The power consumed by the rheostat is 30 W, the voltage at its terminals is 15 V.

The power consumed by the rheostat is 30 W, the voltage at its terminals is 15 V. Determine the length of the nickel wire required for the manufacture of the rheostat if its cross-section is 0.50 mm2.

Initial data: N (power consumed by the rheostat) = 30 W; U (voltage at the rheostat terminals) = 15 V; S (cross-sectional area of nickel wire) = 0.5 mm2.

Constants: ρ (specific resistance of nickelin) = 0.4 Ohm * mm2 / m.

1) Determine the resistance of the rheostat: P = U ^ 2 / R and R = U ^ 2 / P = 152/30 = 7.5 Ohm.

2) Calculate the length of the nickel wire: R = ρ * l / S and l = R * S / ρ = 7.5 * 0.5 / 0.4 = 9.375 m.

Answer: To make a rheostat, you need 9.375 m of nickel wire.