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I am studying this circuit:

enter image description here

and I tried to simplify it like so:

enter image description here

I'm confused since there is a wire atop A surrounding the "square" wire. I calculated its maximum resistance as 63.77 ohms. Any help especially how to simplify it is greatly appreciated.

Edit: Thank you for all the responses! It's quite clear that I still am confused about the topologies of series and parallel, can anyone recommend videos for it so I can understand is better?

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    \$\begingroup\$ Hint: What's the lowest resistance path from the 100 ohm resistor to A and B. How have you redrawn it? Does it still look right? What's the shortest distance from 90 ohms ..., from 12 ..., from ... There's a built-in schematic tool on the editor toolbar if it helps. Double-click to edit component properties. 'R' to rotate. Tip: Draw a horizontal rail across the top of your schematic for 'A' and one at the bottom for 'B'. \$\endgroup\$
    – Transistor
    Commented Mar 19, 2021 at 14:34
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    \$\begingroup\$ Your approach to simplify first - it is excellent. There are three resistors completely shorted out - you can eliminate those entirely from your solution. \$\endgroup\$
    – glen_geek
    Commented Mar 19, 2021 at 14:42
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    \$\begingroup\$ I think you should finish your redrawn schematic. Label all nodes on both schematics. Label all resistors on both schematics. See if something doesn't pop out at you when you do that. \$\endgroup\$
    – Elliot Alderson
    Commented Mar 19, 2021 at 14:48
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    \$\begingroup\$ Yes, this is definitely a "puzzle" schematic to make sure students can understand parallel/series topologies, as well as identify "degenerate" components. \$\endgroup\$
    – W5VO
    Commented Mar 19, 2021 at 15:11

3 Answers 3

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Here's your schematic, re-drawn:

schematic

simulate this circuit – Schematic created using CircuitLab

Think you can work it out, now?

Always, always, always get into the practice of re-drawing schematics. Especially, when they look like yours.

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It's a silly artificial puzzle question meant to teach you how schematics work.

The point A exists all along the line where it was drawn. As long as you can follow a line in a schematic without passing a component, you are still in the very same spot electrically. So for example the dot in the upper right corner is as much 'A' as the dot where the signal 'A' is connected. So we have:

schematic

simulate this circuit – Schematic created using CircuitLab

This means you also have B all over the schematic. The only resistors with any current passing through them are those who go between A to B. From there on, you can calculate the parallel resistance of those resistors that go between A and B. Those which don't are just there to confuse.

1/R1 + 1/R2 + ... 1/Rn = 1/Rp

Then you can replace the whole schematic with a single resistor between A and B.

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There are only 3 nodes. The nodes 3, 18, 6 meeting are not connected to the current flowing path.

Answer is therefore 9 ohm. enter image description here

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    \$\begingroup\$ Hello mr Mitu Raj, you have edited my answer, thanks for that. Please note that units are only singular. You can't say ohms, you should say 9 ohm. That is a law in 1 st lesson in High school physics. The 1 st lesson is "measurments" . You can't say Newtons. It is Newton. No plural. Only singular. Sory if my spelings are wrong. Keep in touch. Thanks. \$\endgroup\$
    – upali
    Commented Mar 20, 2021 at 5:52
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    \$\begingroup\$ common usage in electronics has always been that plural was perfectly acceptable: volts, amps, ohms, watts, kilowatt-hours, mhos, siemens... \$\endgroup\$
    – John R. Strohm
    Commented Mar 20, 2021 at 8:19
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    \$\begingroup\$ The symbols do not have a plural form but the spelt out versions do. "Plural unit names are used when they are required by the rules of English grammar. They are normally formed regularly, for example, "henries" is the plural of henry." See NIST rules and style conventions. You don't say "5 apple" do you? \$\endgroup\$
    – Transistor
    Commented Mar 20, 2021 at 10:47
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    \$\begingroup\$ @upali You would not say "10 Newtons", that is incorrect. You would say "10 N" and no one would think you are talking about people. The name of the unit is not used when you also have a quantity; only the symbol is used. NIST Pub 811 says: "Plural unit names are used when they are required by the rules of English grammar. They are normally formed regularly, for example, “henries” is the plural of henry." \$\endgroup\$
    – Elliot Alderson
    Commented Mar 20, 2021 at 12:23
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    \$\begingroup\$ And please stop handing out solutions to homework problems. We don't want this to become a homework solution service. \$\endgroup\$
    – Elliot Alderson
    Commented Mar 20, 2021 at 12:24

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