currentresistors
I am studying Kirchhoff's Law on my book and I noticed some examples.
As seen on both pictures, there are + and – signs on resistors and they decide current's direction. I don't understand how can we decide their signs? Current flows through + to – but in first picture ic flows in opposide direction. In second picture they look like assigned randomly. We just randomly assign these signs? I am really confused.
You are measuring the current through the circuit. By Ohm's Law you know the current through a resistor is proportional to the applied voltage. But what happens when you don't know the voltage across the terminals of the device (i.e. what's the voltage at the middle node of the voltage divider?).
You can figure that out simply enough if you know the current through the network. Simply combine the resistor into a single component and apply the same rules as if it were a single resistor.
$$i = \frac{V}{R_3}$$
Where
$$R_3 = R_1 + R_2$$
the current will spread out across the whole conductor, there will be more curent in the centre but as the path that skirts the edges is only a litttle longer the current density there will be only a little lower
Best Answer
You can choose any polarity for your analysis. Basically, you are choosing a reference direction for the current. The result is relative to that polarity -- so if you get a positive answer, then current is actually flowing in that direction (w.r.t. your initial choice of reference). If you get a negative answer, then 'negative' current is flowing in that direction, or 'positive' current is flowing in the opposite direction.