The mains supply transfers 2400 J of energy electrically to a phone charger. The phone battery's chemical store increases by 1800 J. During charging, the charger and the nearby air become warmer.
Which statement correctly describes the energy transfers?
All 2400 J is transferred usefully because every joule supplied reaches either the phone or the nearby air.
The warming of the charger is useful, while the increase in the battery's chemical store is unwanted.
The 1800 J chemical-store increase is useful, and the remaining energy is dissipated to thermal stores.
The energy causing the warming stays in the charger and becomes useful when the phone is next charged.
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.