What law is used to find the polarity of a circuit?

Answer 1

It is possible that Kirchoff's law will answer this question. But why make it hard? It is customary to find the polarity of a circuit by simple inspection. Let's do a basic circuit and see how it works. Draw a square. Put a battery in the left side, and put the positive terminal down and the negative terminal up. Put a resistor in the right side. Two components - a simple series circuit. The battery will be "sending out" electrons from the negative terminal (the top one), and it will be "collecting them in" at the positive terminal (the bottom one). Put a "-" by the top of the battery to label that terminal. Now put a "+" by the bottom terminal. You've specified the polarities. Anyone looking at the circuit now can "find the polarity" of the circuit by inspection. Let's go one step farther. The electrons will be flowing out the top of the battery, and over and down through our resistor. Then they'll flow out the bottom of the resistor and across and into the bottom of the battery. Very simple, very easy. Now focus on this. The polarity of the voltage drop across the resistor is negative on the top and positive on the bottom. The end of the resistor into which current flows is said to be the negative end of the component, and the end out of which the electrons flow is said to be the positive end. Is this clear? Electrons flow out the negative terminal of a battery, and electrons flow into the negative end of our resistor. Be super clear about this. That's the way voltages are labeled in a circuit diagram. (The resistor "drops" voltage, while the battery represents a voltage "rise" in circuits.) For our circuit, the voltage drop across the resistor, which can be though of as the measure of voltage across the resistor, is negative on top and positive on the bottom. If you were going to measure the voltage drop across the resistor, the black meter lead would be on top of that resistor and the red lead would be on the bottom. If anyone encounters the circuit, the polarity will be obvious by inspection. We must be given a voltage somewhere, or we must be given a direction of current flow so we can make our inspection and determine polarity. If we were given nothing about our circuit except the direction of current flow, we'd know the rest by inspection. Take the simple circuit we made and take off all references to polarity. Now draw an arrow parallel to the top line of the circuit, and make it point to the right. If that arrow represents the direction of current flow, it would be telling us current was flowing clockwise around our square circuit. The polarities of all the other components (there are only two) would then be known. We would know by simply looking at the circuit what was happening, know by inspection. [This answer has a couple of simplifications, but not where it makes any difference. The answer was designed to supply fundamental knowledge or introductory information. It is unnecessary to say, for instance, that modern digital multimeters (DMMs) will measure voltage no matter what lead is placed where - the only difference being whether on not the "minus" sign appears on the DMM's display. Save that for later. We're keeping this simple.]