#176508 - 24/08/2003 11:42
Re: Empeg power requirements
[Re: genixia]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Here's a simple kitchen experiment that may help you to really get a grasp; Get an old soda bottle or clear juice bottle, about 2L is ideal. Okay, that experiment shows me the relationship between current and resistance. But where in that experiment do I see the relationship between volts and amps? What would I change to represent volts, and what would I change to represent amps?
When someone says "current", they mean the combination of volts and amps, right? In all these analogies, you guys keep saying "current", and I'm looking for a way to grasp what the two different halves of "current" represents. I haven't yet seen anything in the water analogies that shows me this. To me, it's all the same. More water= More pressure, even in the soda bottle analogy. So in that analogy, if the height of the water represents volts, then what represents amps?
Sorry if I'm being dense. These are all very good and detailed explanations, and I really appreciate them. Everyone is really giving it their best shot, and my puny brain is still not seeing the difference between the two...
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#176509 - 24/08/2003 11:44
Re: Empeg power requirements
[Re: pca]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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The larger the differential between the input and output voltages, the more heat, hence the difference you notice with different mains adaptors. Ah, excellent.
So then, there's something to be said for getting a PSU that's got an output voltage closer to 12-13 volts?
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#176510 - 24/08/2003 13:14
Re: Empeg power requirements
[Re: tfabris]
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pooh-bah
Registered: 02/06/2000
Posts: 1996
Loc: Gothenburg, Sweden
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When someone says "current", they mean the combination of volts and amps, right?
I don't. Current is measured in amps. Voltage over a resistance causes a current to flow through the resistance according to Ohms law: U=RxI (or I=U/R) (where U = voltage, I = current, R = resistance)
In all these analogies, you guys keep saying "current", and I'm looking for a way to grasp what the two different halves of "current" represents. I haven't yet seen anything in the water analogies that shows me this. To me, it's all the same. More water= More pressure, even in the soda bottle analogy. So in that analogy, if the height of the water represents volts, then what represents amps?
The water flow through the opening (volume per time unit), and the size of the opening represents the inverse of the resistance.
A taller water reservoir corresponds to higher voltage.
Connect a water reservoir to a straw and you get a fairly small flow at some speed. The straw is high resistance. The flow is the current. Connect a 3' pipe and you get a much larger flow from the same height water reservoir and at the same speed... The larger pipe <-> less resistance.
If you increase the pressure enough (water pillar) you can get the same flow through the straw (at a much much higher exit speed) as you got out of the 3' pipe above
Effectively, current (amps) can't exist without voltage: you need a voltage difference over a resistance to cause current to flow. Or one can see it like current passing through a resistance giving rise to voltage. In the water analogy, water flow passing through a small/rough pipe would cause a pressure drop...
/Michael
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/Michael
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#176511 - 24/08/2003 13:20
Re: Empeg power requirements
[Re: tfabris]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Okay, that experiment shows me the relationship between current and resistance. But where in that experiment do I see the relationship between volts and amps? What would I change to represent volts, and what would I change to represent amps?
When someone says "current", they mean the combination of volts and amps, right? Not if they're being at all precise -- in the language of electricians and physicists, current is measured in amps. Fuses, for instance, are marked with the maximum current they can take: those measurements are in amps. The complete mapping onto Genixia's experiment is:
Water pressure (psi) ~= Voltage (volts)
Water flow (pints/sec) ~= Current (amps)
Constrictedness of the hole (no sensible unit) ~= Resistance (ohms)
So increasing the head of water (which increases the pressure) is like increasing the voltage: it forces a greater current (more amps) through the same resistance (a greater flow through the same hole). To increase the head of water without increasing the flow (or increase the voltage without increasing the current) you'd have to make the hole smaller (increase the resistance).
Likewise, widening the hole will cause a greater flow for the same pressure: i.e. decreasing the resistance will cause a greater current (more amps) for the same voltage.
In other words, if you know any two of voltage, current, and resistance, you can work out the third. (And the same for water pressure, water flow, and "hole resistance" -- the only problem being that the relationship between "hole resistance" and hole size isn't very neat; factors such as viscosity and friction complicate it.)
The combination of volts and amps, since you mentioned it, is called power and is measured in watts. A high voltage at a low current delivers lots of power (think the jet of a firehose) and so does a low voltage at a high current (the Thames through London isn't going very quickly, but it'd be a devil of a job to stop it).
Peter
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#176512 - 24/08/2003 19:07
Re: Empeg power requirements
[Re: peter]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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A high voltage at a low current delivers lots of power (think the jet of a firehose) and so does a low voltage at a high current (the Thames through London isn't going very quickly, but it'd be a devil of a job to stop it). I guess I'm still not getting it. Whether the water is coming from a hole in the side of a dyke along the river, or whether it's coming from a same-sized hole in the side of a fire hydrant, the amount of flow is still determined by the pressure behind that hole. How can there be two different kinds of pressure? There's either X pressure behind that hole or there isn't. I don't see how the water can represent both volts and amps.
