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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.

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    $\begingroup$ Are you able to reproduce this? Especially in combination with this phone? Do you sometimes experience electrostatic shocks when leaving your desk? $\endgroup$
    – Bernhard
    Jan 20, 2012 at 15:37
  • $\begingroup$ I've never been able to repeat this $\endgroup$
    – Paul Hutchinson
    Jan 23, 2012 at 14:22

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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.

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  • $\begingroup$ I had wifi on also.. and it could have been downloading a large update. $\endgroup$
    – Paul Hutchinson
    Jan 23, 2012 at 14:43
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    $\begingroup$ In fact the wi-fi is more likely to be the cause of the heating than the GSM transmissions because it uses higher frequency microwaves. One of the main wi-fi bands at 2.4 GHz is close to the 2.45 GHz used in microwave ovens. What length was the key? $\endgroup$
    – Philip Gibbs - inactive
    Jan 23, 2012 at 17:35
  • $\begingroup$ I think the optimum key length for heating would be one wavelength because impedance is highest, so 12.5cm for 2.4 GHz (or 6cm if they use the 5 GHz wi-fi band). A dipole antenna should be half the wavelength to minimize impedance so a key at 6.25cm (or 3cm for 5GHz) would just resonate and re-radiate without heating. $\endgroup$
    – Philip Gibbs - inactive
    Jan 23, 2012 at 22:28
  • $\begingroup$ I have some images for you of the key. plus.google.com/112684779468811505659/posts/UKk2k5Liyqf I wonder if the chain an other keys played a part in the induction however. $\endgroup$
    – Paul Hutchinson
    Jan 25, 2012 at 9:06
  • $\begingroup$ So it is about 7cm long which is half a wavelength for 2.4 GHz or a full wavelength for 5 GHz. Does the manual tell you what wi-fi frequency it uses? Might be both. $\endgroup$
    – Philip Gibbs - inactive
    Jan 25, 2012 at 9:20
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The cell phone puts out a maximum of 1W. Even if the phone failed and went to full power AND your keys were a perfectly coupled antennea I can't seem them getting hot.

Could the key have shorted the battery, perhaps the phone has exposed contacts for a charging or docking station?

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  • $\begingroup$ it's micro usb.. so I'm not sure whether or how a key can get in there.. it is possible though.. $\endgroup$
    – Paul Hutchinson
    Jan 23, 2012 at 14:21
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Anecdotally, this has happened to me and my brother more than once.

My hypothesis is that some combination of body grease and pocket lining, together with some metalic object can behave as a decent insulator/Van de Graaff that might hold significant amount of static electricity (behaving as a capacitor). Thus, kinetic energy of the body is transferred to potential electric energy in the capacitor, which occasionally discharges and the resulting current causes the observed heat.

If this is true, the phone doesn't play an special part in this

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  • $\begingroup$ I can see this happening.. but I wasn't jiggling my leg. $\endgroup$
    – Paul Hutchinson
    Jan 23, 2012 at 14:22
  • $\begingroup$ there really is no need to 'jiggle', any natural walk movement might be enough to harvest electricity $\endgroup$
    – lurscher
    Jan 23, 2012 at 15:41
  • $\begingroup$ I was still sitting down at my desk for a good few hours :) $\endgroup$
    – Paul Hutchinson
    Jan 25, 2012 at 9:06
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    $\begingroup$ I don't think the amount of energy created by this effect is enough. $\endgroup$
    – Martin J.H.
    Jul 4, 2013 at 22:25

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