the cylinder, so current shouldn't flow even when fresh charged is supplied because all the electrons are on one side of the copper wire? $\endgroup$ – Leah May 29 at 4:27

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