Current Electricity 2 Question 27

27. An infinite ladder network of resistances is constructed with 1Ω and 2Ω resistances, as shown in figure.

The 6V battery between A and B has negligible internal resistance.

(1987, 7M)

(a) Show that the effective resistance between A and B is 2Ω.

(b) What is the current that passes through the 2Ω resistance nearest to the battery?

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Solution:

  1. (a) Let RAB=x. Then, we can break one chain and connect a resistance of magnitude x in place of it.

Thus, the circuit remains as shown in figure.

Now, 2x and x are in parallel. So, their combined resistance is 2x2+x

 or RAB=1+2x2+x

But RAB is assumed as x. Therefore,

x=1+2x2+x

Solving this equation, we get

x=2Ω

Hence proved.

(b) Net resistance of circuit R=1+2×22+2=2Ω

Current through battery i=62=3A

This current is equally distributed in 2Ω and 2Ω resistances. Therefore, the desired current is i2 or 1.5A.



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