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Power dissipated is I^2*R, I don't know the resistance of my tongue so I can't tell how much current goes through my tongue. A resistor I have does get hot and I'm guessing it has a higher resistance than my tongue which I'm guessing is very good at conducting since it's wet.

So is it because my tongue doesn't have a lot of resistance that it doesn't get as hot? But if my tongue has higher resistance than I should get a higher current which should heat it up more no?

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    $\begingroup$ ...why would a higher resistence mean higher current? Just use $V = RI$ to get $V^2/R$ for the power dissipated in terms of voltage. $\endgroup$ – ACuriousMind Nov 1 '15 at 1:57
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9v batteries are made from 6 small 1.5v cells connected in series. So it has a very high internal resistance which limits how much current you can draw from one (and why they aren't used in high power devices)

The resistance of your tongue is complicated, depends on how wet it is and the resistance changes as the current causes chemical changes.

So you have a very high resistance inside the battery in series with a high resistance in your mouth - which means very little current flows.

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    $\begingroup$ Relative to the resistance of the tongue (~7kΩ wet for the distance between the terminals of a 9v battery), the battery's internal resistance is negligible - for example a common Energizer Battery will discharge at 500mA according to the datasheet - at 9v, that's 4.5W of power - should be plenty to give a nasty zap $\endgroup$ – user2813274 Nov 1 '15 at 3:02

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