Science Questions

Will an iPod weigh more when it's full of music?

Sun, 20th Jul 2008

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Neil Pariser asked:

I have a brand new iPod. Itís never been charged and has no data put on it. Will it weigh more after charging the battery and filling it with music and pictures?



It will, Neil, if you fill it with heavy metal (!)

But really, the answerís no.  The way in which an iPod works depends on which one youíve got.  If youíve got the one which uses solid state memory then all itís doing is storing binary data.  Itís using a memory chip which is storing information as digital information.  If youíve got the older, bigger iPods that have hard-discs in them, this is magnetic binary data. But in either case itís storing a "1" or a "0" by having something Ė such as a piece of magnetic material - pointing one way or the other. 

So it doesnít actually matter whether itís actually got anything stored on it or not, because storing nothing still weighs the same as storing something.  Itís not like a cupboard that youíre putting tins into.  On our forum, Madidus Scientia put it very well saying, "itís a bit like having a handful of coins and theyíre all either heads or tails." Thatís like the "0" or "1" in digital binary.  They weigh the same whether theyíre all showing heads or theyíre all showing tails. So thereís no reason to think there should be a difference in the actual weight.

However, when you charge the battery, you are adding energy and there is a relationship between energy and mass...

As Einstein said E=mc2, so when you boil your kettle or - and this is the best excuse for not doing P.E. at school - when you run, in both cases (the hot kettle or you running) you have more energy.  When youíre running faster you have more kinetic energy. Because E=mc2 Ė thatís E, energy, equals m, mass, times the speed of light, c, squared, and since the speed of light, c, doesnít change, if your E, energy, goes up, your mass must go up. 

So a hot kettle will weigh more, and when you run in P.E. you will gain something like 10-14g.  This is not a prodigious weight-gain but it is nonetheless weight gain due to taking exercise!

Similarly, in your iPod, when you charge it up youíre putting energy into the battery and it will weigh a tiny amount more. In fact, one statistic we did hear is that a thumbprint applied to the front of the iPod in the form of, say, the grease on your thumb will weigh thousands of times more than the increase in the weight of the battery due to charging it...


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It would weigh more by the most insignificant tiny unmeasurable amount after it has been charged, but no, the data has no weight. Think of holding a few coins in your hands, 2 showing heads, 1 showing tails. If you flip them all so that they all show tails they still weigh the same, but show different information. Madidus_Scientia, Tue, 15th Jul 2008

From simple physics I can't see that a battery will add weight when charged.  If it were true then discharging would reduce its weight.  Electrons flow from one plate through the external circuit (although individual electrons don't make the whole trip) and back to the other plate..  The exact same number will return to the battery.  The diluted acid in a lead-acid battery will increase its weight when charged..a hydrometer measures this to check on the state of the battery...but the lead plates will lose weight and there will be no overall change.  More esoteric physics(such as quantum mechanics) may suggest otherwise.

Writing data to 'new' (maybe unformatted) flash memory involves injecting tiny currents so there may be a very very small but finite increase in weight.  Deleting may just prepare the memory for over-
writing and not remove the charge on the gates of the Mosfets. Pumblechook, Tue, 15th Jul 2008

M-S's argument involves the E = mc2 argument and must be right.
A similar argument around the memory elements could take you either way - does a 1 correspond to more or less energy than a 0? That has to depend on what logic technology is used. The total system would have to be electrically neutral before and after your 'injection' of currents so there would be no mass added or taken away.
I think, basically the answer to the original question is "No". lyner, Tue, 15th Jul 2008

As a firm believer in empirical study I drove to a public weigh station with a non charged and empty ipod.......weight of car plus non charged and empty ipod  1431kg..........I then charged up the ipod and filled it with songs and did the same....the result ? 1431kg....voila !!

Glad I could help ! neilep, Tue, 15th Jul 2008

I agree with Sophie & M-S. Using E=mc2, if the energy of the battery increases then m must increase to keep the equation balanced. DoctorBeaver, Tue, 15th Jul 2008


I dont believe YOU

Can i ask ,did you check to see if your car battery was in the same state of charge on both occasions. No i bet you didnt ?

