Science Questions

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? Neil Pariser

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 is solid state memory then all it’s doing is binary data.  It’s just 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.  In each case it’s either storing a 1 or a 0 by having something pointing in one direction – a piece of magnetism pointing one way or the other, effectively.  You can think about it like that.  That 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 this very well when they said it’s a bit like having a handful of coins and they’re 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.  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, this is the best excuse for not doing P.E. at school, when you run in both cases the hot kettle or you running 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.  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.  You could use this as an excuse for not doing exercise.

Similarly, in your iPod when you charge it up you’re putting energy into the battery.  It will weigh a rather tiny amount more. One statistic I did hear is that a thumb print 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 weight of the battery will increasing due to charging it!

July 2008