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Old 06-11-2011, 12:54 AM #49
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Default Re: Rethinking our approach to the DDL?

I understand what HIMNL is saying...
You can not have current without voltage AND resistance...
and yet you are saying that is not so...

Can you produce and measure current without voltage or resistance...
I can easily measure voltage without resistance or current (straight off
a battery) and I can easily measure resistance without voltage (right
across a resistor) with my DMM...


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Old 06-11-2011, 01:27 AM #50
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Default Re: Rethinking our approach to the DDL?

Quote:
Originally Posted by lasersbee View Post
I can easily measure voltage without resistance or current
There's the internal resistance of the voltmeter, and measuring voltage requires drawing a small amount of current.

Quote:
Originally Posted by lasersbee View Post
I can easily measure resistance without voltage
There's the voltage applied by the DMM itself.
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Old 06-11-2011, 07:39 AM #51
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Default Re: Rethinking our approach to the DDL?

@ Cyparagon: yes, but these are external parameters, that you apply for do the measure ..... they does not influence the existence of the voltage and the resistance .....

I mean, as example, a resistor of 100 ohm, have a value of 100 ohm, independently from the fact that you are measuring it or not, AND independently from the fact that you are applying a voltage on its sides ..... same, a power supply that give you 5V, at the output of the PSU the 5V voltage exists independently from the fact that you are measuring it or not, AND independently from the fact that you have a load connected or not ..... but when you connect the 100 ohm resistor to the output of the 5V PSU as load, you have through it a current of 50 mA only until the resistor is connected to the PSU ..... sure, until it's connected. the current too exist also if you are not measuring it, but when you open the circuit, no more current, also if you still have a voltage and a resistance presents ..... the current, opposite to voltage and resistance, cannot exist without an interaction from the first two things.

This is called primary and secondary parameters (maybe the English uses a different word, other than parameters ? ..... values ?) ..... a primary parameter is something that exist alone, and can be used alone, a secondary parameter is something that derivate from the interaction of two (or more) primary parameters, and that cannot exist alone, without the interaction of the first two things.
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Old 06-11-2011, 07:49 AM #52
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Default Re: Rethinking our approach to the DDL?

I'm not sure that's true, HIM.

The statement could be reversed in another universe where batteries supplied constant voltage as the current dropped. And, this wouldn't be a different universe from ours... in fact, it could happen here. We just don't know how yet.

The point is, the mathematics of Ohm's Law states that any one value is completely dependent on the other two.

That said, how do we know that the 100 ohm resistor has that value until we measure it? How do we know that the voltage of a 5V PSU is 5V, until we measure it? In order to do those measurements, we need the other parameters. And we can't be sure they don't "drop to zero" when we aren't measuring them... it's like, what happens if a tree falls in the forest and no one is around to hear it?

EDIT: This is similar to quantum mechanics' Copenhagen Interpretation, now that I think about it, anyway. The only way something is defined is after we observe something. It doesn't have value until it's been observed.

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Old 06-11-2011, 08:36 AM #53
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Default Re: Rethinking our approach to the DDL?

Quote:
..... in another universe .....


^ we know those things are so, cause someone had fun inventing phisic laws (in this universe, so they works only here, afaik ) ..... i don't doubt can exist some different universe (ask Boliay and Lobachewsky ), but we can't (yet) access them, so we're limited on our boring 3 dimensional one, for now, and here things works in that way

BTW ..... quantum mechanic is easy to do ..... you just need quantic tools

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Old 06-11-2011, 08:38 AM #54
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Default Re: Rethinking our approach to the DDL?

The point being, we just can't imagine a way to do it here. The point of mentioning another universe was just to simplify it from having to explain *how* to build that constant voltage battery (I am not a chemistry expert :P ). Point being, I think you're wrong and you haven't disproved me.
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Old 06-11-2011, 08:57 AM #55
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Default Re: Rethinking our approach to the DDL?

Er ..... i hope you're doing this discussion just for the fun, right ? (also cause i don't want to start a war )

Anyway, still for the fun .....

First, i suppose you mean "constant current" battery, cause the ones already existing are, more or less, all "constant voltage" ones.

Second, we already have some sort of "constant current" batteries, also if that was not the goal of the manufacturers ..... those are small, low capacity cells, that have an internal resistance high enough for self-limit the given current (If you want a practical example, think about those led keychain lights powered from 2 Li-Ion CR2016 cells in serie, with no resistance ..... this gives you 7,2V directly connected to the led ..... do this with a power supply and *PAF*, no more led, but these cells have an internal resistance so high that they self-limit the maximum current that they can make flow in the diode)

Third, i don't "need", technically speaking, to "prove" or "disprove" you, cause you're taking as example a totally hypotetic fact ..... if the fact, for its own definition, cannot be tested, proved, demonstrated, or imagined (also only mathematically), then for its own nature is not existent, in the actual context, so it don't need to be disproved ..... no offenses here, i'm just saying that i can (with your same line of thinking and your same words) say to you that you are wrong talking about another universe, and cause you cannot demonstrate me that it exist, you haven't disproved me
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Old 06-11-2011, 10:43 AM #56
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Default Re: Rethinking our approach to the DDL?