This may obvious but in between your trips did you refill your car with petrol so it had the same amounts whilst sitting on the weigh bridge ?

Also was it sunny on your second trip , did you look into whether a car weighed more if its body was warmed by the sun.

And i bet you didn't remove all those the flakes of skin and hairs that fell off your body in between trips.  ukmicky, Tue, 15th Jul 2008

And he'd had a dump! DoctorBeaver, Tue, 15th Jul 2008

If the weighbridge was accurate to 1kg, that would correspond to a possible error of 10e17J. The ipod battery would store something like 1MJ. So . . lyner, Thu, 17th Jul 2008

The first time I heard this podcast, I thought your answer of E=MC2 might be wrong. Because charging the battery only move electron from one chemical to another. So the total mass should not change.

And I thought E=MC2 only works when nuclear reaction taken place.
After searching through Wikipedia, I found out that, the formula works even with normal chemical or physics reaction.

Physics, for instance, when something speeds up, its would gain more mass.
And when you charge the battery, electron moves from a track of an atom to another track of another atom. Then the electron moves faster(probably..), and because it moves faster, it gains weight.

That is my explain to this. ksc91u, Tue, 16th Sep 2008

There is a difference in weight when downloading music files, especially if you are down loading heavy metal or rock;

The equation is quite simple, vis:

M + V(ΘΘΨΦδ x ωλ8 ) Σ + #of downloads : XL5 = Bullshit  Don_1, Wed, 17th Sep 2008

Sorry but it's not this the reason. Even if there weren't any electron movements at all the battery's mass would increase the same because of the flux of electromagnetic field (which carries energy). lightarrow, Thu, 18th Sep 2008

Do you mean that if we let a current flow over a wire, the weight of the wire would also increase?

When a current flow over a wire, electrons are moving faster? than when they are on the track, right? ksc91u, Thu, 18th Sep 2008

Do you mean that if we let a current flow over a wire, the weight of the wire would also increase?

When a current flow over a wire, electrons are moving faster? than when they are on the track, right?
What I mean is that an electromagnetic wave can convey energy as well, you don't need to think about masses or charges exclusively. If you heat the battery with a flame or with a hot lamp, you increase its mass; if you send a light beam inside of it, you increase its mass, and so on. lightarrow, Thu, 18th Sep 2008

Since all chemical electric energy storage (cells and batteries) have some irreversibility, I'd guess mass change (through venting of the cell) would swamp any other effects. Really admired the heads/tails analogy from a previous post. Lawrence Skarin, Wed, 31st Mar 2010

I know this is an older discussion, but this is the second time today I've encountered a mis-aprehension, so I figured I should correct it.

An object has potential energy, including the potential of converting its mass to energy.  This is what E=mc^2 means, it is a method of determining the potential energy that would result from completely converting the existing mass to energy, not a formula for indicating that the mass changes when energy does.  Increasing an object's energy using other techniques (gravitational, electric, thermal, chemical, whatever) does not convert said increase into mass.  Mind you, it is possible that some of these increases might add mass, but only by the expedient of adding something material to the system.  For chemical it might be atoms pulled out of the air, for electrical it might be additional charge carriers.  In neither case is the mass increase related to the E=mc^2 forumla.

Unless you have a nuclear (or anti-matter) reaction occuring there is no conversion between energy and mass, and therefore no change in total mass.  DanD, Mon, 7th Nov 2011

Dan - E=Mc^2 is mass energy equivalence not just mass energy conversion.  Lightarrow is correct in his post and you are misunderstanding the majesty of einstein' vision

-taken from the original paper that caused all the trouble
DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT? By A. Einstein September 27, 1905 link here

there is also a great wikipedia page on mass energy equivalence linked here with great real world practical examples (coiled spring ha greater mass when compressed, spinning bowling ball has greater mass more etc)

and back to the op - there are those that say that the information and entropy involved will also lead to higher and lower energy states which will increase/decrease mass - although that is very debatable and is dependent on the state of the disc before and after imatfaal, Tue, 8th Nov 2011

Surely trapping electrons (1) will amount to mass or weight ? Stu, Wed, 30th Mar 2016

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