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BritElec

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Alright, I'm still stumped about a basic electrical concept (No I'm not an electrician nor do I pretend to be, so don't worry! :))

I've always thought (and been taught) that voltage was comparable to water pressure and that current was comparable to the volume of water that is passing through.

For instance, a large pipe with water flowing slowly would be equal to high current, low voltage (like a huge sewer pipe or something)

And a small pipe with water flowing very quickly would be low current high voltage (say for instance, a waterjet used to cut steel).

But then I thought about something the other day that totally confused me.

If you take a hose, turn the water on, and close the open end (so you just have a pressurized hose with no flow, yet) and you poke one hole in it, the water will flow out with very high pressure. This would be high voltage with low current.

Now if I poked 100 holes in the same hose, each hole would have much less pressure and the water wouldn't fly out as far from the 100 holes as it would if there was only 1 hole.

So this means that the more holes there are, the less pressure there is, but the total volume of water coming out is probably close to the same. This means that with multiple holes, pressure differs, but total volume stays the same. This is like saying voltage differs, but current stays the same.

But isn't that the total opposite of electricity? When you have things in parallel, voltage stays the same and current is different???

Now I'm totally confused.

Is it:

voltage=pressure
current=total volume

or is it the reverse? Or is it sometimes one, sometimes the other?
 
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uncle betty

  • Thread Starter Thread Starter
  • #2
The water analogy only goes so far.

You have to all the while keep in mind the third leg of the equation... RESISTANCE

If you take a hose, turn the water on, and close the open end (so you just have a pressurized hose with no flow, yet) and you poke one hole in it, the water will flow out with very high pressure.

Depends on the size of the hole. One little hole will have very high resistance to current flow, under 'x' volts. One big hole will offer less resistance, so you could in that case have high current under 'x' volts.

You need the third concept of resistance to get the other two in proper relationship.

Now if I poked 100 holes in the same hose, each hole would have much less pressure

In electrical terms it's probably better to think of it as all those holes decreasing the total resistance. With the result that current will rise. That's just Ohm's Law.

The amount of current you get in a circuit is a direct consequence of pressure/volts and resistance/ohms.

So this means that the more holes there are, the less pressure there is,

No. The more holes there are, the less resistance there is. And the more current there will be.

but the total volume of water coming out is probably close to the same. This means that with multiple holes, pressure differs, but total volume stays the same. This is like saying voltage differs, but current stays the same.

With multiple holes, the pressure will be the same at all points. The total volume will be the same as for one hole equal in size to all the little holes combined. In either case, the volume will be determined by the pressure and the resistance.

But isn't that the total opposite of electricity? When you have things in parallel, voltage stays the same and current is different???

Different to what ? If you have eg 12v at source, then that's what you have, whether series or parallel. Volt drop (in a series/one hole circuit) is a slightly different thing.

Anyway - if you have one little hole, that's a high resistance. And that will dictate your current flow at your given voltage.

One big hole, or a number of little holes will give you a very much lower resistance. And that resistance will dictate the amount of current flow at your given voltage.

Is it:

voltage=pressure
current=total volume

Current = the result of the voltage's ability to overcome the resistance in the circuit.

volume = pressure / resistance
resistance = pressure / volume
pressure = volume x resistance

Ohm's Law, in other words. The three components all depend on each other.

Hope that helps. :)

ps - here's a technical point that I moved to the bottom, to avoid confusing the thing...

This would be high voltage with low current.

Well, ''high voltage'' actually means something specific and is not related to this.

Let's put numbers on it - if I have a 12v battery, and my circuit has 12 Ohms of resistance, then this dictates that I will have 1 amp of current. (12/12=1)

If I make a load of holes in the hose, and therefore lower the resistance to say, 1 ohm - this dictates that I now have 12 amps (12/1=12).

Think about this though - big hole or small, your voltage supply will be the same in both cases.

With 12v and 1 amp, proportionately I have a ''high'' voltage compared with a ''low'' current, but it's not technically correct to call that high voltage.
 
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Grumpygrouter

  • Thread Starter Thread Starter
  • #3
Good post Uncle Betty, a great help for the non-electricians amongst us.:)
 
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Dave

  • Thread Starter Thread Starter
  • #5
WOW...very informative reply uncle betty....thanks for taking time for that detailed response...and britelec...don't think too hard into theories ..lol you will get a headache...lol lol....welcome to both of you..
 
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BritElec

  • Thread Starter Thread Starter
  • #6
Wow, thanks for the awesome reply. I see where I went wrong now.
 
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