# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Paul Tennant on 23/08/2008 15:52:50

Title: If a charged battery weighs more, where does the mass go over time as the charge diminishes?
Post by: Paul Tennant on 23/08/2008 15:52:50
Paul Tennant  asked the Naked Scientists:

Dear Chris
I'm a regular listener to your Podcast (http://www.thenakedscientists.com/HTML/podcasts/) -- yours and ABC's Science Show are my two favourites.

Recently you explained that energizing a battery produces a slight increase in its mass; my questions are:

1) What does the added/created mass consist of and what is its distribution in the battery?

2) If a charged battery is left unused its charge diminishes -- I assume its mass diminishes also, but in this case what is the corresponding production of energy? (Perhaps it's in the form of heat within the battery?)

Sincerely,

Paul Tennant

What do you think?
Title: If a charged battery weighs more, where does the mass go over time as the charge diminishes?
Post by: lightarrow on 23/08/2008 16:19:25
Paul Tennant  asked the Naked Scientists:

Dear Chris
I'm a regular listener to your Podcast (http://www.thenakedscientists.com/HTML/podcasts/) -- yours and ABC's Science Show are my two favourites.

Recently you explained that energizing a battery produces a slight increase in its mass; my questions are:

1) What does the added/created mass consist of and what is its distribution in the battery?

2) If a charged battery is left unused its charge diminishes -- I assume its mass diminishes also, but in this case what is the corresponding production of energy? (Perhaps it's in the form of heat within the battery?)

Sincerely,

Paul Tennant

What do you think?
1. The energy you give to the battery, and its mass as consequence, since the battery stays still, goes in increasing the chemical potential of its compounds; PbO2 for example has a greater chemical potential than PbO or PbSO4. Essentially the chemical potential is related with the electronic potential of the chemical species. Simplifying furthermore,  the increase in mass goes in the increase of the electromagnetic fields of the chemical's electrons.

A simple example: you give electromagnetic energy (of the right frequency) to an hydrogen's atom. The atom absorbs a photon and as a consequence its electron jumps up to the first excited level. At this moment the hydrogen atom has not the mass it had before:  its mass is increased (almost exactly) of E/c2 where E is the photon's energy.
"Almost exactly" because the atom don't stay exactly still after have absorbed the photon (anyway the difference is very small).

Where does the atom's mass increase has gone? It has gone in the increased electromagnetic field in the space between the nucleus and the atom's outer edge (the excited atom has a different spatial distribution of the electron).

2.The battery's charge goes away through an internal, little but constant, discharge current. That current dissapates energy through the production of heat. In absence of phase transforms of the chemical inside (which could be, e.g. a water evaporation), the battery's temperature increases, so it gives off heats, that is energy, and as a consequence its mass decreases.
Title: If a charged battery weighs more, where does the mass go over time as the charge diminishes?
Post by: syhprum on 24/08/2008 10:12:15
Although a completely sealed battery would gain a tiny amount of mass when fully charged due to E=MC^2 in real life it is more likely there would some loss of H2 generated by electrolysis that is very difficult to contain
Title: If a charged battery weighs more, where does the mass go over time as the charge diminishes?
Post by: Alan McDougall on 27/08/2008 13:25:22
The energy used would be dissipated the process of entropy and the mass become minutely less