Phy circuit qn

wishboy SG

[O-lvl standard]

can someone pls enlighten mi on how to go about doing this qn?

it would be good if u can include some workings (such as re-drawing the circuit)

eagle SG

I have divided the circuit. The top white coloured resistor has no current passing through it, because the potential is the same between two ends. There's no potential difference across the resistor, and hence no colour.

The bottom resistor is left white now, because it is only to be considered in the end.

The next diagram shows the top resistor removed.

Follow the circuit along the route of the green resistors. Notice that the centre is also at the same potential as the blue resistors. Hence, the wires can be separated as shown below.

The resistors in the red box are in parallel, and thus can be simplified as follows:

The rest of the resistors are 1 ohms. Hence, total resistance is

(1/2 + 1/2 + 8/3)^(-1) = 8/11 ohms = 0.73 ohms

Regards,
Eagle (Strategic Tuition)

wishboy SG

Follow the circuit along the route of the green resistors. Notice that the centre is also at the same potential as the blue resistors. Hence, the wires can be separated as shown below.

I don't really understand about the centre having the same potential as the blue resistors and why can the wires be separated.

I think there's an error in the working, it should be
(1/2 + 1/2 + 3/8)^(-1) = 8/11 ohms = 0.73 ohms

Thanks for ur help !

Twincat SG

fantastic...thats thinking out of the box... it is same potential, by drawing an imaginary line equadistant between p and q you will realise the resister on the left and right is the same number, meaning voltage is divided equally at the center too, since voltage is the same as each center no current will flow through top white resisters. if no current flow is equavalent to open circuit...

lucky it is in mcq...else i would put nobody in right mind would connect the circuit like that...

eagle SG

I don't really understand about the centre having the same potential as the blue resistors and why can the wires be separated.

I think there's an error in the working, it should be
(1/2 + 1/2 + 3/8)^(-1) = 8/11 ohms = 0.73 ohms

Thanks for ur help !

Yup your workings is correct. I typed too fast :p

We concentrate only in the red box. The green resistors and the white resistor both have equal potential difference since they are in parallel. So we don't bother about that bottom white resistor.

Then you following the current path of the green resistors, you will realise that the centre is at the same potential as the centre of the blue resistors. This only works because the ratio of the resistance from between P&centre and between Q&centre is the same for both the blue and the green path.

Since they are of same potential, joining together with wires or not (or even if we put a resistor in between, like what happened to the top white resistor) doesn't matter. We can safely separate them without any effect.

eagle SG

lucky it is in mcq...else i would put nobody in right mind would connect the circuit like that...

Actually such questions are much more common in A levels, but can ocassionally come out in O levels.

There's actually one other theorem that can help solve this much faster, but out of syllabus for both Os and As.

wishboy SG

oh ok now i understand better
but i still hope this kind of qn dun come out for any exams

thanks