Question

In the circuit of Fig. 1.57, find the value of supply voltage V so that 20-Ω resistor can dissipate 180W

Answer 1

I₄² × 20 = 180 W; I₄ = 3A 

Since 15 Ω and 20Ω are in parallel, 

I₃ × 15 = 3 × 20 

∴ I₃ = 4 A 

I₂ = I₃ + I₄ = 4 + 3 = 7 A

Now, resistance of the circuit to the right of point A is 

= 10 + 15 × 20/35 = 130/7Ω 

∴ I₁ × 25 = 7 × 130/7 

∴ I₁ = 26/5 A = 5.2A 

∴ I = I₁ + I₂ = 5.2 + 7 = 12.2 A 

Total circuit resistance 

R_AE = 5 + 25 || 130/7 = 955/61 Ω 

∴ V = I . R_AE = 12.2 × 955/61 = 191V