Quote:
Originally Posted by Benm View Post
I don't want to push it into academic discussions too far, but current actually is a base unit that is simply define as the number of electrons per unit of time. To use a water analogy: you can quite simply make a tap that spills a liter of water per minute(~current), regardless of how high that water falls from (~voltage) after leaving the tap.
Not true. Current is always and fundamentally bound to voltage and resistance by Ohm's law. That's why it is a law.

In a pipe, you cannot have flow without an energy potential between two points. You can also measure the water's potential by determining the water flow and the resistance of the pipe. Given that water is a relatively incompressible medium, the only way to cause more water to pass through a given pipe diameter is to increase its flow rate ("current"), which requires more pressure (potential).

Likewise, a diode provides a voltage drop, but it provides no current control due to its low resistance. It may seem like a magic situation circumventing Ohm's Law, but it really means that the diode will die if connected up directly to a high current source like any wire. I have no idea what the water-equivalent of a diode would be.

Quote:
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The statement could be reversed in another universe where batteries supplied constant voltage as the current dropped. And, this wouldn't be a different universe from ours... in fact, it could happen here. We just don't know how yet.
We do have that. It's called a voltage regulator. While the regulator is not a battery, it doesn't matter because it's a voltage source. The voltage regulator will provide (up to the hardware limits) any amount of current to maintain a specific voltage. If the current needs are beyond the capability of the voltage regulator something will break because V = IR in all cases.

Ohm's law always applies even with non-linear devices such as voltage regulators and boost/buck circuits. An example: you have a boost circuit that has a 3V input, and a programmed 1Amp output. An example would be Dr.Lava's Microboost. Connect that up to a 1ohm resistor. It will not function. Why? Because V = IR = 1A * 1Ohm = 1V and boost circuits don't operate if Vout < Vin.

Another example that really made Ohm's Law hit home for me with passive and active devices: we were trying to use a MOSFET as a voltage-controlled switch for some high powered LEDs that would run about 30 Amps max. However, just for testing, we had the MOSFET connected in line with an incandescent light bulb. The test apparatus used a 6V lead-acid battery for the power supply.

When we turned the circuit on, the light bulb lit up, but the MOSFET was running extremely hot. We measured about 9 Amps flowing through the circuit which should have been perfectly fine for both the light bulb and MOSFET to handle, especially with the MOSFET's extremely low Rds_on (~7 micro-ohms). Unfortunately it was not. The light bulb, because it was a passive resistance (~0.2ohm), only dropped 1.8V at 9 Amps. That meant, to make up the voltage difference, the MOSFET had to eat 4.2V; at 9 amps, that was nearly 40W being converted directly to heat through our MOSFET. It didn't matter at all what the Rds_on was, the MOSFET had a 4.2V potential across it at 9A and had to drop all that power as heat.

A good rule with Ohm's Law and active devices: if you think you're skirting Ohm's Law with active devices, you can soon expect the magic blue smoke to disappear from the device.

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Old 06-11-2011, 11:36 AM #57
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Default Re: Rethinking our approach to the DDL?

Please explain for noobs in electronics what you are trying to make?
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Old 06-11-2011, 12:54 PM #58
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Default Re: Rethinking our approach to the DDL?

Quote:
Originally Posted by personx910 View Post
Please explain for noobs in electronics what you are trying to make?
I would suggest you start by reading the 1st Post on
this Thread to get up to speed....


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Old 06-11-2011, 07:10 PM #59
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Default Re: Rethinking our approach to the DDL?

I still think that current, voltage, and resistance are all equally fundamental. Because of the way they are defined, we can state that resistance is nothing without voltage and current; voltage is nothing but resistance and current. Those definitions alone make the claim that current is fundamental, as is voltage and resistance.

@HIM: Yes, this is all for fun :P
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Old 06-12-2011, 08:32 AM #60
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Wink Re: Rethinking our approach to the DDL?

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I still think that current, voltage, and resistance are all equally fundamental. Because of the way they are defined, we can state that resistance is nothing without voltage and current; voltage is nothing but resistance and current. Those definitions alone make the claim that current is fundamental, as is voltage and resistance.

