The teaser for the evening news just showed a clip of cellphones purportedly popping popcorn, and asking the question of whether it’s a hoax. My money’s on yes. Wired has a story on it, and there’s more than one video.
Assume a kernel has 0.1 g of water in it. You need to heat it from ~20 ºC to 100 ºC and then boil it, which is what happens when you pop popcorn. The heat capacity is 4.18 J/g, and the heat of vaporization is 2260 J/g. So this requires 80*4.18*0.1 +2260 * 0.1 = 260 Joules. This happens in a few seconds, so the absorbed power is somewhere around 50-100 Watts, per kernel. The transmitted power of the phones would have to be much, much larger, since it’s not focused on the popcorn.
Not.
Google also tells me that Zapperz beat me to the punch here.
Update: It looks like I overestimated the water content by a factor of about 5 (see link in comment 2; I assume medium-large kernels, though, not small. Orville has standards). Doesn’t change the overall answer. It takes my microwave oven ~3.5 minutes to pop my popcorn. A few hundred Watts for a (few) hundred kernels so let’s call it 1 Watt per kernel for 270-330 seconds (onset of popping is at about 2.5 minutes). That matches up pretty well with the numbers above, which we now know are overestimated. I see no reason to hypothesize that only infinitesimal boiling is happening.
Great minds think alike! I wrote a post on this yesterday, but it was a few places back in my posting queue and so it won’t be coming online until tomorrow morning. Publish or perish, I guess. 😉 I wrote about it from a slightly different angle though so I think they’ll be complementary.
Popcorn is about 1/8 water and small kernels weigh less than 0.1 grams. This gives the water at around 0.01 grams and you’re over estimating the water heating requirements by 10x. That cuts the Joules required down to 26.
Next, your heat calculation assumes that ALL the water in the kernel is heated to steam. It is in the heating to steam that the vast majority of your 260 Joules comes from. I think the calculation is assuming a rather optimistic estimate of the strength of the kernel; you’re supposing that it doesn’t pop until absolutely all the water in the kernel is turned to steam.
I would think it would pop as soon as the first water reaches boiling as the kernel is not very elastic (and water cannot be compressed). I get 3.2 Joules, about 2 orders of magnitude less than your figure, LOL.
Saying that water cannot be compressed is incorrect here. Liquid water is nearly incompressible, but that certainly does not apply to water in the gas phase.
“Saying that water cannot be compressed is incorrect here. Liquid water is nearly incompressible, but that certainly does not apply to water in the gas phase.”
Before the water boils it is not compressible. Converting all the water to steam takes a great deal of energy and therefore time. So when the boiling first begins, the water will be almost entirely liquid and incompressible. In fact, all physics students know that as the pressure increases, the temperature at which water boils increases.
On the other hand, the microwave heating will apply to all the material of the corn, not just the water in it.
The physics explanation is fine as far as it goes, but the engineering answer is much simpler: the mobiles, while being paged, aren’t transmitting.