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Question
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.
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Solution
Given:
Power of the bulb, P = 100
W = 100 J/s
We know,
Energy = Power × Time
Hence, total energy emitted in 1 year is given by
Etotal= 100 × 3600 × 24 × 365
Etotal= 3.1536 × 109 J
This energy is converted into mass. Thus,
Increase in mass \[= ∆ m = \frac{E_{total}}{c^2} = \frac{3 . 1536 \times {10}^9}{9 \times {10}^{16}}\]
\[ = 3 . 504 \times {10}^8 \times {10}^{- 16} kg\]
\[ = 3 . 5 \times {10}^{- 8} kg\]
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