Herculi said:
Homework Statement:: .
Relevant Equations:: .
I have been studying electronical circuit. In general, it is not difficult and is more about to solve EDO. The main problem is, really, the signs.
See as follows:
View attachment 286374$$V_{L} = L dI/dt, V_{c} = +- Q/C$$ ?
Should i put a minus sign there? Or shout i maintain the signs? When do i know what sign to use? Even about the voltage dro pacross the inductor, couldn't it be ##V = -L dI/dt##?
The signs you apply are quite arbitrary, it doesn't matter how you define voltage and current polarities initially. However. it is ESSENTIAL that you are then consistent when you write out the circuit equations and that you correctly account for the sign conventions that you initially chose.
In your schematic, the current polarity has been documented, but not the voltage polarity, so we can't tell you if ##V_{c} = Q/C## or ##V_{c} = - Q/C##, you get to choose that. This is why it's really important when you are communicating your results to others that you include your polarity definitions.
Most all EEs prefer the "passive sign convention" for components. This is where the current polarity flows from the higher potential side of the component to the lower potential side; i.e. V=I*R, not V=-I*R.
Other common conventions are that all currents flow into a node (note, this guarantees that some are negative, which is OK). Also people seem to prefer currents that flow clockwise around loops. However, these last two conventions usually can't be consistently applied to anything but the simplest networks.
Honestly, I think people might be better off just assigning all of these randomly. Then you'll know they could be backwards and you will be more careful about consistency.
Finally, when you solve your circuit equations, some results will may be negative. This is fine, it just means the polarity you initially chose was backwards from reality. That doesn't matter because when ever there is a polarity involved, YOU HAVE TO go back and refer to the definitions.