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How long does it take for water to boil?
Answer 1:

This is a breadbox physics/engineering question.

The process is a fairly simple one:

A container with some amount of water (lets say 1 kg for simplicity) is heated using some source (let's say an electric heating element that outputs Q (energy)= 2,500 Watts or Joules/second)(Joules is an energy unit).

All we need now is the starting temperature of the water (we'll say 25 Centigrade) and the heat capacity (4186 Joules/kg*Kelvin)(Kelvin is a unit of temperature degrees).

So, how much heat do we need to add to 1kg of water in order to heat it from T0=25 Centigrades to T1=100 Centigrade?

Well, this is precisely what heat capacity means, it's been measured, and it is ~4186 Joules/kg Kelvin.

Q_Required = (100 - 25) Kelvin * 4186 Joules/ (kg Kelvin) * 1kg = 314,000 Joules

We said earlier that our stove supplies ~ 2,500 Joules/second, so 314,000 Joules / (2,500 Joules/second) = 126 seconds.

Time it! Remember 1 kg is about 1 Liter of water. Our estimate is likely low (based on personal experience) and is likely due to an over estimate of the heat output of the stove (not 100% of the stove's heat goes directly to the water; much of it is wasted in hot air escaping around the sides of the kettle).



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