Defend the statement that 100% of the electrical energy that goes into lighting a lamp is converted to thermal energy. Are the first and the second laws of thermodynamics violated?
Solution 34E This statement is trivially wrong. From everyday experience we can say that 100% of electrical energy the lamp gets does not convert to thermal energy. Because most of the energy does convert into light energy. The remaining energy gets converted into heat energy, some of the energy is lost due to the resistance of the wire and all. If all the energy gets converted into heat energy, the bulb will not be able to give a little bit of light too. But if it happens, that all the electrical energy gets converted into thermal energy, then, these things will happen. STEP 1: The bulb will not glow at all first of all. The 1st law is the law of conservation which is not violated by this process. The change in internal energy is still the heat added to the system plus the work done by the system. So, the 1st law is safe. STEP 2: The mathematical definition of the efficiency of a heat engine is, T1T2 efficiency = T . 1 This efficiency is always less than one as T must be positive. 2 But as it is mentioned that 100% of the electrical energy got converted into thermal energy, the efficiency became zero. As everything converted to heat which is the unused energy. So, T & T must be same. 1 2 And we know that zero efficiency does not violate the 2nd law of thermodynamics. 2nd law also will not be not affected by this.