IIT JEE Advanced 2007 Paper 1

Using Wheatstone bridge principle, we have the following for the two resistances mentioned here:

2

l

=

R

100−l

R

1+20

=

2

80−l

where R is the unknown resistance. Dividing the aboveequations, we get

l+20

100−l

=

80−l

l

l2

20

= 8000 - 100 l - 80l + l2
⇒ 200l = 8000
⇒ l = 40 cm
Therefore, the unknown resistance is
R =

2×60

40

= 3 Ω