Not sure where to ask this question – thought you guys would probably have the best idea!
Today a single key on my keychain in my pocket heated up so that it was too hot to handle and scalded my leg. Any ideas what could possibly have done this?
I always keep a prison guard style set of keys attached to my trousers and in my pocket – one of these keys is very large and thick (longer and thicker than my finger) – this is the key that heated up somehow.
In my pocket was my phone (an htc desire hd).
I was sitting at my desk as I do all the working day – when I felt my pocket getting hotter rather alarmingly quickly. I stood up and pulled everything out of my pocket – and I can't work out how it happened. The phone was not hot at all and worked fine.
My best guess is that there was some form of induction going on. Does anybody have any idea what might have happened? I don't think it was friction or bending of the keys as it's never happened before and I was sitting still all morning.
Thanks for any ideas! I'm more curious than anything else.
Best Answer
Mobile phones transmit at microwave frequencies so they can induce currents in metals and other conductors. The energy dissipates as heat. The principle is the same used by microwave ovens. The phone would not itself get hot because (presumably) the microwave radiation would be directed away from metal components in the phone. Your key must have been close to the antenna and was perhaps aligned with it in a way that maximized the effect, or perhaps its length was just right for a resonance.
Here is a paper that seems relevant http://www.springer.com/about+springer/media/springer+select?SGWID=0-11001-6-1296921-0
Edit: Let me add a small calculation. Suppose a phone were outputting 1 Watt and 50% was absorbed by the key for 3 minutes. This puts 1W*180s*0.5 = 90 J of heat into the key. If the key weighs 10 g and is made of iron with a heat capacity of 450 J/Kg/K then the temperature goes up by 90J/0.01Kg/450J/Kg/K = 20K So this could raise the temperature of the key from 25C to 45C which would make it feel quite hot to the touch. Actual result depends on how fast the heat is conducted away and how much of the power is absorbed but it is not beyond the bounds of possibility that enough heating is possible to account for the observation. I would not blame anyone for being skeptical though. It would perhaps be unusual for the phone to transmit on full power for that long.