Checking a circuit problem with Kirchhoff's current law

[Doc Al]

I think I did 18A correctly,

What you found is the total resistance of the circuit.

 

[Marcin H]

What you found is the total resistance of the circuit.

Oh, true... Hmm. So how else can I find the resistance? Should I find the voltage drop across the 20 ohm resistor and then subtract that from the source voltage and use that V in the equation? Or is their a different way.

 

[Doc Al]

Well, you found the total resistance. The only unknown is R, so solve for it.

 

[Marcin H]

Well, you found the total resistance. The only unknown is R, so solve for it.

Oh... lol. I keep over thinking everything. 100-20-60=20ohms. I am so used to harder circuits from E&M that now I'm making dumb mistakes over easier problems. It all works out now. Thanks!

 

[Marcin H]

Well, you found the total resistance. The only unknown is R, so solve for it.

Hello again! Sorry have one last question! I thought I had this right, but then I started thinking about it and I'm not sure anymore. My thought was since the voltage was changing and the current was constant, that the source was a voltage source. I kinda thought of it as you changing the voltage on a power supply or something keeping the current constant. Is that right? I feel like it's not.

 

[Doc Al]

Hello again! Sorry have one last question! I thought I had this right, but then I started thinking about it and I'm not sure anymore. My thought was since the voltage was changing and the current was constant, that the source was a voltage source. I kinda thought of it as you changing the voltage on a power supply or something keeping the current constant. Is that right? I feel like it's not.

You had it right the first time. Read this: Voltage and Current Sources

 

[Marcin H]

You had it right the first time. Read this: Voltage and Current Sources

I am very confused right now. So are you saying that the graph shows a constant voltage source? My professor said that if you have a constant current, then it is a current source, but i don't think that's right. The slope for the graph on that problem would be zero which means zero resistance. The hyperphysics link agrees with that too. Here are my professors notes:

Edit: Also, I asked this here yesterday and gneill seems to say otherwise.
https://www.physicsforums.com/threads/voltage-or-current-source.854007/#post-5356009

 

[Doc Al]

So are you saying that the graph shows a constant voltage source?

No. Current is on the vertical axis and is constant.

My professor said that if you have a constant current, then it is a current source, but i don't think that's right.

Your professor is correct.

The slope for the graph on that problem would be zero which means zero resistance.

Note that is is a Current vs Voltage graph, so be careful how you interpret the slope.

The hyperphysics link agrees with that too.

Not sure what you mean here. Hyperphysics agrees with your professor (and with me).

 

[Marcin H]

Note that is is a Current vs Voltage graph, so be careful how you interpret the slope.

How can you interpret the slope then? Slope is rise over run, so i/v. Oh, which is not resistance. REsistance is v/i. So what does the slope tell us exactly?

 

[Doc Al]

So what does the slope tell us exactly?

You can think of it as 1/R going to zero.

Or just imagine how you'd draw the v/i graph.

 

[Marcin H]

Ohhhhhhh. Ok, that makes sense now. I had it flipped. Now makes me question why show the graph like that -_- Oh well. Thanks for clarifying!