QotW: Does potential energy have mass?

Would charging your phone make it heavier?
26 April 2022


Battery full indicator



Listener Tibor wanted to know the answer to the question, "Does potential energy have any mass? If I was to charge a phone battery, would it become heavier?"


It's commonplace to describe batteries as full or empty depending on their charge. So surely it to make sense for them to get heavier. Right? Dr. Israel Temprano from the Univserity of Cambridge explains to Otis Kingsman about this battery baffler...

Israel - Potential energy is a term in physics related to the distance that a certain mass has to in any potential well. This relates to how high an object is. If you have an object that is standing very, very high and you let it go, it will start moving. Then it will trade its potential energy to kinetic energy. That's not how we store energy in batteries.

Otis - It turns out these are two very different questions Tibor. The potential energy of objects held up doesn't give it any extra mass. But then what happens when we charge a phone battery?

Israel - The common batteries that we use in our everyday electronic devices, is what we call a lithium-ion battery. lithium-ion batteries work by just moving lithium ions, which are positively charged lithium atoms, from one side of the battery to the other. AA on AAA batteries, they have a different architecture inside, but again, all you're doing is moving components from one side of the battery to the other.

Otis - As the ions move to the other side of the battery, electrons move out of the battery and into the phone where it generates electricity before going back into the battery but on the opposite side.

Israel - In order to balance the charge, electrons have to follow these ions. What we do is force these elections to go outside of the battery. This is why we call this type of battery, 'a rocking chair', because we're just moving things from one side to the other. It's this movement of electrons that we use to power our portable devices.

Otis - When we charge a battery, we are moving the electrons out of the device and into a wire before electrons go back in on the original side of the battery that it started on. This resets for battery, and the process can start all over again. Overall, there will always be the same amount of electrons and ions within the battery.

Israel - So no. When you charge one of these batteries, all you're doing is moving components from one part of the battery to a different part of the battery.



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