You have to get to pretty big quantities for it to matter. The space shuttle main engines, in the process of burning 1.62M pounds of H2 and O2, converted about a gram of mass into energy.
Yes, a static electric field is a thing, and yes, it has mass.
Rubber bands heat up when wound and cool down when released. You can wind a rubber band and let it sit until it drops back to room temperature. When it unwinds it cools off below room temperature.
ohmy, that’s terrible.
I’ll take your word for it on the space shuttle example. (And I get it that the quantities we’re talking about are vanishingly small.) But my question is the same, in this case you combine hydrogen and oxygen and energy is converted from chemical to heat and light. How/why does conversion of matter to energy come into it? Is mass converted to energy every time energy goes from one form to another?
Or in the battery case that started this all, is energy converted to mass whenever energy changes form? Or is it sometimes one way and sometimes the other? I’m getting confused.
The chemical energy is part of the mass of the original fuel. It’s just a very, very small part of the total mass, too small to be measured.
Likewise, in uranium, say, the nuclear binding energy is also part of the mass of the uranium. It’s still a fairly small part, but in this case, it’s large enough to be measured.
One can also add the surprising result that the binding energy inside protons and neutrons is the dominant source of their mass, greatly exceeding the contribution of the quarks themselves.
Perhaps the important point when thinking about chemical energy and mass is that in a closed system energy is conserved. If you have an ideal insulated box containing some hydrogen and oxygen, if you light the mixture and it burns, creating water, the mass of the box contents does not change. But some of that mass is now coming from the kinetic energy of the water molecules whizzing about, and less from the hydrogen and oxygen.
Charging a battery isn’t a closed system, energy came in from outside, so its mass increases. Energy leaving reduces it mass.
If, again in an ideal box, you are exchanging say the potential energy of a rock with chemical energy powering your muscles, the total system energy is constant, and the box’s mass remains constant.
Eventually we changing the form of the energy. In chemical reactions there is energy in the electric field binding the atoms together. That energy contributes to the overall mass. If we allow that energy to be converted to kinetic energy, that energy contributes the same mass to the overall system. In a very real sense mass has not been created or lost. Using a the battery, same deal, energy moves about and may eventually convert in form. Wherever that energy ends up, it brings its contribution of mass with it.
So, if I’m reading this correctly, energy actually HAS mass? Not just mass and energy can be converted one to the other, but energy HAS mass? (As I said, I’m no physicist. Thanks for fighting my ignorance.)
At this level, that’s a sufficiently-correct description. I’d further add that all mass is energy.
I kind of love this—if you walk through the experiment, you end up with an intuitive understanding of hysteresis, which I think is otherwise hard to come by.
Have you actually done this experiment?
I’ve done it with a balloon. Stretch the balloon and touch it to your lips, which are very temperature-sensitive. Let it equilibrate while stretched, then let it relax and touch it to your lips again.
You can do the same thing with a rubber band. Stretch it, touch it to your lips, and then let it go back to its original size. You’ll feel the coolness.
That is true, but the potential energy stored in the rubber band is not due to the heat. Once the heat dissipates, the potential energy is still there. The heat is a product of the internal friction of the molecules as the band is twisted.
So how is the potential energy stored and how does is increase the mass of the rubber band?
To a large extent, the potential energy is actually the heat. When the band contracts and cools off, it’s drawing energy from heat, and converting it into kinetic energy. The really interesting things happening inside a rubber band all involve the entropy, more than the energy.
In any event, if you insulated the rubber band such that it stayed hot while stretched, the heat would also represent an increase in the mass of the band.
But what is going on when it cools prior to when it contracts? It still has all the same potential energy it had before it cooled off.
How about cells like AA batteries?
I can attest that the “bounce test” of whether a battery still has charge indeed works. But more than that, a bouncy battery feels lighter in the hand. I haven’t gone to the level of actually weighing them to see if it’s an illusion or real though.
Not exactly. It’s not actually storing energy in the band; it’s storing the ability to extract energy from thermal energy. You can usually get away with approximating this as storing energy in the band, but the detailed description is much more interesting.
If done under controlled conditions there is no way you would feel the weight difference in your hand and pretty unlikely you have anything around your house that could weigh it at the precision of picograms (see earlier posts)
I wasn’t claiming to be able to feel the difference of picograms lol
I was simply saying that spent batteries do seem to feel lighter so asked the question of why that might be, and whether there are any differences between typical AA batteries and phone batteries that might be relevant to this question. If not then I would conclude it was all in my head.
But on Googling, it seems that there might be. A fresh alkaline battery contains some water, and liquid will leak out when cut open. A spent battery is dry, and bounces because one of the chemicals produced in the reaction is quite elastic.
So anyway, since a new battery is somewhat more liquid, if that liquid can move around in the battery then this may give a slightly different feel when picking the battery up. Not actually more heavy, just a slightly different response to accelerations.