WBJEE 2019 Physics Solved Paper

The circuit diagram is as shown below.According to question, when given pair of point in options are connected through a resistance R, then equivalent resistance between point A and B (RAB) remains unchanged. It is only possible when current does not flows through resistance R and circuit become Wheatstone bridge as shown in the figure below,At the balanced condition, ‌

P

Q

=‌

R

S

The current flow in CD branch will be zero.
Now by checking each option,
from option (a), circuit is,Now, ∵

P

Q

=

R

S

⇒

2r

4r

=

r

2r

⇒

1

2

=

1

2

Hence, option (a) is correct.
From option (b), circuit isNow, ‌

P

Q

=‌

R

S

⇒‌

3r

3r

≠‌

r

2r

So, option (b) is also incorrect.
From option (c) circuit is,Now. ‌

P

Q

=‌

R

S

⇒‌

4r

2

=‌

2

r

⇒‌

2

1

=‌

2

1

So, option (c) is also correct.
From option (d), circuit is