Whilst it’s still cold working from home, I’ll often use a hot water bottle to keep warm. I’m often curious if it’s better to fill it more or less and whether it makes any difference to the bottle staying warm. In consistent and matching conditions, would it make any difference how much water was put in the bottle, and why?
In: Chemistry
well, if you put more in, there is more total heat in the bottle.
But it has the same surface area, so losses heat at the same rate.
More Heat+same loss rate=stays hot longer.
Heat is energy. By putting more hot water in you have increased the total energy in the hot water bottle. The hot water bottles surface area will stretch a little probably but not by much so let’s ignore that for convenience. So with the discharge of energy being the same, having more energy will allow it to discharge for longer.
A good analogue would be a AA battery will power something longer than a AAA battery as it has more energy.
Yes. You are increasing the mass but not the surface area, so it will retain more energy for a longer period.
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Fantastic! Thanks for the answers. I had an idea that a filler bottle was hotter for longer but didn’t know the science.
To help understand it, imagine putting just a tiny bit of water in there. A thimble full of water would not even make the bag hot. A shot glass of water would be a little warm but only for a minute.
There’s another aspect not yet discussed, the shape of the vessel. a sphere has the highest possible ratio of volume to surface area. all other shapes have lower ratios.
As you fill a water bottle it bulges, making its shape closer to spherical, meaning the ratio of volume to surface area reduces, so its heat retention improves.
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