Trying to reconcile contradictory statements about parallel circuits

[fourthindiana]

TL;DR Summary

Will electrical current flow through all the resistors in the parallel circuit in the attached photograph?

Preface to thread: The "closed switch" in this ladder diagram is a normally closed contactor. Sometimes when I post photographs of ladder diagrams with this symbol on them, people sometimes mistakenly assume that the switch is a variable capacitor. That's why I am letting everyone know that the normally closed switch is a normally closed contactor.
___________________________________________________________________________________________________________________________________________I'm a student attending a HVAC program at a trade school. In the photograph of a ladder diagram attached to the top of this post, my instructor says that electrical current will only flow through the 200-Ohm resistor. My instructor says that no electrical current whatsoever will flow through the 100-Ohm resistor. My instructor says that "electricity is lazy", and my instructor says that the electricity will take the shortest possible path from line 1 to line 2. Therefore, since electricity will take the shortest possible path from line 1 to line 2, electricity will bypass the 100-Ohm resistor and go straight from line 1 through the 200-Ohm resistor and to line 2.

We have an electrical wiring training board at the trade school. My instructor wired up a parallel circuit that my instructor said was exactly the same as the ladder diagram I attached at the top of this post. I put my amp clamp on the wire that was equivalent to the wire that goes to the 100-Ohm resistor, and my amp clamp measured 0.0 amps. But perhaps my meter resolution was too low to pick up on current. However, my instructor told me that not one electron of current will flow through the 100-Ohm resistor.

I thought I understood what my instructor was saying about how electricity would flow in a circuit with the attached ladder diagram until I thought about an exchange of messages I had on Physics Forums back in December of last year on a thread I created with a title similar (but not exactly the same) to this title of this thread. The thread I created back in December 2018 is titled "Reconciling an apparent contradiction about parallel circuits". My "Reconciling" thread back in December was about parallel circuits with digital multimeters in them. In my "Reconciling" thread back in December, pf member Borek told me that electrical current will flow through all resistances in a parallel circuit. Nobody else on the thread disagreed with what Borek said.

What Borek told me indicates that current would flow through the 100-Ohm resistor. My instructor says that no current whatsoever would flow through the 100-Ohm resistor.

Who is correct, Borek or my instructor?

In the parallel circuit in the ladder diagram in the photograph attached at the top of this post, will any electrical current flow through the 100-Ohm resistor?

Please explain your answer.

If you say that electrical current will flow through the 100-Ohm resistor, how can I prove this to my instructor using the electrical wiring practice board at the trade school?----------------------------------------------------------------------------------------------------------

Here is a link to the "Reconciling" thread I created back in December of last year. Borek's post is post #4 in this thread.

 

[berkeman]

Could you please post a diagram that uses normal circuit schematic symbols? If your variable capacitor is something else, that's not a useful diagram, IMO. And the circles would normally be voltage sources in electrical schematics, but if they are resistors, please use more normal symbols to ask your question in this thread. Thank you.

 

[essenmein]

Ok so technically speaking with the switch closed there will be current in both the switch and the 100Ohm resistor. This is because there is no such thing as zero resistance, its just a convenient simplification. You could waste a lot of time calculating all the individual actual resistances and their resulting series/parallel values, but it would not yield you any more useful information for your design. Its much simpler to assume switches are zero ohm, and cables are zero ohm, this will how ever bite you if you start pushing the limits of cables or conductors etc.

Lets say the switch has a very low, but non zero resistance, maybe its a few milli ohm depending on the rating of the switch. So when you do the math and put 1mOhm in parallel with 100Ohm you get a value ever so slightly less than 1mOhm. Basically the switch if it is 1mOhm has 100,000x lower resistance than the 100Ohm resistor, so therefore the 100Ohm resistor will flow 100,000x less current. If there is 10A flowing in the switch, the resistor would see about 100 micro amps which results in heating power of about 1 micro watt, which in the context of a heating element most of us would be quite happy to round that to zero.

So to answer your questions, Borek is correct, and no your amp meter with 0.0A does not have enough resolution to see the 100Ohm resistor current, you would need an amp meter good down to 0.00001A.

 

[fourthindiana]

berkeman, here is a new wiring diagram with the schematic symbol for resistor that you like.

 

[berkeman]

berkeman, here is a new wiring diagram with the schematic symbol for resistor that you like.

Thank you, but you're still saying that your variable capacitor is a switch? In that case, I agree with @essenmein and @Borek (who is on holiday at the moment).

from wikipeda:

 

[AZFIREBALL]

The amount of current that will flow through a switch that is in parallel with a resistor will be inversely proportional to the relative resistance values in the two devices. If the switch has zero resistance, while the resistor has some resistance, there will be no current flow through the resistor.

