A heater transfers 900 J electrically in 10 s. During this time, 720 J is transferred to the water and the rest heats the surroundings. Which gives the useful and unwanted power transfers?
useful 90 W90\ \mathrm{W}90 W; unwanted 18 W18\ \mathrm{W}18 W
useful 72 W72\ \mathrm{W}72 W; unwanted 90 W90\ \mathrm{W}90 W
useful 72 W72\ \mathrm{W}72 W; unwanted 18 W18\ \mathrm{W}18 W
useful 18 W18\ \mathrm{W}18 W; unwanted 72 W72\ \mathrm{W}72 W
105 exam-style questions on Edexcel GCSE Physics 10.3 Electrical energy and power, covering 10.3.1 The heating effect of an electric current, 10.3.2 Reducing energy transfer in wires, 10.3.3 Advantages and disadvantages of the heating effect, 10.3.4 Energy transferred by an electrical circuit, 10.3.5 Electrical power, and 10.3.6 Energy transfers in domestic devices. Each one has a worked solution and a mark scheme showing where the marks go.