(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A rechargeable AA battery with a mass of 25.0g can supply a power of 1.20W for 50.0mins.

What is the difference in mass between a charged and an uncharged battery?

2. Relevant equations

W = Pt

W = K = gmc^2 - mc^2

where g = 1/(1-(v/c)^2)^(-1/2)

E (total) = gmc^2 = K + E (rest) = K + mc^2

3. The attempt at a solution

W = 1.20 * 3.00x10^3 = 3600J = K

I am unsure of where to go from there, or even if this is the right way to start. The question does not make it clear (for me) if the 25.0g of the battery relates to the rest energy or the charged energy. Assuming it was clear which the mass relates to, I do not know what to use for the velocity in g.

There is also the possibility (in my mind, at least) that in part this is a trick question. That there is no mass difference, as while the battery is not in use the electrons are not moving. But I'm sure that's not what the question means, as it goes on to ask what fraction of the total mass the mass difference is.

Thank you

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Mass difference between charged and uncharged battery - Special Relativity

**Physics Forums | Science Articles, Homework Help, Discussion**