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Post by Lex Of Sydney Australia on Aug 10, 2013 16:27:44 GMT
Ok I have a friend who insists on using hot water from the tap in her kettle when she wants to boils water. She claims it heats up faster but I think (& have argued with her) that your basic electric kettle heats the water using a pre set timer, & that using pre heated water from the tap only -
A) is a waste of power & time
& B) can create super heated water that can potentially injure you.
I'd love to peoples thoughts on this one.
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Post by GTCGreg on Aug 10, 2013 17:18:34 GMT
The hotter the water is when it starts out, the sooner it is going to boil.
Want the scientific reason? Water starts to boil when it reaches 100ºC (212ºF). It takes 4.18 Watts of electricity to rise 1 gram of water 1ºC in one second. Now let's say you have a very small kettle holding 1 gram of water and a 4.18 Watt electric heater. If the water starts out at room temperature, (25ºC) it will take 75 seconds to start boiling that 1 gram of water. But if you start out with water that is already heated to 50ºC, it will only take 50 seconds to start the 1 gram of water boiling. So the pre-heated water will boil faster.
The total amount of energy would be the same for both the 25º and 50º water if you also included the energy it took to preheat the hot tap water. The amount of energy the kettle had to provide would be less for the 50º preheated water.
As to question "B", the only way to make superheated water is to keep it under pressure, as in a pressure cooker, or start out with very pure distilled water which may reach temperatures above 100ºC before it starts to actually boil. I doubt your friends hot tap water is any purer than her cold tap water so superheated water shouldn't be an issue if heated in an electric kettle.
Hope that makes sense.
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Post by Cybermortis on Aug 10, 2013 17:51:53 GMT
Nope, no timer involved. Just a simple thermostat that cuts the power when the water reaches the desired temperature.
I'm guessing that the thermostat is a small metal strip consisting of two metals which expand at different rates when heated - causing the strip to bend in one direction when heated. If the bar is resting on a connector and part of the power line going into the heating element, then when it bends upwards it will break the connection. (This explains both the 'click' you hear when the kettle turns off, and why you have to wait a few moments before you can turn the heating element back on.)
Of course this does make me wonder if your average electric kettle would work at high altitudes, where the boiling point of water is lower....
The reason for not using hot water from the tap for cooking and drinking is because the water is much more likely to have bacterial contamination and to have dissolved metals etc in it. This was probably only really true for older systems and water pipes, the former being tanks that had hot/warm water sitting in them and the latter possibly being lead pipes. Today it probably isn't going to make much difference beyond being a waste of energy. Your kettle is going to be a lot more efficient at heating water than any heating system found in a house - if only because you are heating less water and not wasting water while you wait for the warmer stuff to pass through the pipes.
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Post by Lex Of Sydney Australia on Aug 10, 2013 18:03:21 GMT
Thanks for the science lessons guys, I’ve always used cold water to save power, but I think I’ll keep on using it after hearing about dissolved metals. Nice - how to get ALL your daily minerals (& a few more) in one cuppa tea. YUCK!  No thank you think I’ll pass the fluorite in the water is enough for me. |
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Post by the light works on Aug 10, 2013 20:05:34 GMT
If I'm in a rush, I will start with hot tap water. because of the efficiency of my central water heater, it will actually heat water more efficiently than the open coils on my stove - which I would estimate only transfer about 2/3 of the applied energy to the water. but normally, I start with cold tap water, because I have other things to do while the water comes to a boil, anyway.
the thing I did for both speed and efficiency when I was using an electric "kettle" to heat water for hot chocolate (and now do when I heat it in a stovetop kettle) is to use only as much water as I need, and remove it from the heat as soon as it reaches the temperature I intend to drink it at.
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No thank you think I’ll pass the fluorite in the water is enough for me.