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So i'm really lost and I have to solve some problems from circuits regarding mainly resistors. I have this circuit and I am a bit lost because I don't really understand what to do to find the value of the resistors as well as the Voltage on N1 and N2. I think I need to use Kirchhoff's Laws. The thing that bothers me is do I need to pretend that the circuit is connected to itslef on the bottom portion ? (the three bars appearing 4 times at the bottom).

Thank you in advance and sorry if my question seems dumb but I really am lost here.

Here is the circuit : enter image description here

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    That's a Ground symbol. Treat them all as connected together and (normally) label that point 0 Volts. –  May 04 '20 at 12:02
  • Is this homework? Also can you explain in more detail what you don't understand about the circuit? Voltages N1 and N2 are already given to you, and currents too, so you only need ohm's law if you want the resistances. Note that there are four resistances you can't calculate because there is not enough info in the picture. – Justme May 04 '20 at 12:05
  • Thank you brian, so is this circuit valid ? And what do I have to label 0 volts ? https://imgur.com/a/RteMQzZ –  May 04 '20 at 12:10
  • Yes it is homework, the circuit is only what I have. I have to find the values for each resistor as well as I(tot), V of N2 and V of N2. –  May 04 '20 at 12:12
  • See https://electronics.stackexchange.com/a/427625/73158 for more on ground and earth symbols. – Transistor May 04 '20 at 12:14
  • there is no "pretend" ... those four points are connected together – jsotola May 04 '20 at 14:56
  • Can you state KVL and KCL in your own words and show an example of each in the circuit. Add this to your question as a first step. We can help you with understanding, not with homework. Take it one step at a time... – skvery May 06 '20 at 17:50

2 Answers2

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Yes you treat each node with the "three" bars as connected. Those are ground symbols and are treated as the 0v reference when making voltage measurements. It is also more convenient to show the end of each branch off of the battery with a ground symbol instead of drawing a line all the way back to the battery's negative terminal.

The ground symbol gets its name because it can be used to represent a connection that actually connects to the earth via some conductor buried into the ground. Literal earth connected grounds are usually labeled as such.

*Addition:

You are aware of Kirchhoff's Laws, so we will use the KVL to gain some insight. Imagine a loop going up R3,R2 and back down R4,R5. We know the sum total voltage must be zero. Following that loop we have -30 + VR2 + 35 + 40 = 0. From this we have VR2 = 45V. From this notice that VR2+VR3 = VR4 + VR5, That makes sense they are in parallel. As for the voltage at N1 remember it is referenced to ground. So trace a path to ground and see what the voltage drop is. Starting from N1 down R4 we drop 35V, continuing down R5 we drop another 40V for a total of 75V. So N1 = 75V, Notice this works if you go down R2 and R3.

Some Sorta EE
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  • Okay, I have heard about ground and know I understand it thank you ! –  May 04 '20 at 12:11
  • And so this circuit (https://imgur.com/a/RteMQzZ) essentially is the same as the one I sent right ? –  May 04 '20 at 12:13
  • @HectorBertucat, That's right, from that drawing ohms law will give you R3 and remembering the properties of voltage with parallel circuits will give you N1. – Some Sorta EE May 04 '20 at 12:22
  • Okay so from what I understand, because we are in a parallel circuit the voltage is the same though the whole circuit, meaning the Voltage is 125 V on N1 and N2 because the battery is 125V right ? And regarding the. value of R3, it is 1200Ω because 30V/0.025A = 1200Ω I think this is correct but I am not sure –  May 04 '20 at 12:29
  • And do I have any way to determine the values of the 6 other resistors ? –  May 04 '20 at 12:51
  • R3 is correct, however N1 is not 125V. Notice that R1 makes it so the rest of the resistors are not in parallel with the battery, there will be a volt drop across R1. Also notice that the R2, R3 branch is in parallel with the R4,R5 branches, node N1 is the top of all three branches. – Some Sorta EE May 04 '20 at 13:10
  • Okay I understand. But to find N1 I need to find the voltage drop of R1 because it is the resistors "before" N1. But I have no information on R1 itself, for me to find V of R1 I would need the R value and then use the V = IR but I only have i1 = 45 mA here. And regarding the other resistors, if I knew i5 I could then find R4R as well as R5R because I have both voltage drops of the twe resistors given. But because I don't know anything about R4L and R5L I can't -more accurately don't know how to- find i4 and i5. Thank you already for your help, you really help me understand what I'm doing ! –  May 04 '20 at 13:26
  • Glad to help Hector, I've added more to my answer and now I must get to my day job. – Some Sorta EE May 04 '20 at 14:19
  • Thank you a lot for that, I now have everything but R for R4L,R5L,R4R and R5R. I can’t determine the values in this current state right ? Here is the circuit with updated values ans what I already know : https://imgur.com/a/spZ0757 –  May 04 '20 at 23:40
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More specifically, that 3-lines symbol is "GND" in the low voltage, silicon electronics meaning of the word. It means it is the common, or the zero-voltage reference, and the place where current normally "returns".

The word is also used in AC mains power distribution (the high voltage power to your home that runs appliances). In that case, common/current return is called "neutral". The word "ground" is the safety shield (on which power should never flow, except during a fault condition obviously) typically seen as the 3rd pin on the connector.

Harper - Reinstate Monica
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