Question

null point is observed at 40cm from A.
If a 2 ohm resistor is added in parallel to s the null point shifts to 25 cm. find r and s
​

Answer 1

Answer:

Explanation:

When 12Ω resistance is not connected in parallel to  S than we can write,

R=S  

100−l  

1

​

 

l  

1

​

 

​

 

l  

1

​

=40cm

∴R=S(  

60

40

​

)=  

3

2

​

S __(I)

Now when 12Ω is connected in parallel to S, then  

The effective resistance can be written as

S  

1

​

 

1

​

=  

S

1

​

+  

12

1

​

=  

12S

12+5

​

⇒S  

1

​

=  

(12+5)

12S

​

 

Now we can write

R=S  

100−l  

′

 

l  

′

 

​

⇒l  

′

=50cm

i.e. R=  

(12+5)

12S

​

×  

50

50

​

=  

12+5

12S

​

 __(II)

Using eq (II) and (I)

3

2

​

S=  

12+5

12S

​

⇒(12+5)2=36

⇒24+25=36⇒25=12⇒S=6Ω

and R=  

3

2

​

×6=4Ω