Current Electricity 1 Question 7

7. A wire of resistance R is bent to form a square ABCD as shown in the figure. The effective resistance between E and C is [E is mid-point of arm CD]

(2019 Main, 9 April I)

(a) 764R

(b) 34R

(c) R

(d) 116R

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Solution:

  1. Let the length of each side of square ABCD is a.

Resistance per unit length of each side =R4a

Now, R1=R4a[EC]=R4a×a2=R8

Similarly, R2=R4a[EDABC]=R4a×7a2=7R8

Now, effective resistance between E and C is the equivalent resistance of R1 and R2 that are connected in parallel as shown below.

REC=R1R2R1+R2=(R/8)×(7R/8)(R/8)+(7R/8)=7R264×88R=764R



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