Electronics > Beginners
Some noob questions
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ArthurDent:
Measuring resistance is a variation of what I stated above: “You measure the current flowing THROUGH a known resistance and convert that to VOLTAGE”; only to measure resistance you pass a known current through an unknown resistor and measure the voltage and convert that to resistance. If ohm’s law only has 3 terms and you are trying to solve for one, the other two must be known.

R=E/I
Brumby:

--- Quote from: Mr D on July 21, 2018, 01:53:56 pm ---Wait, nope, i still don't understand. Is it possible to measure the current without knowing BOTH the voltage and resistance?
--- End quote ---
Well, yes it is - but not with a DMM.  You can do this using an analogue meter movement.  If the basic sensitivity of the meter is 50uA, then you can push through anything from zero to 50uA and it will give you the current.  You don't need to know the resistance or the voltage.  Of course, since the coil of the meter will have some resistance, there will be a voltage drop across it, but you don't need to know either of these to measure your current.

However, if you want to use this meter movement to measure currents higher than 50uA, then you WILL need to know voltages and resistances in order to build the appropriate shunt circuit.  Ohm's Law all the way!!


--- Quote ---Or put another way: to measure either, voltage, current or resistance, you ALWAYS need to know the other two? Or is one ever enough?

--- End quote ---
With resistance, you have a passive quantity and you need to "excite" the component and determine its response in order to calculate that quantity ... So, yes, voltage and current are both needed.  Always.

With current, you can - in some cases - measure it directly, but with any sort of conditioning network, Ohm's Law is a must.  In the case of a DMM, you absolutely need it.

Voltages are an interesting case and while there are some techniques that can be used to measure a voltage without using Ohm's Law, they are not the sort of thing you would see very often, if at all.  So, in practical terms, Ohm's Law is applied almost universally.


The one overruling fact, though, is that if you cannot directly measure the value (as in the 50uA meter movement mentioned above) then you must have the other two values for Ohm's Law to be used.  This is fundamental mathematics - whichever way you write V=IR (you can also see it written E=IR), there can ONLY ever be ONE unknown.
drussell:
I believe I do understand some of Mr. D's confusion from many years ago when I was a beginner...

You do, indeed, need to understand that Ohm's Law is a constant LAW and that you can convert and figure out the voltage drop across a known resistance as the current just as well as you can use the current through a circuit with a known resistance to infer the voltage.  (edit: or the current and voltage to calculate the resistance...)

It simply depends on the instrument you are using to actually measure the quantity you're observing.  A small current measured by a galvanometer of some sort or other current meter like you would find in a VOM, (typically the 5k, 20k, 30k or 50k ohm-per-volt meter movement) in series with (for voltage) or parallel with (for current) measurement of that quantity actually measures by physical deflection due to the current inducing a magnetic field.

On the other hand, as mentioned previously, the analog to digital converter (ADC) in a typical DMM actually measures the voltage potential (or Electro-Motive Force, EMF) being pushed/pulled between those two points, despite the fact that the actual input resistance may be very high (10,000,000 ohms, 10,000,000,000 ohms or even more on some bench meters) or even essentially completely open.

They're not using the same method to measure the specific quantity but it can always be calculated if you know one of the two other variables.  (Well, for DC, anyway...  :) )
Mr D:
Hi folks,

During my summer holiday to the Alps i re-read through this thread a few times, and also thought about Ohm's law quite a bit and i think i got my head around it.

I've also discovered the MathsTutorDVD lectures and they're really helpful.

Now i think the next thing for me to understand is ground.

I have some questions about this, but first i need to clear something else up: While searching Youtube for info about ground, i came across this guy's vids:



Can anyone tell me: why is he dipicting flow from neg. to pos. in this video?
Mr. Scram:

--- Quote from: Mr D on August 16, 2018, 11:05:24 pm ---Hi folks,

During my summer holiday to the Alps i re-read through this thread a few times, and also thought about Ohm's law quite a bit and i think i got my head around it.

I've also discovered the MathsTutorDVD lectures and they're really helpful.

Now i think the next thing for me to understand is ground.

I have some questions about this, but first i need to clear something else up: While searching Youtube for info about ground, i came across this guy's vids:



Can anyone tell me: why is he dipicting flow from neg. to pos. in this video?

--- End quote ---
When electricity was discovered they found out that it flows from one end to the other. They couldn't actually see or measure which way it flowed however, so they picked a direction and went with it. When science finally advanced enough to actually measure the direction of the flow, it turned out that the convention is actually wrong. Electrons flow from the negative pole to the positive and not the other way around. It doesn't really matter though, because the traditional model works perfectly fine as it is unless you go into some detail most people will never deal with. It's too much trouble to swap everything around, so people just work with the "wrong" model.

"Conventional current" is the name of the traditional model we all know and tend to work with where electricity flows from positive to negative. This turns out to be wrong, but works perfectly fine. "Electron flow" is the name for using a model with the actual direction of the electrons. People don't use this as often and when you see a generic schematic, you can assume it to be conventional current.
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