# Weight Of A Battery

1. Dec 31, 2005

### ukmicky

Firstly Happy New Year

secondly does a charged battery weigh more than an uncharged one

I presume the charged one will weigh more.

Last edited: Dec 31, 2005
2. Dec 31, 2005

### krab

That's a really interesting question! I say Yes.

3. Dec 31, 2005

### Ace2005

My GUESS is no

http://www.energizer.com/learning/howbatterieswork.asp *requires Flash

Have a look at powering the device, third tab, electricity is the flow of electrons, when the circuit is completed the electrons flow from the cathode to the anode, the electrons are therefore displaced and not removed, so if all else remained the same the mass should not change

but this is just a guess

4. Dec 31, 2005

### GOD__AM

Something interesting is, that if you drop a battery (with a full charge) from about 6 to 10 inches onto a concrete floor, it will bounce much less (almost seem to stick to the floor) than an empty one, which bounces quite high comparitivly. Not saying this has to do with the weight of the battery, but maybe someone could offer an explanation?

5. Dec 31, 2005

### chroot

Staff Emeritus
Energy is stored in a battery in chemical form. In other words, the energy is stored in the chemical bonds between atoms, which involve electrons. A charged battery has many such bonds already formed, while a discharged battery contains the same atoms, but with the bonds almost all broken.

The question, then, really boils down to this one: does a molecule weigh more or less than the sum of its constituent atoms?

A molecule certainly contains more energy than does a collection of its constituent atoms. It is also true that energy and mass are both affected by 'gravitational fields,' as can be seen by the deflection of starlight as it passes near the Sun, or by the redshifting of light as it travels upwards from the floor of a laboratory.

A charged battery, of course, contains more energy than a discharged one -- a trivial statement. A charged battery must necessarily weigh more than an uncharged one. In the same vein, a box of mirrors full of photons will weigh more than a similar box without any photons inside.

However, the difference in weight is incredibly tiny, due the very large value of c2, the constant that associates quantities of mass with quantities of energy in the famous equation E = mc2.

To give you an idea, consider my laptop battery, which produces a potential difference of about 12V and can supply about 4 amp-hours of current. This battery contains about 172,800 joules of energy when charged. The equivalent mass of that energy, however, is tiny: only about 2 billionths of a gram:

- Warren

Last edited: Dec 31, 2005
6. Dec 31, 2005

### Ivan Seeking

Staff Emeritus
E=MC2 can be used to predict the increase in mass due to heating as well.

7. Dec 31, 2005

### Danger

This one weirds me out a bit, Chroot. It might just be because of the 6 Keith's that I've injested in the past 2 hours, but I don't think so. While photons follow the spacetime curvature of a gravitational field, they are by all physical rules massless. How can something of zero mass affect the weight of its container? If you're referring to photonic pressure, it should be equal in all directions. Please clarify this for me.

8. Dec 31, 2005

### krab

6 Keith's in 2 hrs? you getting warmed up for something? Anyway, massless does not mean weightless.

9. Jan 1, 2006

### Ivan Seeking

Staff Emeritus
They have no rest mass, but they are never at rest. Consider that photons colliding with a screen [or a large sail] impose a force on that screen - the [negative] rate of change of momentum is force.

Last edited: Jan 1, 2006