Q1. Calculate the efficiency of the following device as a decimal and also as a percentage.


  • An electrical appliance rated at 60 W with a useful power output of 48 W





  • A machine supplied with 1600 J, but which wastes 275 J to various useless energy stores.




Q2. What is the efficiency of the following lamp? Its transfers 7.5 kJ of energy, 4.5 kJ of this as useful light, 2.9 kJ is transferred to the surrounding thermal energy stores as heat and 100 J is transferred away by sound waves. State efficiency as a decimal and then as a percentage.


The input energy is 7.5 kJ which is 7500 joules of energy. The output from the lamp is 4500 J light, 2900 J as thermal energy and 100 J as sound energy.



Although the information is given, we disregard the thermal energy and sound energy as they are not useful forms of energy. In this case the only thing that is useful to us is the 4500 J given out as light energy.


Q3. A machine with an efficiency rating of only 47% transfers 697 J of energy to useful energy stores. Calculate the total amount of energy that is supplied to the machine.



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