a few basic questions on voltage, electricity, etc

[lboogy]

Hi everyone,

 

I hope my simplicity isn't frustrating for you all. I just have a few questions:

 

1. When a power source gives energy to the free electrons in the current, do these electrons turn from a negative charge to a positive charge?

 

2. Where do the free electrons come from? Am I correct in assuming that they are in the wires and the volts from the power source energise them to flow? Ar have I got it completely wrong and they actually come from the power source, along with the volts?

 

thanks in advanceeveryone!

[insane_alien]

1/ no, electrons don't change charge. ever. a power source just shunts them round the wires.

 

2/ the 'free' electrons are electrons involved in metallically bonded elements. the proper name for them is 'delocalised electrons' which means they are drifting from atom to atom with no direction or effect. when a voltage is applied, they tend to move preferentially to the positive electrode.

 

hope this helps, if you didn't understand anything just ask.

[Klaynos]

1. No the electrons still have negative charge but they move in the OPPOSITE direction to conventional current.

 

2. They are 'free' electrons which are normally in the vallance band of the metal. Have a look for "free electron gas" on wikipedia. Applying an electric field makes these electrons move.

[lboogy]

Thanks for your help guys! I understand what you've told me, but now I know more I have more questions! I've been looking on wikipedia and came accross the following which doesn't seem to make much sense to me:

 

"When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field".

 

1)Wouldn't this result in all the free electrons being drawn towards the positive end of the battery, rather than going round the circuit? In most circuit diagrams I've seen, the charge goes from the positive end to the negative end.

 

2)Do the electrons carry the voltage, and then release it in the components?

 

3)Is it true to say that an electron will literally move from one end of the circuit to the other when volts are applied, or do they merely rub of volts to other electrons that they touch hence the volts being passed round the circuit that way?

 

I'm conscience of the fact that I'm asking basic questions on what appears to be quite a high level forum, if my questions get annoying at any time please let me know. I do try and find out the answers before asking!

 

thanks for answering though!

[insane_alien]

1/ the electrons would drift towards the positive terminal, yeah. Diagrams use 'conventional current' which is an american thing. and like george bush, its backwards conventional current assume that its a positive charge thats moving and real life is a negative charge thats moving. so they come out in different directions. it doesn't really matter as the equations come out to the same either way.

 

2/ The electrons don't carry voltage, they carry charge. You can think of the voltage difference like a difference in pressure. the electrons flow from the high pressure to the low pressure, kind of like water in a pipe.

 

3/ yes, the electrons move. they don't move spectacularly fast(couple of centimeters an hour IIRC) but they will move from one end to the other.

 

And finally, don't worry about how simple your question sounds. Do you think we learned all we did by keeping quiet?

 

the only time we'd find your questions annoing is if you suddenly go an say 'science is all wrong, everything works with little pink faries moving everything about when we blink and theres nothing you can do to disprove it'. then you'll be annoying.

[Klaynos]

It's not so much that it's an american thing. The conventionally current is historical from before the time we understood what was happening!

 

Questions are good. The drift velocity of the electrons (how fast the electrons move from one end to the other of a length of wire) is very very slow as IA says.

[timo]

I dunno the values but cm/h intuitively seems incorrect. I´d expect something in the order of cm/s or m/s.

[insane_alien]

http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/Current.html

 

okay so i was a little slow with the speed i remembered turns out the typical drift velocity is around 0.1 mm/s thats 6 centimeters a minute or 3.6 meters per hour

[lboogy]

Ok, so going back to the analogy of voltage being like pressure, like water in a pipe, is the pressure caused by the current? So the electrons are moving towards to positive end and this pull also causes the voltage to move throught the circuit to?

 

Does the voltage actually move along with, and intermingled with the current (I'm assuming I have the correct interpretation of current - that current is the negativlety charged electrons?).

 

thanks again guys! I learn in a weird way, the more I know the more I want to get to the bottom of things, I can't just understand how to find the voltage, current etc, I want to know why/how it works!

[insane_alien]

right, the pressure(voltage) isn't caused by the current. its the other way round. The voltage causes the electrons to drift in one direction because it creates an electric feild and the electrons are charged. but the voltage doesn't move.

 

your interpretation of current is good.