This can be verified by measuring the voltage drop across the resistor...which should be zero in this case (if the switch’s resistance is truly zero). Use a very sensitive volt meter or scope to make the measurement and you will be able to see the results.
BTW-This is the normal symbol for a switch. See:(http://www.circuitstoday.com/electronic-circuit-symbols)

 

[Asymptotic]

Thank you, but you're still saying that your variable capacitor is a switch? In that case, I agree with @essenmein and @Borek (who is on holiday at the moment).

from wikipeda:

View attachment 246584

View attachment 246588

They can cause confusion (it threw me the first time I ran into them fresh out of school) but those "capacitor like" symbols are legit NEMA representations for contacts, and often found in industrial and HVAC drawings in the United States. They are also the standard symbols used in IEC 61131-3 ladder logic programming languages.
https://en.wikipedia.org/wiki/Ladder_logic

A tech must become comfortable interpreting them in context as they are the most common contact symbols used in American HVAC drawings, particularly in motor circuits.

From Eaton "Comparison of NEMA and IEC Schematic Diagrams", Cross-reference MZ081001EN
http://www.eaton.com.co/ecm/idcplg?...aveAs=0&Rendition=Primary&dDocName=MZ081001EN

 

[Tom.G]

Here is one way you could demonstrate on the training board.

First establish with your instructor that there must be a current flowing thru a resistor to have a voltage drop across it. Since this is basic Ohms Law, I expect he will readily agree.
(If no agreement, report him to management as incompetent....well, maybe not such a good idea.)

• Connect a 10k Ohm resistor in series with the 100 Ohm.
• Connect a Voltmeter that reads down to millivolts across the the 10K.
• Show him the voltage reading across the 10k resistor.

Have Fun!
Tom

p.s. Thanks to @Asymptotic for the explanation and documentation. I've pointed that out here on PF a few times. I guess we can't all read every post.

 

[Asymptotic]

p.s. Thanks to @Asymptotic for the explanation and documentation. I've pointed that out here on PF a few times. I guess we can't all read every post.

I may have read it in earlier posts, but my memory leaks like a sieve .

 

[Tom.G]

I may have read it in earlier posts, but my memory leaks like a sieve .

Mine too!
Anyhow, my ambiguous wording was intended to actually compliment you for finding and posting the documentation. Between us, and hopefully a few others, people will realize that there is more than one established language in which to write schematics.

Cheers,
Tom

 

[Asymptotic]

Mine too!
Anyhow, my ambiguous wording was intended to actually compliment you for finding and posting the documentation. Between us, and hopefully a few others, people will realize that there is more than one established language in which to write schematics.

Cheers,
Tom

We had an old surface grinder in our shop that was a German design sold through a Spanish firm with the schematic in both languages and a mish-mash of NEMA, IEC, and other symbols they may have cooked up themselves. It was sometimes necessary to follow wires back from known points in the drawing to individual parts to figure out what the symbols meant. :)

 

[Guineafowl]

Summary: Will electrical current flow through all the resistors in the parallel circuit in the attached photograph?

my instructor says that the electricity will take the shortest possible path from line 1 to line 2

The contactor will have some small but finite resistance, so will act as a parallel resistor to the 100 ohm. The resistances will combine as per: 1/R1 + 1/R2 = 1/Rtotal.

Since the contactor’s resistance will be many orders of magnitude less, the vast majority of current will flow through this easier path, but some will flow through the 100 ohm resistor. Too little to register on your amp clamp.

In everyday terms, you can think of the current taking the path of least resistance, but always keep in mind that it really takes all paths in proportion to their resistance (see equation above).

 

[Nik_2213]

Slightly Off-topic, but I remember coming across a robot circuit which had sundry small-value resistors in places I did not expect, and with no immediately apparent function...
Variously they were for...
System analysis, being a convenient way to clip probes and monitor stuff, like those LEDs in 12/24 Volt fuses.
Spike mitigators, dissipating back-EMF from switched inductive components.
Ground-loop limiters.
Load balancing.
Adjusting nearby resistor to 'non-preferred' value under 'limiting' conditions.
And, yes, some I never did figure...

FWIW, my eye-balls nearly fell out the first time I saw a process-control 'Ladder Logic' chart.
Huh ??
IIRC, such evolved from robust, relay-style switch-boards and contactors, rather than wireless / RF etc, so speak a rather different but very concise dialect of 'Electronician'...
Kin to 'Gant Charts', too...