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Open Circuit voltage

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dann11

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Hi! I know this is quite a very simple question, where we all know that once a circuit is open we can measure a voltage equals to the supply voltage while the current is zero, but I want to fully understand why and how? why is the voltage be equals to the supply voltage at components before the portion where is cut. Like for example the simple circuit below. With ohms law, V=IR, if I = 0A, then the voltage should be zero as well, regardless of the resistance value, right?

open circuit.jpg
 

The presence of the power supply changes circumstances, and requires you to keep in mind what your meter is measuring. Your schematic has the meters reading supply V now, but if you add two more meters one across each resistor they'd read 0V.
Then when you close the switch, current flows causing readings across all 4 meters to change.
 

Hi,
I don't think I understand your question, but if you are asking why the voltage shown by XMM2 is Vsupply here is one simple explanation: Suppose we replace the open switch by a resistance (Rsw). Now if we consider a simple voltage divider, the voltage indicated by XMM2 is:
VXMM2 = Vsupply*(R1+Rsw)/(R1+Rsw+R2).
If we consider that Rsw is very, very large (Rsw->inf for open circuit) we can write the following:
VXMWW(when Rsw->inf) = Vsupply*Rsw/Rsw = Vsupply.
 

basically its kirchoffs law
There is no current through the resistors...therefore they have no voltage draped across them.
...therefore your meters just see the voltage of the battery.
To be nauseously academic about it..."the voltages around a loop must sum to zero."

But i fear that you are trying too hard to understand it in absoluteness....and you just have to accept it.......many tell that thats what being an engineer is...ie, you dont necessarily understand it, but you can wangle enough savvy together to design products that work and can be sold...

Only when you actually set out to try and make some electrical device which has a purpose will you start to come to terms with the science....and mostly, you will make compromises, and you will just console yourself that even though you never understood, you still sucessfully ade the product.

If someone had the answers to all engineering, then that person woudl also know the answer to why the universe exists, and when it started, and how it started.
But in fact, the best scientists are just basically clinging to a range of physical Maxims, desperately trying to explain things with them. None of them understand String Theory.

Space aliens are looking at us and laughing their heads off.

You have 2 engineers.....
one understands all the theory necessary, but still cant make a marketable product
one doesnt properly understand, but can make a product that is marketable.
......which one is the better engineer?

Of all the engineers around (years ago) in UK, why did it need Sir James Dyson (who has no formal engineering training), to actually come up with the brilliant Dyson products.

In any case, all the V , i and ohms law stuff is all , stricly speaking , totally incorrect....and is part of what is known as the "low frequency approximation"......because in fact, all electricity flows round circuits as electromagntic fields, mostly outside the conductors, but following them closely....Maxwell's laws etc
 
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