Question

Seven identical lamps of resistance $2200\Omega$ each are connected to $220V$ line as shown in the figure. What will be the reading in the ammeter?

• A. $0.1A$
• B. $0.13A$
  
• C. $0.4A$
  
• D. $0.7A$
  

Answer 1

The correct option is C $0.4A$

Let the left point of ammeter be $B$ and right point be $C$ . Then say the equivalent parallel resistance on left side be $R_1$ and on right side be $R_2$.
$R_1=\frac{2200}{4}:R_2=\frac{2200}{3}$
So the current across ammeter will be the current across $R_1$.
Net current is $I=\frac{220\times 7}{2200}=0.7A$
Thus current across ammeter would be ,
$\frac{R_2}{R_2+R_1}\times I=\frac{2200\times 4}{2200\times 7}\times 0.7=0.4A$