Timeline for Finding current and initial voltages of Inductor, capacitor and resistor
Current License: CC BY-SA 4.0
17 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 3, 2018 at 21:54 | comment | added | Elliot Alderson | "constant" is not the same as "continuous" | |
| Jul 3, 2018 at 21:28 | comment | added | Chu | @ElliotAlderson if \$ i\$ is constant \$\frac{di}{dt}=0 \$. dcromley's comments say VL = 12 V, that's back emf isn't it? How else can you get a voltage across an inductor? Anyway I'm done with this; I suggest you do some work on transient analysis of R/L/C circuits. | |
| Jul 3, 2018 at 20:25 | comment | added | Elliot Alderson | No, I didn't say \$\frac{di}{dt}\$ was zero, I said \$i\$ was continuous. At t=0+ you have to use KVL to find \$V_L\$ and then you can calculate \$\frac{di}{dt}\$. See @dcromley's comments to my answer. | |
| Jul 3, 2018 at 20:02 | comment | added | Chu | @ElliotAlderson In that case there's zero voltage across the inductor, so KVL is violated! | |
| Jul 3, 2018 at 16:32 | comment | added | Elliot Alderson | Yes. The inductor current does not change from t=0- to t=0+, it must be continuous. | |
| Jul 3, 2018 at 16:26 | comment | added | Chu | @ElliotAlderson So are you saying that the current is constant at t=0? | |
| Jul 3, 2018 at 16:09 | comment | added | Elliot Alderson | I don't think so. At t=0+ we know the voltage across the capacitor and the current through the inductor because they are the dc values that existed at t=0-. So we have series circuit and the current is through all elements is known. We use Ohm's Law once and KVL once and we're done...one multiplication and some addition and subtraction. No differential equations. No concern about \$\frac{di}{dt}\$. | |
| Jul 3, 2018 at 12:27 | comment | added | Chu | @ElliotAlderson At the top of the OP: Find the initial current. Find the initial Voltages of the capacitor , the inductor and the resistors. The OP has made a mistake in the KVL - hasn't recognized that there's a back emf at t=0. Ldi/dt explains where the error lies. | |
| Jul 3, 2018 at 12:15 | comment | added | Elliot Alderson | OK, fine, but the OP doesn't care about \$\frac{dI}{dt}\$. The OP cares only about \$i(0+)\$. I wasn't asking you to give a more complete answer, I was pointing out that you were telling the OP to do analysis that was not necessary to answer the question as asked. | |
| Jul 3, 2018 at 12:09 | comment | added | Chu | @ElliotAlderson KVL at \$\small t=0 \$: \$\small -16+(8*10^3*3*10^{-3})+L\frac{dI}{dt}=20\$; hence \$\small L\frac{dI}{dt}=12\:V\$ | |
| Jul 3, 2018 at 11:01 | comment | added | Elliot Alderson | My point was that your suggestion to derive a differential equation is unnecessary. I didn't want the OP to be confused by that. | |
| Jul 3, 2018 at 6:54 | comment | added | Chu | @ElliotAlderson 1st paragraph of answer gives initial current; initial inductor voltage can then be determined, but homework criterion applies so can't give the full answer. | |
| Jul 3, 2018 at 6:39 | comment | added | Chu | @Jaden we shouldn't give complete answers to homework. | |
| Jul 3, 2018 at 2:18 | comment | added | Jaden | This isn't the whole answer, but it is part of it. Still need to do some KVL/KCL to find the other voltages (and currents if that is indeed asked). | |
| Jul 3, 2018 at 1:56 | comment | added | Elliot Alderson | The OP just needs the initial conditions, I don't think much more than algebra is required. | |
| Jul 3, 2018 at 1:10 | history | edited | Chu | CC BY-SA 4.0 |
added 9 characters in body
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| Jul 3, 2018 at 0:47 | history | answered | Chu | CC BY-SA 4.0 |