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| Confused about voltage potentials & polarity in a circuit |
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| Ronan:
Thanks for your comments on trigonometry. I watched several YT videos and since several of you REALLY emphasized it, I'll be signing up for a trig & calc refresher course soon. But I'm still lost in the wind. Maybe I'm making this harder than it should be, but I'm totally confused on at least three things. The first thing is the voltage drops or "voltage potentials add when going from point to point" with respect to polarity of items in a circuit, as see in the attached image. * Going from b to c, we're going from "+" to "-" so we drop 6 volts * Going from b to a, we're going from "-" to "+ so we go up 4 volts"What I don't get is if these are resistors, how is there a polarity since resistors aren't placed in a specific direction like LEDs are? Why would one be a + - and the next one be a - +? Secondly, I'm confused about direction of current. It seems arbitrary for one tutorial to use a "conventional flow theory" where the current is coming out of the positive end of a DC circuit, and then in another video tutorial the speaker will use the "electron flow theory" where the current is coming out of the negative terminal. I understand that there are positive and negative charges where the flow of electrons is the negative current, but is the conventional current the flow of protons? I hear it's a negative current, so then I get all confused on how to envision the current and apply it to figuring out voltage drop across an item. Thirdly, what does it mean if power is consumed or generated in these two examples? * current is going from "+" to "-" which has a positive voltage and positive current so power is consumed. * current is going from "-" to "+" which has a negative voltage so we get negative power so power is generated.I don't even know if I'm explaining myself clearly with respect to positive and negative current, and voltage and power generated or consumed. The instructor used this terminology but didn't explain the definition. I get the water reservoir analogy for voltage, but negative voltage confuses me. I'm just friggin' lost on these specifics. Thanks in advance. |
| Brumby:
1. Voltages. The mountain explanation where voltages are equivalent to height is a very good one. Let's pay a bit more attention to it.... If you have a circuit that is joined - such as the example given - then if you traverse from one point to the next, then the next, etc. until you get back to your starting point, then you have a net change in voltage of exactly zero. This is the exact same thing as traversing the mountain. You first pick a reference (eg up is positive and down is negative) and you have to keep using this same reference throughout your travels. Now whether the changes are positive or negative will depend on which way you decide to go. For example, if point 'x' is higher than point 'y' then if your journey goes from point 'x' to point 'y', then (using the previous reference) you will be travelling down - but if your journey takes you from point 'y' to point 'x', then you will be travelling up. When applying this to a circuit, imagine yourself walking around it. Defining your reference direction between two points will have you connect the red lead of your DMM to one point and the black lead to the other. Let's say that as you walk between these two points, you will have the red lead in front of you and the black lead behind you - then you must always have the red lead in front of you and the black lead behind you. (The maths works exactly the same if they were the other way around, but once you pick a direction, you have to stick with it.) If we look at your circuit, using the "red lead in front, black behind" reference, let's look at what happens when travelling from point 'a' to point 'b'. Firstly, the black lead will be on point 'a' and the red lead on point 'b'. There is a difference in potential of 4V with point 'a' being higher than point 'b' and with our "red lead in front" reference, the DMM will show -4V. When you start walking from point 'b' to point 'c', you have to move both leads so that the red lead stays in front - which means the red lead will be on point 'c' and the black on point 'b'. The difference in potential here is 6V with point 'c' higher than point 'b' and with our "red lead in front" reference, the DMM will show +6V. Using this same process of "red lead in front and black behind", when you go from 'c' to 'd' you will get a reading of +3V and from 'd' back to 'a' you will get -5V. To prove you got this right, just add up all four measurements you took: -4V +6V +3V -5V ---- 0V Your answer should always be zero - since you are back where you started. This is Kirchhoff's Voltage Law. Extension: Your circuit could have many other connections and components weaving in and around, but as long as everything was in steady state (Electrically this means DC) then you could do this exercise around ANY closed path and you will always get a total of zero. |
| Ronan:
Thanks Brumby, I get that each loop has to add to zero, but I'm still confused about the polarity of a resister which causes an addition or subtraction. How can there be a polarity for a resister, and how can that polarity change? As seen in this attachment. |
| ArthurDent:
Sometimes I find it helps me to redraw the circuit so it's clearer to me. This circuit with 4 resistors can be visualized as two resistors in series across a 9 volt battery and a second two resistor in series across the same 9 volts. for the first pair, the top resistor on the left in my drawing drops 3 volts, and the lower resistor drops 6 volts. If you measure the voltage across the top resistor with your negative (black) meter lead on the top lead of the top left resistor (point d) to the bottom lead on the top resistor (point c), you will measure -3 volts because point c is 3 volts more negative than point d. The resistors do not have polarity like a diode and can only have a voltage drop across them but you have to consider where your point of reference is for measuring the voltage drop. |
| Brumby:
1a Polarity Polarity is a term that is used in several ways. In basic terms: For a circuit element that provides voltage, it is used to identify which connection has the higher potential and which has the lower. Traditionally, a higher potential is called positive and the lower is called the negative, but this is not an absolute truth - it is a relative truth. What makes things "positive", "negative" or zero depends on where you put your meter probes! For a circuit element that "uses" electricity, there can be two situations. The first is where that circuit element will work properly no matter which way it is connected. Resistors are the classic example, but so are low value capacitors and neon bulbs. The second is where that circuit element will only work properly when it is connected a particular way around. These - especially those with only two connections - are described as having a "polarity". Diodes and electrolytic capacitors are classic examples. Same word - but two very different meanings. The fact they are interrelated in electronics makes for some confusion. |
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