RL Circuit Analysis

My question is how would I get the equation for the voltage across R3 if the switch moves from position A to position B.

by Fal96
December 30, 2021
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1 Answer

Answer by vanderghast

Not sure of the "level" of the question. Is it an elementary question or a question from an advanced mathematical teaching?

In the first case, the answer is ZERO since the close loop with L1-SW1-L2 and R3 has then no power when SW1 completes that loop. If advanced math, that small space is not enough to explain how to solve the collapsing electromagnetic fields or L1 and L2. Fortunately, we can use the simulator graciously available to us.

Note that I change SW1 for a time switch, so the switch will close itself (without rebound). Click on it so see its settings, should be self documented. I also added ground. And a node with a name since I intend to run a "transient" simulation.

Open the circuit, run the simulation, without modifying the parameters. Nothing exciting after the first four milliseconds (ZERO everywhere, or almost zero).

It should look like the curve of a charging capacitor, but this time, with the current. In fact it is the energy stored in the em field which must dissipate through the resistor R3. And it is a noticeably short phenomenon, a "transient".

+1 vote
by vanderghast
December 31, 2021
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