What you Need
If you actually want to charge your Ipod you will also need a USB extension cable, you are not attached to, and a lot more fruit and nails...
What to do
If you just want to make a battery (or technically a cell - a battery has more than one cell)
Push the zinc object into the fruit
Push the copper object into the fruit close to the zinc one (but make sure they don't touch or they will short out), if you are using a citrus fruit, try to get it in the same segment
Measure the voltage between the zinc and copper objects.
Now measure the current it will produce.
Try using different metals, see what happens.
If you actually want to charge your MP3 player
Beware this may damage your mp3 player!
String at least 12 of these cells together and probably use multiple nails in each fruit.
Cut your USB cable open, and work out which wires should carry current by plugging the male end into a computer and measuring voltages.
Connect the battery to the USB cable which you have cut open to reveal the wires that carry the power.
Plug in your MP3 player
Hope you are not going to get into trouble for ruining all the fruit in the house.
What may happen
You should find that there is about 1 volt between the copper and zinc objects in the fruit, which sounds good, but it will only support a tiny current. Ours would only produce about 0.1mA at the most.
You should find that different metals will produce different voltages, for example Steel to Zinc or Copper will produce about 0.5V.
If you ruin a whole fruit bowl and have enough metal objects you can just about get enough current to cause an mp3 player to think it is charging. Whether the charging circuit is actually using more power than you are putting into the system is arguable, but it does definitely work in principle if you can string enough nails and fruit together..
Why does it happen?
When you put the two different metals in the fruit a chemical reaction will try to take place. If Zinc metal can dissolve to from Zinc Ions it will release energy, but it also has to loose electrons. If the zinc is connected to the copper by an electric circuit these electrons can flow around the circuit and neutralise copper ions in the lemon. This process releases energy which you can use to charge your mp3 player.
You have built a battery (or technically a cell) which works on the same principle as the ones you buy in the shops. This will often use different metals , a much more optimised design and fewer fruit, but they will all involve a similar reaction that requires electricity to flow for the reaction to occur.
Why are there copper ions in the lemon?
There will be a few there naturally, but most of them will be created by the tarnish on the copper dissolving in the acid (see the cleaning coppers experiment) releasing all the copper ions in the copper oxide tarnish.
Why doesn't the reaction happen without the circuit?
If a zinc atom is to dissolve it must from Zn2+ ions, this involves loosing 2 negatively charged electrons. If these electrons can't go anywhere the Zinc object will become so negatively charged it will attract as many positively charged Zinc ions back as are dissolving, so the reaction will stop.
What happens when you complete the circuit?
Electrons can flow around the circuit to the copper electrode, here copper ions are attracted to the negative electrons and when they meet they are neutralised forming copper metal. Because Zinc is more reactive than copper, this whole process releases energy, about 1 joule of energy for every coulomb of charge that is moved, which is the same as saying the battery produces about 1 volt. This voltage is related to the difference in reactivity between the two metals you are using, so if you change the metals you will change the voltage.
Why does the battery produce such a small current?
To produce a large current both chemical reactions have to be able to take place fast, so the larger the surface area and the more reactants there are in the solution the faster the reaction will occur. There is a very low concentration of Cu2+ ions so the reaction is likely to be quite slow, limiting the current that can be produced.
The other limiting factor is that if it is difficult for ions to move around in the fruit, as after a while the region around the Zinc will get positively charged and the region around the Copper will get negative. This will mean that there is less difference in voltage between the Zinc and Copper Nails, so instead of giving out 1 V the cell may only produce 0.5V. But if you wait for a while ions will flow through the fruit to cancel out this effect.
This is why we needed to use 12 1V cells to produce the 5V to charge the mp3 player, each cell was actually producing less than half a volt. This is also why commercial batteries when they are almost flat can produce a good voltage, right up until you draw a current.
Part of the show Gauging Age, Virtual Life, Reading Emotions and Cyber-Forensics from the 24th Feb 2008