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QPage 458: Q21: HC Verma: Concepts of Physics - Volume 2 The Special Theory of Relativity Page 458: Q21 Solution: A 100 W bulb together with its power supply is suspended from a …

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often fail to recognize that the system's mass decreases because energy is radiated away, leading them to either calculate the mass of the energy itself without subtracting it or ignore the mass-energy equivalence entirely. Remember that $\Delta m = -E/c^2$ for the source, so the balance reading drops by the mass equivalent of the total energy emitted.

A 100 W bulb together with its power supply is suspended from a sensitive balance. Find the change in the mass recorded after the bulb remains on for 1 year.

Step-by-Step Verified Solution

**Answer:** Change in mass =

3.5 \times 10^{-8}

kg. **Step-by-Step Explanation:** Energy radiated:

E = Pt = 100 \times 3.156 \times 10^7 = 3.156 \times 10^9

J. Mass equivalent:

\Delta m = E/c^2 = 3.156 \times 10^9/(9 \times 10^{16}) = 3.5 \times 10^{-8}

kg.

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