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# Application of the fundamental laws (solve)

We solve the equations created by direct application of the fundamental laws: Ohm's Law and Kirchhoff's Laws. (Part 2 of 2).

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• Is there a voltage drop across Is? There must be, right? 1 Volt in this case, the same voltage drop we have across R2. V=IR, so 1V=3mAxRs... So the resistance of the current source must be 333.33 ohms (1V/3mA).... Is this correct?
• You are correct that there is a 1 volt across the current source, Is. It is tempting to apply Ohm's Law to the current source. But, you have to be careful when you do this, because a current source is not a resistor. The current in this current source will be 3 mA no matter what the voltage is, so it's not very resistor-like. Voltage is not proportional to current. When you do the calculation of 1 V / 3 mA = 333 ohms, you get what seems like a valid "resistance" for the current source, but it is misleading.

It may help to look at the article "Ideal elements and sources" (https://www.khanacademy.org/science/electrical-engineering/ee-circuit-analysis-topic/circuit-elements/a/ee-ideal-circuit-elements). In the section on Ideal Current Sources there is an i-v plot of a current source (a current source plots as a horizontal line). Compare that to the i-v plot for a resistor. (which plots as a sloped line). Try drawing the i-v plot for the 3 mA current source from the example circuit above, then mark the point where i = 3 mA and v = 1 volt. Then draw an i-v plot of a 333 ohm resistor and mark the same point. Can you see why its a bit awkward to talk about the resistance of a current source? The current is always the same, and the voltage can be anything, so Ohm's Law doesn't apply very well to a current source.
• hi , i wanted to ask that when i have the equation in the video I1-I2=Is
, how did you delete I2? why did you chose R2 in R2(I1-I2=Is)?
• Why did you include a current source? It threw me in a little " loop".
• Ha! Current sources are a little mind bending. They are not as familiar in everyday life as voltage sources but they are an essential electrical concept used in every transistor circuit.
(1 vote)
• why is 6 volt when 2K.3mA= 6v ?
• In all of the schematics drawn, it shows the current moving away from the positive side of the voltage source. Is this correct? Shouldn't the current flow into the positive side?
• We always point the current arrow in the direction of movement of positive charge. This is a very old habit in electricity, dating back to Ben Franklin. We do this even though we all know the negative electrons carry the current (in metals).
(1 vote)
• What happens if your current source is flowing against current
• A current source _defines_ the current magnitude and direction. The voltages and currents in the rest of the circuit adjust themselves to account for that definition.
• Two voltage sources are connected to a parallel circuit,how do I apply superposition method?
• Hello Varney,

This is one of my favorite techniques because it works so well for unbalanced thee phase systems...

Regarding your question. Start by turning on power supply #1 and turning off (shorting) power supply #2. Calculate the current for the circuit and set it to the side. Now turn on power supply #2 and short power supply #1. Once again calculate the current. Now all you need to do is add them together - the total current is the sum of the individual currents.

This site has a good tutorial: http://www.allaboutcircuits.com/textbook/direct-current/chpt-10/superposition-theorem/

Regards,

APD
(1 vote)
• why didn't you find the REQ of the two resistors?
(1 vote)
• Before we solved this circuit, was there some way to know ahead of time that current is flowing down from b into c rather than up from c into b? Wouldn't it be possible for the current to flow up from c into b if the values were different?
(1 vote)
• As much as you want to know the current directions before you do the analysis, you should not be worried about that when you start working on the problem. Your job at the outset is to clearly label the circuit with current arrows and voltage polarity labels. That establishes the frame of reference for the equations you are about to generate. If, at the end of the analysis, a voltage comes out negative, or if a current comes out negative, that just means they are negative with respect to your labeled framework. It doesn't mean you made an error in the beginning. It just means the answer has a negative sign. There's no problem with this. There is no need, (or benefit) to have anxiety when you assign current and voltage labels. Trust the math (Ohm's Law and Kirchhoff's Laws to lead you to the correct result.