Technical problem with the program.

Thread Starter

Xenon02

Joined Feb 24, 2021
355
Hello !

I've got a quick question.
Is it me or is the program wrong :

1660086199840.png

Left diagram shows the voltage on the source and right one shows the voltage on the resistor.

I am wrong maybe but shouldn't resistor have 5V instead of -5V ?
 

MrChips

Joined Oct 2, 2009
26,807
Depends on how you label your currents and voltages.

If you apply Kirchhoff's voltage law, the sum of all voltages in a closed loop is equal to zero.
See if you can work with that.
 

Papabravo

Joined Feb 24, 2006
19,023
In going around a loop it is conventional to regard a voltage rise, going from ground to +5V as positive, and a voltage drop across a resistor, going from +5V to ground as negative. KVL is compatible with the opposite convention if you wish. That is voltage drops are positive and a voltage rise is negative. Also you can go around the loop in either direction just as long as you are consistent.
 

WBahn

Joined Mar 31, 2012
26,932
Nothing weird about it.

What does it mean for the voltage across a resistor (or any other two-terminal device) to be 5 V?

It means that the voltage on one of the terminals if 5 V higher than the voltage on the other terminal. But which terminal? Let's say that the voltage on the top terminal if 5 V higher than the voltage on the bottom terminal. That might be how YOU picture it, but there is nothing that prevents someone else from saying that the voltage on the bottom terminal if 5 V lower than the voltage on the top terminal and thereby claiming that the voltage across the resistor is -5 V. Both are correct, but each is using a different reference.

What reference is your simulator using? It HAS to use something. More importantly, YOU need to understand what reference it is using so that you can correctly interpret what it is telling you. It is exactly the same thing when you use a voltmeter. If you swap the leads, the sign of your reading changes. You can work with either reading, but you need to know how your leads are connected.

Virtually all simulators use the same convention when reporting the voltage across a two-terminal device. They report the voltage at Pin 1 relative to Pin 2. Unfortunately, few of the symbols for two-terminal devices make it clear which pin is Pin 1 (they don't want to create the impression that the device is somehow polarized). Often, when you insert a part, the insertion point is Pin 1, but this isn't always the case.

There is a similar issue for current. By nearly universal convention, the current at any pin on any device is the current flowing INTO that pin. If you ask it to tell you the current in a two-terminal device, it will report the current flowing into Pin 1.
 
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