How long does it take for a soldering iron with a resistance of 100 Ohm at a current of 0.2 A to release 20 kJ of heat.

Task data: R (resistance of the used soldering iron) = 100 Ohm; I (current) = 0.2 A; Q (released heat) = 20 kJ (20 * 10 ^ 3 J).

The duration of the used soldering iron is determined using the Joule-Lenz law: Q = I2 * R * t, whence t = Q / (I ^ 2 * R).

Calculation: t = 20 * 10 ^ 3 / (0.2 ^ 2 * 100) = 5000 s = 1 h 23 min 20 s.

Answer: The soldering iron used will release 20 kJ of heat in 1 hour 23 minutes 20 from work.