Battery

[David90]

Does a fully charged battery weigh more than the same battery when discharged albeit by a very minuscule amount ?

[heathfield]

Yes. According to Einstein an increase in the potential energy of an object will bend space and time in such a way as to effectively increase the mass of the object. The potential energy of a charged battery has been increased by drawing energy from elsewhere, (e.g. the charger). But the increased weight will be the teensiest fraction of a zillionth of a gram, so there's no practical way of measuring it.

[Clanad]

I'm very cautions about questioning an answer from heathfield, but in this case, I'll throw caution to the wind...

In fact the only thing that happens when recharging the typical lead acid battery (used in automobiles for example) is that the electron flow is reversed. Ions of the battery's plates move into/out of the electrolyte, and electrons flow one other direction. When one recharges a battery (usually) by forcing electrons in the opposite way, you reverse that chemical reaction, essentially storing energy back up for later useage. In either discharge (when the battery is being called upon) or recharging, the result is the conversion of electric energy into chemical energy within the cell or battery with no change in mass. (With assistance from University of Washington, Department of Electrical Engineering))

[Rev. Green]

Sorry Clanad, my money's on Heathfield - a charged cell is heavier.
It is easier to demonstrate with a capacitor. A charged capacitor contains the same number of electrons, protons, etc. as an uncharged one but it is more massive by the equivalent of the energy used to charge it.

[Clanad]

You may well be right, Rev... however the most basic prmise of my argument is that the recharge is not adding energy, it's simply reversing a chemical process that has just undergone a change in electron orientation... nothing is really added to the battery... but, since the opposing premise can't be challenged due to weighing limitations, ours is a futile but noetic stimulation, no?

[heathfield]

Aha! I note that Clanad uses the words 'essentially storing energy' in his description of the battery, and thus the battery's potential energy is increased.
I'm not skilled enough to examine Einstein's reasoning in concluding that increased potential energy results in increased mass, (well, not beyond quarter-way down page 1) - but I'm prepared to take his word for it. ;-)

[ChuckFickens]

Surely you can increase the potential energy of a battery by simply lifting it off the ground, but nobody would think this would increase the mass of it. I would say that increasing it's chemical potential energy is just moving the atoms about a bit, not adding or removing anything so the weight stays the same.

[heathfield]

Very true Chuck. Nobody would think that simply lifting the battery would increase its mass, yet Einstein says it does! Remember E=MC²? Or put another way, M=E/C². The battery gains potential energy equal to the force required to lift it, so E is increased and since C² is constant, M must increase. See the last section of an Interesting article on energy and mass here...

http://galileoandeins...es/mass_increase.html

[matt_london]

Yes it does. Likewise a wristwatch wthat has been wound up will weigh more than one of the same model that is not wound up.
A spring pulled at both ends weighs more than one in it's relaxed state.
(When we say weigh, we actually mean more mass because you could weigh one on the moon and one on earth which would cause them to be different)

[David90]

I thought the answer would depend on Einsteins equation. You can't get owt for nowt, as they say.

[jake-the-peg]

I think you're wrong because I think that other factors are likely to be more dominant.

Calculate the numbers

How much mass are we talking about?

I suspect you will lose more than that in other ways

[jake-the-peg]

take a car battery say 3MJ

that is 3 x10^-11KG so actually quite a lot compared to the mass of a proton

But say 2x10^17 hydrogen atom's worth or 3x10-7 mol or 7 millionths of a litre of hydrogen gas

Now I'm naot an expert in batteries but if you charge a battery up I reckon you're likely to get at least that much hydrogen given off