Question

The equivalent resistance of circuit across points A and B is equal to —

[A] 22.5 Ω
[B] 25 Ω
[C] 37.5 Ω
[D] 75 Ω

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Answer 1

The above circuit can be modified as shown in the diagram fig(i).

Resistor between CD are in series.

So, Resistance between CD = 30+20 = 50ohm

Resistor between EF are in series.

So, Resistance between EF = 30+20 = 50ohm

Resistor between GH are in series.

So, Resistance between GH = 30+20 = 50ohm

Now circuit can be modified as fig(ii)

Resistor between CD are in parallel.

So,

1/R = 1/50+1/50+1/25

1/R = 1/50+1/50+2/50

1/R = 4/50

R = 12.5 ohm

Resistor between AB are in series.

So, Resistance between Ab = 10+12.5+15 = 37.5 ohm

Answer 2

The current through the 40Ω resistor can be found using Ohm's law: i40Ω = vo ... resistors must be equal. .... Req = (35 + 15 + 25)‖(10 + 40) = 75‖50 = 30 Ω ...... (10) = 22.5 ...