Basic Question on voltage

Thread Starter

msn56

Joined Apr 9, 2010
15
I am just getting started in electronics. I have been looking at voltage in parallel DC circuits and using the water analogy of pressure for voltage and flow of water for current . Say I have a simple circuit starts with 9 volts and goes to a resistor of say 50 ohms now it splits into a parallel with two resistors one at say 100 ohms and the other at 5 ohms - ok its easy enough to calculate the voltage drop across that system - but here is what I find confusing - say I keep ramping up the 100 ohm resistor - ok its going to use more and more voltage - if i replace it with say 5 Mohm its going to use up most of the voltage. The water analogy would be the pipe is getting smaller and smaller. Now replace it with 500Gohm - ok again even more of the voltage is used up the pipe is getting extremely small - so lets just keep making the pipe smaller and smaller and smaller - it keeps using up more and more voltage ... we keep making it smaller now only a few molecules of water get through it has a huge resistance - now only one water molecule can get through the pipe - monstrous resistance now none can get through and the resistance drops to zero just seems odd that as we close down the pipe resistance gets higher and higher until its totally closed and then we have "no resistance" MY only answer is that we have no flow and if we have no flow (no current) that you cant calculate voltage - but if you let just one water molecule through then you have flow and voltage is used up

so to push millions of water molecule through ( ie low resistance ) you little voltage - to push thousands you use some voltage to push one one water molecule through you use up most of your voltage but to push none through you use no voltage

millions of water molecule ++++++++ less voltage used +
thousands of water molecules ++++ some voltage used ++++
one molecule + most voltage used ++++++++++
zero molecule 0 No voltage 0

In the left hand column the numbers go to zero if the right hand numbers head towards infinity - but as you continue the process they both drop to zero essentially destroying the patter that was developing
 

MrChips

Joined Oct 2, 2009
22,912
Your flaw is when you said "no resistance".

As you keep increasing the resistance the limit is infinite resistance, not zero resistance.
When you have infinite resistance, the current is zero. Then it does matter how many volts you apply, one volt or one million volts, the current is still zero.

btw, never think in terms of voltage used. Resistors do not "use up" voltage.
A resistor can experience a voltage drop across its leads. It does not "use up" the voltage.
 

MrAl

Joined Jun 17, 2014
7,979
I am just getting started in electronics. I have been looking at voltage in parallel DC circuits and using the water analogy of pressure for voltage and flow of water for current . Say I have a simple circuit starts with 9 volts and goes to a resistor of say 50 ohms now it splits into a parallel with two resistors one at say 100 ohms and the other at 5 ohms - ok its easy enough to calculate the voltage drop across that system - but here is what I find confusing - say I keep ramping up the 100 ohm resistor - ok its going to use more and more voltage - if i replace it with say 5 Mohm its going to use up most of the voltage. The water analogy would be the pipe is getting smaller and smaller. Now replace it with 500Gohm - ok again even more of the voltage is used up the pipe is getting extremely small - so lets just keep making the pipe smaller and smaller and smaller - it keeps using up more and more voltage ... we keep making it smaller now only a few molecules of water get through it has a huge resistance - now only one water molecule can get through the pipe - monstrous resistance now none can get through and the resistance drops to zero just seems odd that as we close down the pipe resistance gets higher and higher until its totally closed and then we have "no resistance" MY only answer is that we have no flow and if we have no flow (no current) that you cant calculate voltage - but if you let just one water molecule through then you have flow and voltage is used up

so to push millions of water molecule through ( ie low resistance ) you little voltage - to push thousands you use some voltage to push one one water molecule through you use up most of your voltage but to push none through you use no voltage

millions of water molecule ++++++++ less voltage used +
thousands of water molecules ++++ some voltage used ++++
one molecule + most voltage used ++++++++++
zero molecule 0 No voltage 0

In the left hand column the numbers go to zero if the right hand numbers head towards infinity - but as you continue the process they both drop to zero essentially destroying the patter that was developing

Hi,

You are deep thinking into this and that's good. You just might want to take that thought to the next level where you imagine that things are NOT the way that you imagined in the first thought. Thus you ask yourself more and more questions some of which may actually contradict what you think is true in the first place and try to answer them too.

