JEE Main 2025 · 2 April · morning · Single correct · +4 / −1
The battery of a mobile phone is rated as 4.2 V, 5800 mAh. How much energy is stored ir it when fully charged?
- (a) 87.7 kJ
- (b) 43.8 kJ
- (c) 48.7 kJ
- (d) 24.4 kJ
Answer and solution
Answer (a) 87.7 kJ
Solution
Physics sir · 3 steps
mAh is a charge, so change it to coulombs and multiply by the voltage to get the energy.
- Rating is charge. Current × time is charge, so 5800 mAh tells you how much charge the battery can deliver. Convert it to coulombs: 5800 mAh = 5.8 A × 3600 s = 20880 C.
- Energy from charge. The battery pushes every coulomb through the full 4.2 V. The voltage stays the same as it discharges, so the stored energy is charge times voltage.
E = QV = 20880 C × 4.2 V = 87696 J
- So. E ≈ 87.7 kJ, which is option (a).
The idea to keep. A battery's mAh rating is charge, not energy. Convert it to coulombs (1 mAh = 3.6 C), then multiply by the voltage: E = QV.
Answer from the NTA final answer key. Solution by PrepWiser.