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Physics Question 35 – JEE-MAIN 2025

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The battery of a mobile phone is rated as 4.2 V, 5800 mAh. How much energy is stored in it when fully charged ?

The energy stored in a battery is related to its voltage and charge capacity.

🥷
Ninja Strategy•Order of Magnitude Estimation

Quickly estimate the energy in Watt-hours (V * Ah) and then convert to kiloJoules using the approximation that 1 Wh is about 3.6 kJ (since 1 Wh = 3600 J). This helps eliminate options that are too far off.

● Video Walkthrough
Step 1: Identify Given Values and Formula✦ Active

The given voltage of the battery is V=4.2 V. The charge capacity is Q=5800 mAh. The energy stored in a battery is given by the formula E=V×Q.

Step 2: Convert Charge Capacity to Standard Units○ Expand

First, convert the charge capacity from milliampere-hours (mAh) to Ampere-hours (Ah), then to Coulombs (C) or directly to Watt-hours (Wh) for easier calculation of Joules. Using Watt-hours:

Q=5800 mAh=5.8 Ah

The energy in Watt-hours is EWh=V×QAh.

EWh=4.2 V×5.8 Ah=24.36 Wh

Now, convert Watt-hours to Joules. We know that 1 Wh=3600 J.

💡 Teacher's Secret Hint

Ensure all units are consistent before multiplication. mAh needs to be converted to Ah or Coulombs.

Step 3: Calculate Total Energy Stored○ Expand

Multiply the energy in Watt-hours by 3600 to get the energy in Joules:

E=24.36 Wh×3600 J/Wh=87696 J

Convert Joules to kiloJoules (kJ) by dividing by 1000:

E=876961000 kJ=87.696 kJ

Rounding to one decimal place, the energy stored is approximately 87.7 kJ.

💡 Teacher's Secret Hint

Pay attention to the final unit requested (kJ) and round appropriately.

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