# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: SimplyElectronics on August 30, 2016, 07:07:35 pm

Title: Negative Voltage/Current?
Post by: SimplyElectronics on August 30, 2016, 07:07:35 pm
Hi Guys,

I am at another hurdle :/ I am struggling to get my head around the concept of negative voltage/current.

Watch this quick video to see what I am looking at:

I know once I wrap my head around this I will get it. But this concept is difficult for me to grasp.

Basically, what I expected was when I close the switch, the voltage across the cap would read 12v and when viewing current, I expected a spike of +25mA rather than -25mA and -12v that I actually saw.

Could someone possibly help a newbie understand why this happens?

Title: Re: Negative Voltage/Current?
Post by: madires on August 30, 2016, 07:29:16 pm
I don't know that software, but I assume that the green arrows are the currents. And since the sum of all currents in a junction is zero (Kirchhoff's laws), and the green arrows both point away from the switch, one has to be negative. Or in other words, the direction of the green arrow at the battery is the wong way around. But when you consider the "wrong" direction and the negative current, everything is perfectly fine. I hope this isn't too confusing.
Title: Re: Negative Voltage/Current?
Post by: Galaxyrise on August 30, 2016, 08:10:20 pm
Looks like the voltmeter is attached "backwards"--it's + terminal is going to the battery's - terminal--so that's where the -12V is coming from.

It's not obvious to me where the current measurement is coming from.  There's two green arrows on there.  One of them is facing opposite the direction that current will flow, and the other is pointing in the direction that current will flow.  I suspect your negative current is a similar issue to the negative voltage; you're measuring it with something that thinks "positive current" is in the opposite direction from how current is flowing in the circuit. (Positive current flows from higher voltages to lower voltages.)

Voltage measurements are always relative.  Strictly speaking, you can't point at a node and say "that's 12V". You say "that's 12V higher than this node over here".  If you do just say "That's 12V", you are referencing some previously agreed-upon 0V node.  Current measurements do typically have a fixed 0 (0 coulombs/s) but the direction is still arbitrary, so the sign will be based on which direction you measure it in.
Title: Re: Negative Voltage/Current?
Post by: Nexo on August 31, 2016, 05:26:24 am
Quote
Looks like the voltmeter is attached "backwards"--it's + terminal is going to the battery's - terminal--so that's where the -12V is coming from.

I also so that and, for me, is the only explanation to why would the voltage measurement is negative although I have one question, how is this software measuring the current? I mean, the voltage measurement is quite obvious, it's connected in parallel to the CAP but I don't see a amp-meter connected in series within the circuit. Am I missing something?