Question

In the given circuit the current flowing through the resistance $20\Omega$ is $0.3A$ while the ammeter reads $0.8A$. What is the value of $R_1$?

• A. $30\Omega$
• B. $40\Omega$
• C. $50\Omega$
• D. $60\Omega$

Answer 1

The correct option is D $60\Omega$

Given Current through $20$ Ohm resistance is $i_{20}=0.3A$
All resistances are parallel, so potential is same.
$V=0.3\times 20=6\text{V}$
Current through $15$ Ohm resistance is $i_{15}=\frac{6}{15}=0.4A$

Ammeter reading is given as $i_A=0.8A$

Current through $R_1$ from kirchhoffs junction law is $i=i_A-i_{15}-i_{20}=0.1A$

from Ohms law $R_1=\frac{V}{i}=\frac{6}{0.1}=60\Omega$