For example, if we calculate voltage from:
V=I*R

with a small current I and some resistance R we get some voltage V, and if the current goes down we still get some voltage V so we can still calculate that, or divide by the current and get the resistance R. But what happens if the current I goes to zero, then we get:
V=I*R
0=0*R
and if we try to calculate R:
R=V/I
and I goes to zero:
R=V/0
which makes it look like R went to infinity.

But what really happens when I goes to zero is V also heads toward zero so we have:
R=0/0

and the then we have to look at what is called the "limit" as the current I goes to zero.

To start, lets say we have a 10 ohm resistor and 1 amp current:
V=1*10=10 volts

now we reduce the current to 0.1 amps and have:
V=0.1*10=1 volt

and now reduce the current to 0.01 amps:
V=0.01*10=0.1 volt

In those three cases we had:
R=V/I=10/1=10
R=V/I=1/0.1=10
R=V/I=0.1/0.01=10

so we see that the resistance never changed.
Now in the extreme case when the current goes to zero, we need to set up an expression that shows this so we can test it with lower current. An expression that fits this idea is:
R=10*x/x

and that is because whenever the current is 'x', the voltage is ten times 'x', and R=V/I again.
What we can do now is take the limit as 'x' goes toward zero:
R=limit(10*x/x) [as x goes toward zero from the right]

and what we get after evaluating that limit is:
R=10

which is the same as before.

Note also that we dont have to do this mathematically like this, we can use extrapolation. That's when for example we measure the voltage with a real meter and then extrapolate to the open circuit voltage. What this means is that the current is never zero, but we calculate for zero current using two non zero currents, If we take two readings instead of one we can figure out more information using math to figure out what happens at other values of test currents.
 

noweare

Joined Jun 30, 2017
115
You said 100 ohms in parallel with 5 ohms. so the parallel resistance is slightly less than 5 ohms. Making the 100 ohms larger and larger will not affect the circuit since the 5 ohms dominates.
 

dl324

Joined Mar 30, 2015
12,515
I have been looking at voltage in parallel DC circuits and using the water analogy of pressure for voltage and flow of water for current .
I think the water analogy does more harm than good.

If the concept of current flow, voltage, and resistance are so confusing that you need to use that analogy, here are some links that should be correct:

https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law/all

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html

https://en.wikipedia.org/wiki/Hydraulic_analogy

It would be helpful if you used paragraphs to organize your thoughts and proper grammar. Your posts will be easier to read.
 
Last edited:

Thread Starter

msn56

Joined Apr 9, 2010
15
Hi,

You are deep thinking into this and that's good. You just might want to take that thought to the next level where you imagine that things are NOT the way that you imagined in the first thought. Thus you ask yourself more and more questions some of which may actually contradict what you think is true in the first place and try to answer them too.

For example, if we calculate voltage from:
V=I*R

with a small current I and some resistance R we get some voltage V, and if the current goes down we still get some voltage V so we can still calculate that, or divide by the current and get the resistance R. But what happens if the current I goes to zero, then we get:
V=I*R
0=0*R
and if we try to calculate R:
R=V/I
and I goes to zero:
R=V/0
which makes it look like R went to infinity.

But what really happens when I goes to zero is V also heads toward zero so we have:
R=0/0

and the then we have to look at what is called the "limit" as the current I goes to zero.

To start, lets say we have a 10 ohm resistor and 1 amp current:
V=1*10=10 volts

now we reduce the current to 0.1 amps and have:
V=0.1*10=1 volt

and now reduce the current to 0.01 amps:
V=0.01*10=0.1 volt

In those three cases we had:
R=V/I=10/1=10
R=V/I=1/0.1=10
R=V/I=0.1/0.01=10

so we see that the resistance never changed.
Now in the extreme case when the current goes to zero, we need to set up an expression that shows this so we can test it with lower current. An expression that fits this idea is:
R=10*x/x

and that is because whenever the current is 'x', the voltage is ten times 'x', and R=V/I again.
What we can do now is take the limit as 'x' goes toward zero:
R=limit(10*x/x) [as x goes toward zero from the right]

and what we get after evaluating that limit is:
R=10

which is the same as before.

Note also that we dont have to do this mathematically like this, we can use extrapolation. That's when for example we measure the voltage with a real meter and then extrapolate to the open circuit voltage. What this means is that the current is never zero, but we calculate for zero current using two non zero currents, If we take two readings instead of one we can figure out more information using math to figure out what happens at other values of test currents.

Thanks I like your explanation - it is indeed a limit problem and dividing by zero is undefined
 
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