Q1. If an electric motor transfers 10 kJ of energy in four minutes, what is the power of the motor?


Remember first of all to look back at the equation to see what units are expected. In this particular case you are given kilojoules and minutes, both of which will need to be converted to Joules and seconds before you do any calculations.


10 kJ is 10,000 J and 4 minutes is 240 seconds, and now we have these values we can enter them into the equation to work out the power of the motor:



The actual result was 41.6 recurring, and you'll normally be told to how many decimal places you need to state your answer. In this particular case I chose to round to one decimal place.


Q2. A 600 W motor runs for twenty seconds, during that time how much work does it do?


Again, this is a straightforward use of the formula. As you have done previously make a note of what you're given, and what you need to find out because you may (and this particular question is no exception) have two modify or transpose the equation.



Q3. How long does it take for a 525 W motor to do 1344 J of work?


This is a straightforward manipulation of the formula shown in the answer to question two. If power output is the work done divided by the time taken then the time (in seconds) must be the power of the motor (in Watts) divided by the work done (in Joules):


Q4. How long does it take for a 1.43 kilowatt appliance to transfer 2860 J of energy?




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