How much energy does it take to heat a cup of coffee or tea?


So, just the other day, I was thinking about how much energy we need to destroy hair follicle and blood vessels. Typically we talk about 1 or 2 joules per pulse. Then I thought, what is 1 joule of energy?

I had no idea what this actually meant in the ‘real’ world!

So, I wondered, how much energy does it take to heat a cup of water from 21 C to boiling point, 100 C – to make a cup of tea or coffee?

Well, its quite simple to work out….


It tuns out that water needs precisely 4.184  joules of energy to raise one millilitre by one degree Celsius (at sea level!).

A typical cup holds around 250 ml of water and, given that room temperature is around 21 C, I want to raise it 79 degrees to boiling point.

So, the total energy required to heat this cup of tea/coffee is:

4.184  *  250  *  79   =   82,634 joules.


In other words, it takes more than 82.6 kJ to heat a cup of water to boiling point!! We usually only need between 1 and 10 joules to heat blood/hair sufficiently to destroy their proteins.


Ain’t that a surprise!!



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