1. The problem statement, all variables and given/known data A string of electric lamps for decorating a Christmas tree consists of 20 12 V lamps connected in series across the 240 V mains. The power consumption of the whole string is 24 W. (a) What is the resistance of each lamp? (b) If one lamp becomes short-circuited, what is approximately the new power consumption of the string? (c) Explain why, when one of these lamps is tested by applying a potential difference of 0.1 V, it passes a current of 10 mA. Answers: (a) 120 Ω, (b) 25.3 W. 2. The attempt at a solution (a) First find the power consuption of 1 lamp: 24 W / 20 = 1.2 W per 1 lamp. Then we find the resistance: P = V2 / R → R = V2 / P = 122 / 1.2 = 120 Ω (resistance of each lamp). (b) In that case we'll have the same voltage consumption (20 lamps on 12 V = 240 V), but we'll get only 19 lamps with resistance of 120 Ω (2280 Ω). So the new power consumption of the string is: P = V2 / R = 2402 / 2280 = 25.3 W. (c) This part I don't understand completely. The resistance should be R = V / I = 0.1 / 0.01 = 10 Ω. But what should I compare with what?