Unless you're trying to tell me that the amperage is simply a measurement of the flow... and not related to the water pressure at all. Like... say, a paddle wheel with an RPM counter inserted into the stream coming out of the side of the dyke. And the amperage is merely the number on that RPM counter.
Is that what you're saying?
If so, then my whole problem was not realizing that amperage was just a flow measurement. In other words, amperage is determined by the resistance in much the same way that your speed is determined by how hard you press on the gas pedal.
I was thinking amperage was some ethereal secondary property of the electricity itself, somehow altering how "powerful" the voltage was. I thought that the amount of amperage was something that existed behind that dyke. If it's just a scale of measurement representing the amount of water after it's gone through the hole, and not a property of the electricity itself before it's gone through the hole, then I think I finally get it. And the concept is pretty simple.
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#176513 - 24/08/2003 23:34
Re: Empeg power requirements
[Re: tfabris]
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enthusiast
Registered: 19/04/2001
Posts: 369
Loc: Seattle, WA (formerly Houston,...
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Unless you're trying to tell me that the amperage is simply a measurement of the flow... Amperage and voltage are both measurements of properties of the electricity.
To continue the analogy, amperage, yes, is simply a measurement of the flow, just as voltage is just a measurement of the pressure on the water at a certain point.
[Amperage is] not related to the water pressure at all. Well, indirectly, yes, it is. The more water pressure (voltage) the higher the rate of flow (amperage), given a constant resistance to flow. What is hard to see at first is that all three factors are related, based on the equation V = I * R.
Look at that equation for a second, and notice how changing one factor affects the others.
For example, if you were to make the hole smaller (increase R) you'd decrease flow (I) given the same pressure (V) on the water at the hole.
But if you were to pour the correct amount of water in, increasing the pressure at the hole (V), then you could increase flow (I) back to the same rate before the hole became smaller.
Thus any factor is determined by both other two.
Any clearer at all?
John
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1998 BMW ///M3
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#176514 - 25/08/2003 00:17
Re: Empeg power requirements
[Re: tfabris]
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enthusiast
Registered: 19/04/2001
Posts: 369
Loc: Seattle, WA (formerly Houston,...
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I was thinking amperage was some ethereal secondary property of the electricity itself, somehow altering how "powerful" the voltage was. Well, this is almost an entirely differently topic, but as Peter said above, both the voltage and current determine the power the electricity delivers, where P = V * I. Power can be thought of as the ability to do work, but explaining that is a whole other discussion.
So, yes, they are both properties of the electricity, which effect the power the electricity (not volatage) delivers.
I guess I'm still not getting it. Whether the water is coming from a hole in the side of a dyke along the river, or whether it's coming from a same-sized hole in the side of a fire hydrant, the amount of flow is still determined by the pressure behind that hole. How can there be two different kinds of pressure? There's either X pressure behind that hole or there isn't. I don't see how the water can represent both volts and amps. You've held the size of hole constant (R). The water doesn't represent volts or amps. It represents the electricity itself.
Remember, measurement of pressure = volts, measurement of flow = current.
John
_________________________
1998 BMW ///M3
30 GB Mk2a, Tuner,
and 10 GB backup
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#176515 - 25/08/2003 00:17
Re: Empeg power requirements
[Re: tfabris]
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enthusiast
Registered: 19/04/2001
Posts: 369
Loc: Seattle, WA (formerly Houston,...
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I was thinking amperage was some ethereal secondary property of the electricity itself, somehow altering how "powerful" the voltage was. Well, this is almost an entirely differently topic, but as Peter said above, both the voltage and current determine the power the electricity delivers, where P = V * I. Power can be thought of as the ability to do work, but explaining that is a whole other discussion.
So, yes, they are both properties of the electricity, which effect the power the electricity (not voltage) delivers.
I guess I'm still not getting it. Whether the water is coming from a hole in the side of a dyke along the river, or whether it's coming from a same-sized hole in the side of a fire hydrant, the amount of flow is still determined by the pressure behind that hole. How can there be two different kinds of pressure? There's either X pressure behind that hole or there isn't. I don't see how the water can represent both volts and amps. You've held the size of hole constant (R). The water doesn't represent volts or amps. It represents the electricity itself.
Remember, measurement of pressure = volts, measurement of flow = current.
John
_________________________
1998 BMW ///M3
30 GB Mk2a, Tuner,
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#176516 - 25/08/2003 02:59
Re: Empeg power requirements
[Re: tfabris]
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carpal tunnel
Registered: 13/07/2000
Posts: 4180
Loc: Cambridge, England
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Unless you're trying to tell me that the amperage is simply a measurement of the flow... and not related to the water pressure at all. Like... say, a paddle wheel with an RPM counter inserted into the stream coming out of the side of the dyke. And the amperage is merely the number on that RPM counter.
