a] Derive an expression for the equivalent resistance of three resistors RI, R2 and R3 connected in a parallel. b] Fuse of 3 A, 5 A and 10 A are available, calculate and select the fuse for operating electric iron of I kW power at 220 V line.

a] Derive an expression for the equivalent resistance of three resisto

1.	a] Derive an expression for the equivalent resistance of three resistors RI, R2 and R3 connected in a parallel. b] Fuse of 3 A, 5 A and 10 A are available, calculate and select the fuse for operating electric iron of I kW power at 220 V line.
Answer» Solution : It is observed that the total current I, is equal to the sum of the separate currents through each branch of the combination. `I =I_(1)+I_(2)` Let `R_(p)` be the equivalent resistance of the PARALLEL combination of RESISTORS. By APPLYING Ohm.s LAW to the parallel combination of resistors, we have `I = V//R_(p)` On applying Ohm.s law to each resistor, we have `I_(1)= V//R_(1), I_(2)= V//R_(2)` From eqns., we have `V//R_(p) = V//R_(1)+V//R_(2)` or `1//R_(p)= 1//R_(1)+1//R_(2)` Thus, we may conclude that the reciprocal of the equivalent resistance of a group of resistances joined in parallel is equal to the sum of the RECIPROCALS of the individual resistances.