Charge vs. Voltage: What's the Difference?

[Evil Bunny]

removed

 

[Evil Bunny]

If you had an isolated ac source (meaning, not connected to Earth ground), like a generator in the back of a pickup truck, for example... you could put one volt meter lead on the "hot" lead of this source and put your other meter lead on the ground rod at your house and you would not read any voltage. The electrons are always trying to get back to the "source". They are never trying to go into the earth. Therefore, these electrons would have no desire whatsoever to make their way from the generator to the earth...

My question is... Why?

I am confusing something here and I think it has to do with charge. If there were a "charge" on this wire that was different from the "charge" on the earth, these electrons would move and try to equalize themselves, and you would read a voltage, right? Since this doesn't happen, the voltage measured between these two portals of the AC source are never being "charged" in relationship to the earth.

Is this correct? Is there any relationship whatsoever between "charge" and "voltage"?

By "energizing" a wire, are you ever "charging" it?

 

[lorenb]

this doesn't sound right...

Electricty is always trying to get to a lower potential energy (not necessarily back to the source)?

So if your generator is producting a voltage (potential energy) higher than that of ground (around 0V). Then you connected these two points there would be a current created between the two. So i'd assume you'd be shocked... I don't know if I'm getting all of what you're saying...

I do know that if you had two sources creating let's say 500V and you touched both of them without touching anything else. there would be no current created between the two points and you would not be shocked... because the potential energy difference = 0

 

[rcgldr]

You could more safely use an ac voltmeter between either line of the generator and Earth ground to see what the potential difference between generator output and Earth ground is.

My guess is that at 110 or 220 volts, the amount of charge (which is cycling), would relatively small, since the effective capactance of the generator would be relatively large, but I wouldn't recommend human testing.

Voltage is an electrical potential. As an analogy for an object close to the surface of the Earth where the force from gravity can be assumed to be constants, gravatational potential would correspsond to the height of that object.

Voltage is an electrical potential
http://en.wikipedia.org/wiki/Electric_potential

Charge is related to the excess or absence of electrons in an object:
http://en.wikipedia.org/wiki/Electrostatic_charge

 

[Evil Bunny]

Ground is only 0V with respect to the "hot" AC lead if you physically connect it there. Otherwise it is not in the circuit at all and you have an open. No current.

 

[Studiot]

If you had an isolated ac source (meaning, not connected to Earth ground), like a generator in the back of a pickup truck, for example... you could hold one of the leads from this source with one hand and with the other you could grab hold of the ground rod at your house and you would not receive an electric shock. ...

That will be one bunny less then.

DON'T TRY IT

 

[Studiot]

Why have you posted two threads, offering dangerous advice?

https://www.physicsforums.com/showthread.php?p=3143064#post3143064

 

[Evil Bunny]

I posted this in the electrical engineering section of this forum after I posted it here because I thought maybe that was a more appropriate place for it and I didn't know how to delete this one.

but anyway... I guess I shouldn't have used human testing as an example. Obviously nobody should attempt this, but I was just trying to make a point that theoretically no current would flow because current is trying to flow from one side of the source to the other side. Many people believe that the Earth is a giant electron sink when it is not...

What I am trying to understand is why.

I guess the question is... if we could freeze time and this 120V AC source is at the voltage peak of 170-ish volts. Is there an excess or an absense of electrons in that wire at that instant in time? I would have to assume that the answer is no, but it sure seems like the answer to that question would be yes. What am I getting confused here? I'm missing something.

 

[Evil Bunny]

That will be one bunny less then.

DON'T TRY IT

Nobody should try it.

Very bad example. I am sorry.

Are you saying that there would be a significant voltage present in that scenario if we but a meter between the (unreferenced) ground rod and the "hot" lead of that generator?

 

[Evil Bunny]

Why have you posted two threads, offering dangerous advice?

https://www.physicsforums.com/showthread.php?p=3143064#post3143064

Sorry for the bad example. I edited it to a safer scenario. I explained in the other thread why I posted it here. Don't know how to delete the other one.

 

[Evil Bunny]

I edited the original post to remove any dangerous suggestions.

 

[Studiot]

PM a mod, explain and ask him/her to combine them.

 

[Evil Bunny]

So if your generator is producting a voltage (potential energy) higher than that of ground (around 0V). Then you connected these two points there would be a current created between the two.

If you take a 9V battery and put a volt meter lead on either one of the terminals of the battery and put the other lead into the earth, what voltage do you think you would read?

I'm telling you that you will get a reading of about 0V on your meter.

The same principal applies to AC. A volt is a volt. The only reason current EVER wants to flow into the ground is because in the United States we pound large ground rods into the Earth and then connect them to one side of our SOURCE. If we didn't connect the one side of our source to the earth, there would never be any reason for the electrons to be attracted to it. Our electrons only want to get back to the source (transformer on your utility pole) they came from. They only use the Earth if they have to. They would much rather travel along a low resistance wire (usually the neutral on your transformer).

If you had a source and did NOT connect one side of it to the Earth (like the generator in the pickup truck example), you would have electrons that had no desire to venture into the Earth at all.

 

[rcgldr]

If you take a 9V battery and put a volt meter lead on either one of the terminals of the battery and put the other lead into the earth, what voltage do you think you would read?

As mentioned in the other threads about this, the equivalent of capacitance for the battery is huge compared to the voltage, so the amount charge at the terminals is tiny, but it's not zero.