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Sign convention for passive components

Video transcript

today we're going to talk about the sign convention for passive components it's a big mouthful but it's a fairly simple idea first of all let's look at this word passive passive is the way we describe components that do not create power or components that do not amplify signals so particularly resistors as we have one here does not amplify a signal capacitors and inductors do not either so they're both all three of those are passive components and sometimes we say something like a battery or a voltage source is a passive component because it can't amplify a signal it does provide power but it does not make signals bigger it can't make a voltage larger than it already is so we're going to start with the way to label this resistor with its current and voltage first we'll go resort to Ohm's law our friend voltage equals current which is I times R and when we label this when we label this device here's here's resistor so we'll give that a label and we're going to put a voltage across this something outside of this is going to put a voltage across it and this end will be positive with respect to this end and the question is the question we want answer is which direction does the current go when there's positive voltages here and the negative voltages here which ways the current flow through this resistor does it go from top to bottom or bottom to top and the answer is the positive current goes this way goes down so that's I in this and the current is going into the end with the positive sign on the voltage now I'm going to draw another one of these same same sort of thing and we'll do it a different way we'll do this one sideways and I'll label the voltage here's R and I'll put a voltage on it and the voltage let's label it this way let's say this we've labeled a plus in this way and label the minus end this way the question is which way is the current go well the current goes in the positive end so it goes in that way and that's the sign convention for positive for that that's per passive components the current goes into the current goes into the voltage on the positive terminal of the voltage the arrow over here like this that would still mean the same thing it's still going in the positive and coming out the negative side so when we draw this for capacitors and inductors let's draw a capacitor over here and there's C and if I label this with a voltage plus and minus V then the passive sign convention tells me that I label the positive current going in I the same for an inductor we'll draw our curly inductor like that we'll put a voltage on this time I'll put the voltage on this side plus minus V plus minus V this is our inductor like that and the current the current goes in the top like that so it goes in the same side as the plus and the current comes out on the minus side you can draw it either way you want let's practice some more this time we'll do it with a resistor again here's another resistor and this time I'm going to label the current first I'm going to say I'm going to say the current goes this way through the resistor and now using the passive component sign convention how do we label the voltage I want to label the voltage where's the plus sign plus sign is on the side that the current goes in the currents going in to this terminal so that is the plus side of the resistor and that's basically the idea of a sign convention for passive components when we're setting up a circuit if we have a complicated circuit we're setting it up we have to give names to all the voltages and all the currents and this is the way you do it to make sure that Ohm's law comes out with the correct sign that's what why we have this sign convention