A phone is connected by a charger to the mains supply. The energy stored by the phone battery increases, and the charger and surrounding air become warmer.
Which statement explains this?
The warming shows that the charger transfers more energy than it receives from the mains supply.
Energy from the mains supply is transferred to the battery's chemical store and to thermal stores, so it is conserved overall.
Energy is transferred from the battery's chemical store to the thermal stores of the charger and air.
All the input energy is stored in the battery; the higher temperatures are not caused by an energy transfer.
104 exam-style questions on AQA GCSE Physics 1.2 Conservation and dissipation of energy, covering 1.2.1 Energy transfers in a system and 1.2.2 Efficiency. Each one has a worked solution and a mark scheme showing where the marks go.