Thoughts on the power bill of the third little pig

Physicist Marcus Wilson from the University of Waikato in New Zealand takes issue with the children’s fairy story The Three Little Pigs.

Not being an engineer, he has little to say about the structural integrity of the various houses or the likelihood of a big bad wolf being able to blow them down.

But he is concerned about how the third little pig managed to heat a wolf-sized pot of water to a high enough temperature that would scald the animal – and that all in a minute… 

One litre of water takes 4200 joules of energy to raise its temperature by 1 degree C. That’s called the ‘specific heat capacity’. To raise 100 litres by 75 degrees, we therefore need 4200 times 100 times 75 = 31 500 000 joules. This happens in sixty seconds – thats about half a million joules per second. What does that mean? One joule per second is one watt of power. So here we have about 500 kW of power – a kW (kilowatt) being a thousand watts.  

Bedtime is obviously an improving experience in the Wilson household…

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