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

Water in an electric kettle starts boiling in $15 \ min$ after being switched on. If the length of the heating wire is reduced to $2/3$ of its original length,the time taken to boil the same amount of water will be ........... $min$.

Two bulbs are rated $(40 \; W, 200 \; V)$ and $(100 \; W, 200 \; V)$. What is the correct relation for their resistances?

If the resistance of the filament increases with temperature,what will be the power dissipated in a $220\, V- 100\, W$ lamp when connected to a $110\, V$ power supply?

Two wires have resistances of $2 \ \Omega$ and $4 \ \Omega$ and are connected to the same voltage. The ratio of heat dissipated in the resistances is:

If $10$ bulbs of $50\,W$ are used for $10$ hours a day,how many units of electricity will be consumed in a month ($30$ days)?