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

20 July 2008

Question

I have a brand new iPod. It’s 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?

Answer

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

But really, the answer's no.

The way in which an iPod works depends on which one you've got. If you've got the one which uses solid state memory then all it's doing is storing binary data. It's using a memory chip which is storing information as digital information.

If you've got the older, bigger iPods that have hard-discs in them, this is magnetic binary data. But in either case it's storing a "1" or a "0" by having something - such as a piece of magnetic material - pointing one way or the other.

So it doesn't actually matter whether it's actually got anything stored on it or not, because storing nothing still weighs the same as storing something. It's not like a cupboard that you're putting tins into.

On our forum, Madidus Scientia put it very well saying, "it's a bit like having a handful of coins and they're all either heads or tails." That's like the "0" or "1" in digital binary. They weigh the same whether they're all showing heads or they're all showing tails.

So there's 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 you're running faster you have more kinetic energy. Because E=mc2 - that's E, energy, equals m, mass, times the speed of light, c, squared, and since the speed of light, c, doesn't 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 you're 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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