Over on Boing Boing, Cory Doctorow quotes from from a 'HOWTO' page explaining how to boil an egg with two mobile phones. He does refer to this as a 'claim' but I can't help thinking that a little critical reflection might not have gone amiss before repeating this story.
Let's do the elementary science on this.
An medium egg weighs about 50g, and we'll assume that it is mostly water and thus has a specific heat of about 4200 J/kg/K. In other words, it takes 4200 joules of energy to heat up a kg of egg by 1 kelvin (i.e. 1 degree C).
If we start with our egg at room temperature - say 20 deg C - then to boil it we have to increase its temperature by 80 deg C. The energy required to do this for a 50g (0.05kg) egg is:
E(boil) = 80 x 0.05 x 4200 = 16,800 J
This is how much energy we have to get into the egg to raise its temperature to boiling point. Now, one watt of power is one joule per second. So to do this in three minutes (180s) takes a power transfer of 16800/180 = 93W.
In other words, if you can get about 100W into a medium size egg, you will heat it up to boiling point in about three minutes. Except that:
- This assumes complete energy transfer into the egg. For heating it with microwave energy, this assumes that all the energy from the transmitter is going into the egg and that it is all absorbed. A microwave oven tries to achieve this by putting the food to be cooked in a reflective cavity where the only place microwaves can go is into the food. But even if you put an egg right next to an antenna in the open, only part of the energy will go into it - a lot will go the other way, or even through the egg without being absorbed.
- This also assumes that the egg loses no heat as it warms up. This is unrealistic, as it will be radiating away heat (faster as it gets hotter) and being cooled by air convection.
In summary, the idea that 4W worth of mobile phone power could boil an egg in three minutes is complete bollocks. But then a quick look around the web site in question might have suggested that it's not entirely serious.
Even without the detailed physics argument though, a moment's thought should have shown how unlikely this is. An egg is mostly water. Your ear is mostly water. If a mobile phone next to an egg could boil it in three minutes, what would it do to your ear during a typical phone call? (And this is presumably why the article calls for two mobiles, so that you suspect some sort of weird interference effect. This can happen, but it just localises the heating, not enhances it.)
Why do I care enough to even post about this? Because the general lack of science literacy frustrates me, and because the lack of critical thought or fact-checking on Boing Boing is - especially because the site is in other respects so engaging - really quite depressing.
Whistles from a Small Planetoid
- Boing Boing's Bad Science Day