Question

If value of two resistance are 10 Ω and 20 Ω, and a current of 5A flow through circuit having p.d 12v, find value of third resistor if all 3 are connected in parallel.

Answer 1

Let third Resistor value be R3.

Current = Potential difference / Equivalent Resistance

5 = 12/ Equivalent Resistance

Equivalent Resistance = 12/5 = 2.4 ohm

For parallel connection formula for Equivalent Resistance

1/R = 1/R1 + 1/R2 + 1/R3

R = 12/5
R1 = 10
R2 = 20

Putting values in formula,

5/12 = 1/10 + 1/20 +1/R3

1/R3 = 5/12 - 1/10 - 1/20

1/R3 = 4/15

R3 = 15/4 ohm = 3.75 ohm


So, third resistor is of 3.75 ohm

Thank you.