@HIM: Yes, this is all for fun :P
Ok, then .... i also feel these discussions funny

I never said they are not fundamental or important ..... i've just said that two of them are primary values, and one is secondary value that can only produced from the interaction of the first two (and no, voltages and resistances still exists, also if you are not using them, opposite to currents)

Let me give you a different example: think about "speed measurement" devices, that police use for give you fines (tickets ?) for excess of speed ..... all the time that they write on the document that the speed was "measured" with a "speed meter" instrument, they do a false declaration in an official document, technically, cause the speed cannot be measured, only calculated ..... speed (secondary value) is distance per time, and it does not exist, without these two primary values, so cannot be directly measured ..... you can measure the time you took for do a certain distance, and then calculate the speed, nothing other.

Same for current, you can place a voltage on a resistor and THEN derive the current from this, but not measure it without the first two things (and, if you say me that you can measure it with an amperometer, i can say you that "amperometer" is an unexisting instrument, itself ..... an amperometer is just a voltmeter that measure the difference of voltage caused at the sides of a known resistor from a certain current flowing in the circuit, it cannot measure DIRECTLY the current )
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Old 06-12-2011, 12:07 PM #61
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Default Re: Rethinking our approach to the DDL?

^^^^ I understood exactly what you were saying the first time
you explained it... and I still agree...

BTW... the speed explanation might come in handy the next time
I go to court for a speeding ticket....


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Old 06-12-2011, 08:06 PM #62
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Default Re: Rethinking our approach to the DDL?

You say that the resistance and the voltage still exist without the other parameters... but how do we know?

Via this equation: V = IR, we can also show that R = V/I. Thus, resistance can *only* be measured by using only voltage and current.

As someone said earlier, current actually is a fundamental, not secondary, unit. It's the rate of electron flow per second.

In fact, in the water analogy, we can think of voltage as just the definite integral of the current between the two "end points" of the circuit. That's all voltage is - a difference in charge between two points on a circuit. And, because electrons carry charge, defining voltage is easy enough to say that it's just the average difference of current between two points. Further, "The voltmeter works by measuring the current through a fixed resistor." As per Wikipedia.

Further, voltage is actually not a base unit in the SI system. Once again, as per Wiki, the way to define voltage using base SI units is this:

V = (kg*m^2)/(A*s^3)

The Ampere, in fact, is one of the seven basic SI units. And, by the definition of the Ampere, you can actually measure current without using voltage or resistance. Once again, according to Wiki (I love Wiki, great source of knowledge), the Ampere is defined as: "The constant current which will produce an attractive force of 2 10–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum."

Interestingly enough, the reason why we often measure the Ampere in terms of voltage and resistance is because the methods of reproducing a single volt and a single ohm is relatively easy (without using Ohm's Law), but the experiment mentioned above is relatively difficult to reproduce, even though the volt and the ohm are actually defined in terms of the Ampere. The idea is that the ampere was first defined, and then, via those, the volt and the ohm were defined, which now offer a good reference to access the ampere.

Just to make sure I get it across right, the Ohm is also defined in basic SI units in terms of the Ampere, the kilogram, the meter, and the second.

Ohm = (kg * m^2)/(s^3*A^2).

So there - did I prove my point now?
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Old 06-13-2011, 05:47 PM #63
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Default Re: Rethinking our approach to the DDL?

It's boiling down to a battle of semantics and perceptions. Ohm's law describes a set of relationships to us, but neither controls nor dictates anything beyond those relationships. The relationships are proven by physics: Voltage is the potential (the preasure pushing the electrons), Current is the flow of those electrons (no flow, no current, but you can still have the potential, voltage). Resistance is any force that impeads that flow. Open circuit, no current, seemingly infinite resistance, voltage at the maximum potential of the source. Closed circuit, some resistance, some current, voltage at some potential. Short circuit, High current (to potential of source), low to seeming zero resistance, low to seemingly zero voltage. How can you have high current and 0 volts?? because the value of R is too low to 'measure', so we cannot 'measure', or calculate the voltage,, but it is still there. As HIMNL9 stated, we use various meter configurations to apply Ohm's law to calculate the values of E, I, or R, and call the displayed results of those calculations 'Measures'. Bottom line, though, is that we could NOT, as explained by the laws of physics, have any one without some level of BOTH the other two.

P.S. Is this not the coolest forum which exists??

Last edited by 123splat; 06-13-2011 at 05:48 PM.
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Old 06-13-2011, 07:10 PM #64
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Default Re: Rethinking our approach to the DDL?

123splat, that's what I have been trying to say - we cannot have one without the two others. It's not like you can have voltage without current and resistance. You can't have current without voltage and resistance. You can't have resistance without voltage and current.

And yes! This *is* the coolest forum which exists!
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