A lamp has a rating of 18V 9W. How much energy is transferred to the bulb in 5 minutes? Calculate the current through it when connected to a 18V supply.

The first question comes from the realisation that the power rating, "9W" represents 9 joules being supplied every second and so can be written as 9J/s and so in 5 minutes (or 300 seconds) the energy supplied is:
9J/s * 300s = 2700J 

The second part comes from the equation P=IV and we already have numbers for power and voltage:
9 = 18 * I 
I = 9/18 = 0.5Amps

Alexander N. GCSE Physics tutor, IB Physics tutor

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