Thread: Is this correct?

1. Hi, everybody! This is my first post... glad to have joined this great forum!

I was wondering if anyone could confirm that this sentence is roughly correct. I calculated it myself but, as a non-scientist, I don't have much confidence:

Here's the sentence:

"If you came here by train today, the chances are that you burned approximately 150Kcal for a one hour commute. That is roughly equivalent to the energy needed to boil water for a mug of coffee."

Many thanks in advance!

2.

3. A kilocalorie is the energy required to raise a kilogram of water 1 degree C. If you had a kilogram of water (1 liter) it would take 75 kilocalories to raise it from 25C to the boiling point, 100C. So, yes, if your mug was half a liter, then you could bring it to a boil with 150 kilocalories. If that is the right number for the energy needed for a 1 hour commute, then it would be a true statement.

4. So I am underestimating it? 150Kcal is enough to bring to the boil 2 liters of water, right?!

When I was first trying to calculate it I found some calculations on the net explaining the energy needed to boil water. These seemed to include a final extra amount of energy needed to actually boil the water, calculated using heat of vaporization. However, in my example I don't want to boil off all the water.... just bring it to the boil to make coffee. Is it correct to ignore the heat of vaporization part?

5. yes, it would raise the temperature of a mug of coffee to 100 degrees Celsius. An important note, though, is that you could make (in an ideal world) 4 cups of coffee with 150kcal.

6. Originally Posted by soliton66
So I am underestimating it? 150Kcal is enough to bring to the boil 2 liters of water, right?!
Yeah, you're right
When I was first trying to calculate it I found some calculations on the net explaining the energy needed to boil water. These seemed to include a final extra amount of energy needed to actually boil the water, calculated using heat of vaporization. However, in my example I don't want to boil off all the water.... just bring it to the boil to make coffee. Is it correct to ignore the heat of vaporization part?
Yes. You don't need to vaporize the water to make coffee. You might boil a little bit of it off, not much.

7. Many thanks for your help!

It's much appreciated. :-D

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