RL circuit

Hi!
For t=0, (the switch closes at this point) To find the voltage across, R2, why was the right loop used for Kirchoff's voltage law but when I use the loop on the left side (-Uq+R1*iL+UR2=0)with the voltage source I get a different answer? I attached a pic of the diagram and the answer sheet calculation of UR2. (ps: it's in German so U is voltage and Uq is source voltage :smile:

) thanks

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Reply 1

mqb2766

21

Im presuming you understand the notes? Because of the inductor, IL doesnt change in the right path, but closing the switch will mean there is a current I1 in the middle path so the current flowing through R1 is the sum of those two currents. Assuming Ive not made a mistake
I1 = Uq R1 / ((R1+R2)(R1+R3))
So
UL = I1 R2 - IL R3 = Uq R1 R3 / ((R1+R2)(R1+R3)) - Uq R3 / (R1+R3)
which seems to match the notes.

Reply 2

sarah630

OP

10

Original post by mqb2766

Im presuming you understand the notes? Because of the inductor, IL doesnt change in the right path, but closing the switch will mean there is a current I1 in the middle path so the current flowing through R1 is the sum of those two currents. Assuming Ive not made a mistake
I1 = Uq R1 / ((R1+R2)(R1+R3))
So
UL = I1 R2 - IL R3 = Uq R1 R3 / ((R1+R2)(R1+R3)) - Uq R3 / (R1+R3)
which seems to match the notes.

thank you! I figured the calculation was using the voltages parallel being equal and that is how i got the answer. However I am confused that we considered for t=0 the current IL (Uq/R1+R3)to be the same as when t<0 when the switch was closed which I understand but later there was a calculation for the current IR2 (which from the previous sentence I understood it means no current is flowing to R2) for t=0 and i was wondering why the answer wasn't zero?

Reply 3

mqb2766

21

Original post by sarah630

thank you! I figured the calculation was using the voltages parallel being equal and that is how i got the answer. However I am confused that we considered for t=0 the current IL (Uq/R1+R3)to be the same as when t<0 when the switch was closed which I understand but later there was a calculation for the current IR2 (which from the previous sentence I understood it means no current is flowing to R2) for t=0 and i was wondering why the answer wasn't zero?

An inductor opposes changes in current, so when the swtich in the middle path is flipped, the curreint in the right path does not change (instantaneously). Not sure what you mean by the "later ..." comment? Closing the switch will cause the current/voltage to step/jump in that path.

(edited 9 months ago)

RL circuit

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