Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I am a beginner in electrical engineering. Often times (most cases actually), the underlying physics aren't really explained to us and we are just left to assume that it works "because it works." This is never enough for me in classes etc and I always end up doing a follow up of physics side of the spectrum.

My question is, you have a battery with excess electrons built up on the negative lead, and excess holes on the positive lead, why is it that in our universe then connecting a conductive compound (like.. copper) to just the negative lead does not produce current, or vice versa connecting the wire to just the positive lead.

After all, if we move a magnet passed a conductor, there is a tiny induced current. How can the magnet do this, but an excess of electrons or electron holes repelling each other cannot?

share|improve this question
    
Do typical batteries really have a stored up excess of electrons on one end? I didn't think that's how they worked. It's not like a capacitor. –  BMS Apr 29 at 23:57
    
3  
If you connect a bit of wire to one terminal of the battery, a current will flow, but only for as long as it takes for the charge to redistribute itself around the conductor. –  The Photon Apr 30 at 0:16
    
In addition to what others said, I don't think the definition of electron holes work out for usual metals. A hole is just the absence of electron in an otherwise full valence band. In your case, the valence band is full only because of the excess electrons and holes. –  Waffle's Crazy Peanut May 1 at 2:11

2 Answers 2

Physicist Plug

You may want to consider becoming a Physicist, especially if the ambiguity of engineering classes bother you. :)

Electrons and Holes Need Holes or Electrons to Flow to!

Excess electrons or holes do not cause current to flow because those excess electrons (or holes) need someplace to go to. If the electrons/holes don't "see" any places to go to, you don't get current. You need a path for the electrons and holes to flow. (This path also has to be energetically allowable, that is: the electrons need to have enough energy to follow that path. That's why batteries don't randomly spark and arc.)

Batteries Use Entropy to Make a Potential Difference

It just so happens that many batteries use entropy to keep the holes at one end and the electrons at the other. This trick is complicated, and something you should ask the Chemistry Stack Exchange, or at least in a different question here. Entropy keeps the holes and electrons apart, despite the fact that they would electrically come together.

Additionally, batteries also work by allowing ions (not electrons or holes) to flow from one end of the battery to the other. This allows the electrons to flow through the rest of the circuit instead of having them discharge like capacitors. Once again, this is where chemistry and physics meet, and you should try the chemistry stack exchange for more information.

The Conclusion

Since electrons/holes need holes/electrons to flow to, you're safe touching one end of a battery, but in trouble if you hold both ends. If you're holding only one end of the battery, the holes/electrons don't "see" anywhere to go, so they don't go. If you connect the circuit, the holes and electrons "see" each other at opposite ends of the circuits and then flow through it to meet up. You could also get current if you brought something with a lot of holes or electrons near the appropriate end, although it would likely be in the form of a sudden static discharge instead of the smooth flow from a battery.

Maxwell's Equations, Magnets, and Induced Currents

Finally, if you take a look at Maxwell's equations, you'll find an equation with a term that relies on a change in magnetic fields to produce current. More specifically, it's those last two which explain the relations between magnetic fields and electric fields. Since you're starting your career as an electrical engineer, you may want to wait until you have a good grasp of vector calculus before tackling those.

share|improve this answer
1  
A Potentially Off-Topic Note for Beginning Physicists and Engineers Having a background in both areas, I can relay my experience to you. In engineering and physics classes, especially undergraduate ones and lower, they'll tell you how things work, but not why things work. Usually, they don't bother telling you why because you simply don't have the math skills to fully appreciate it, or the why isn't the point of the class, or the class isn't ready for the why. I suggest going to Teaching Assistants, Professor's Office Hours, or any other mentors you can find to answer hard questions. –  PipperChip Apr 30 at 2:15

If you connect a piece of wire to just one terminal of a battery, the whole wire tends to gain the same potential as that of the terminal it is connected to, this requires an instantaneous current and is achieved very quickly.

The need of connecting the wire to both terminal, is that the internal chemical mechanism of the battery maintains a potential difference across the wire, thus the wire never reaches a desired potential and continues to try to reach it by flowing current.

The moving magnet also provides a stable potential difference between 2 points on a conductor which just by transference of electrons isn't diminished or nullified, because it is controlled by the moving action of the magnet.

Whereas, the excess/lack of electrons, provide a potential differece only till redistribution takes place and whole conductor gets to the same potential.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.