Mass Loss in Batteries

1. Dec 29, 2011

Eugbug

I just want to confirm an error I have seen on another website about discharging of batteries.
The poster suggested that the mass of a battery decreases because matter is being converted into energy according to the equation E=mc2 as the battery discharges. They said that because c is so large, the weight loss would be imperceptible.My understanding is that any loss in weight is due to vented gases from the chemical reaction and E=mc2 only applies to nuclear reactions and not chemical reactions.

2. Dec 29, 2011

Drakkith

Staff Emeritus
Nope, E=MC^2 applies to all reactions. Consider a simply molecule such as water. Take the two hydrogen atoms and the oxygen atom and determine their mass before they combine. Now, combine them into a water molecule and measure the molecules mass. It will have LESS mass than the combined mass of the free hydrgoen and oxygen atoms. The missing mass was released in the energy that the reaction gives off.

3. Dec 29, 2011

Dr_Morbius

The mass change is tiny and unmeasurable on a standard chemistry balance. You can use E=mc^2 and apply it to the energy released in the reaction and you'll come up with a miniscule change in mass.

4. Dec 30, 2011

syhprum

The nuclear bomb that deverstated Hiroshima converted about 1 gm of matter into energy.

5. Jan 4, 2012

Naty1

Check out BINDING ENERGY and MASS DEFICIT for some insights on the above posts.

http://en.wikipedia.org/wiki/Binding_energy#Mass_deficit

You'll note that it is "binding energy mass" is what changes rather than traditional "mass".

As an example, if you heat a substance it will have more "mass" and more gravitational effect
than before heating...The kinetic energy after heating of the constitutent particles increases and contributes to rest "mass" as noted above via E = mc2......

In a nuclear reaction, some actual constitutent particles are destroyed and converted to energy...radiation....again via E = mc2

6. Jan 4, 2012

kmarinas86

Chemists usually assume conservation of mass from reactants they combine to the chemical products they form. It's a useful assumption in many applications where the energy release is below nuclear levels. It's not strictly true however. The heat energy that is released does correspond to a very small loss of mass, usually of an amount of no practical consequence, that is, if the reaction doesn't go nuclear.