0
\$\begingroup\$

I have the following circuit from the book Electric Circuits Nilsson & Riddle chapter 7. The switch in the circuit in picture has been closed for a long time, which means that the inductor behaves as a short circuit. From theory the current through a short circuit is 0. But after the short circuit the current will "swirl around" (excuse me for the unofficial phrase) and will go to 75 ohm resistor. But!!!!!. I have understood that if a resistor is parallel to a short circuit this resistor is excluded because is parallel to 0. My question is: in the book the writers exclude the 3, 60, and 15 Ω resistors (right part of the circuit). But why? Should it eliminate the left part as well? How does it work when there is a short circuit with parallel resistors?

enter image description here

Also here (2nd circuit): When the change the inductor to short circuit the right part is excluded.Why the right part and not the left part ? At the left the short circuit is parallel to the 60 ohm resistor.

enter image description here

\$\endgroup\$
6
  • 4
    \$\begingroup\$ "From theory the current through a short circuit is 0". No. The voltage across a short circuit is zero, the current can be anything. How much current do you think would flow if you put a short circuit across your mains supply? \$\endgroup\$
    – Finbarr
    Commented Sep 26, 2023 at 9:10
  • \$\begingroup\$ @Finbarr ok .thank you for that.But the question remains. \$\endgroup\$
    – Homer Jay Simpson
    Commented Sep 26, 2023 at 9:11
  • 3
    \$\begingroup\$ The question was only there because your understanding of a short circuit was wrong. \$\endgroup\$
    – Finbarr
    Commented Sep 26, 2023 at 9:13
  • 1
    \$\begingroup\$ If the diagrams have come from a textbook or other website, please reference the source. \$\endgroup\$
    – colintd
    Commented Sep 26, 2023 at 11:37
  • \$\begingroup\$ @colintd I said it in the beginning \$\endgroup\$
    – Homer Jay Simpson
    Commented Sep 26, 2023 at 11:51

1 Answer 1

Reset to default
2
\$\begingroup\$

Once the inductor becomes a "short", the circuit is effectively reduced to this:

enter image description here

As the whole right hand side is now shorted out it can be ignored completely. No current will flow through it and there is no voltage across it.

Note that the term "short circuit" is normally used for an undesired connection between circuits, such as a fault condition. This is simply a case of the resistance dropping to zero.

\$\endgroup\$
1
  • \$\begingroup\$ ok so you are ignoring the right part because there is no current flows through this. \$\endgroup\$
    – Homer Jay Simpson
    Commented Sep 26, 2023 at 18:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.