Question

The two electrical subnetworks $N_1$ and $N_2$ are connected through three resistors as shown in Fig. The voltages across 5 ohm resistor and 1 ohm resistor are given to be 10 V and 5 V, respectively. Then voltage across 15 ohm resistor is

• A. -105 V
• B. +105 V
• C. -15 V
• D. +15 V

Answer 1

The correct option is A -105 V

Current through $5\Omega$ resistor,
$i_5=\frac{10}{5}=2$ Amp.
Current through $1\Omega$ resistor,
$i_1=\frac{5}{1}=5$ Amp.
So, the current through $15\Omega$ resistor,
$i_{15}=-(i_1+i_5)=-(5+2)=-7$ Amp.
Voltage across $15\Omega$ resistor.
$V_{15}=15\left(i_{15}\right)=15(-7)=-105V$