Is that what you're saying? Yes!
If so, then my whole problem was not realizing that amperage was just a flow measurement. In other words, amperage is determined by the resistance in much the same way that your speed is determined by how hard you press on the gas pedal. Exactly. (Well, the gas-pedal one is a fairly indirect connection, but yes.)
Peter
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#176517 - 25/08/2003 07:34
Re: Empeg power requirements
[Re: peter]
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carpal tunnel
Registered: 20/12/1999
Posts: 31597
Loc: Seattle, WA
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Thanks very much, everyone, for helping me to learn a basic concept that I needed to understand very much.
I'm trying to think of a better analogy other than the water thing, something that's less confusing... 
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#176518 - 25/08/2003 10:22
Re: Empeg power requirements
[Re: tfabris]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Well, if it helps, ampere (the unit for current) is actually defined as a certain amount of charge being transferred per second.
So yes, current is equivalent to the amount of water flowing past in a certain unit of time. Voltage is the pressure. And power (the amount of work the electricity can do), measured in watts, is the amount of current multiplied by the amount of voltage.
If we ignore all the complications of current and voltage being out-of-phase in AC, one watt is the work done by one volt of voltage andf a current of one ampere.
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#176519 - 25/08/2003 10:27
Re: Empeg power requirements
[Re: julf]
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pooh-bah
Registered: 02/06/2000
Posts: 1996
Loc: Gothenburg, Sweden
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one watt is the work done by one volt of voltage andf a current of one ampere
That'd be the power developed and measured in watts; if it kept up for a second it'd do 1 Joule [Watt*second] worth of work.
/Michael - nitpicker...
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/Michael
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#176521 - 25/08/2003 11:12
Re: Empeg power requirements
[Re: julf]
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pooh-bah
Registered: 02/06/2000
Posts: 1996
Loc: Gothenburg, Sweden
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Have we stretched the metaphor far enough?
Probably...
/Michael
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/Michael
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#176522 - 25/08/2003 13:44
Re: Empeg power requirements
[Re: mtempsch]
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pooh-bah
Registered: 25/08/2000
Posts: 2413
Loc: NH USA
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We don't even want to go near inductance with the water metaphor.
-Zeke
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WWFSMD?
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#176523 - 25/08/2003 14:18
Re: Empeg power requirements
[Re: Ezekiel]
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pooh-bah
Registered: 13/01/2002
Posts: 1649
Loc: Louisiana, USA
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In reply to:
We don't even want to go near inductance with the water metaphor.
Not power factor or phase angle either?
Stu
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If you want it to break, buy Sony!
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#176524 - 25/08/2003 22:04
Re: Empeg power requirements
[Re: Ezekiel]
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Carpal Tunnel
Registered: 08/02/2002
Posts: 3411
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We don't even want to go near inductance with the water metaphor.
Why ever not? Just look in your toilet bowl - the purging action that occurs at the end of a flush is due to the 'inductance' of the pipes.
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#176525 - 26/08/2003 02:18
Re: Empeg power requirements
[Re: genixia]
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veteran
Registered: 01/10/2001
Posts: 1307
Loc: Amsterdam, The Netherlands
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Just look in your toilet bowl - the purging action that occurs at the end of a flush is due to the 'inductance' of the pipes.
And what is the electrical equivalent of the stuff going down the drain?
Yea, I know... Couldn't help it...
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#176526 - 26/08/2003 07:57
Re: Empeg power requirements
[Re: Ezekiel]
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stranger
Registered: 10/09/2002
Posts: 48
Loc: Austin, TX
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So how about a new metaphor? I always liked marbles rolling down an inclined plane, myself: Voltage = gravity, current = # of marbles/sec rolling down, resistance = width of the inclined plane. Induction is then easily modeled by showing that putting another inclined plane adjacent, but imagine that its marbles are all magnets that pull on the marbles on the first inclined plane... so when the marbles roll down the 2nd hill they 'pull along' the flow on the first hill, making it go faster than it would just due to gravity. or making them move at all if they're still. Hrm, analogy needs work. oh well.
---pj
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#176527 - 26/08/2003 17:53
Re: Empeg power requirements
[Re: paulj]
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carpal tunnel
Registered: 08/07/1999
Posts: 5548
Loc: Ajijic, Mexico
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so when the marbles roll down the 2nd hill they 'pull along' the flow on the first hill, making it go faster than it would just due to gravity
My God, man! Don't publish such dangerous information. Don't you realize what you've done?
The magnets/marbles in the second plane accelerate the marbles in the first plane, which in their turn accelerate the marbles in the second plane -- an open ended positive feedback loop. As the marbles approach the speed of light, their mass will increase, eventually reaching enough density to turn into a black hole which will swallow up the whole world and everybody on it.
It's very dangerous to even talk about such things! If some lunatic bent on world destruction were to read this forum, I shudder to think of the consequences.
tanstaafl.
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