An example of how trying to teach someone exposes gaps in your knowledge: I was talking to someone about why we have line, neutral and earth conductors. We got on to the fact that the earth is an enormous sink and source of charge, so anything referenced to it can be considered 0 volts. Then he asked what would happen if he connected a voltmeter between +ve of a 12V battery and ground. I said 0 volts, since the battery is not referenced to ground. But, he said, the battery as a whole is electrically neutral, but with separators so that an imbalance of charge is created between the two terminals. With this is mind, the positive should be +6V above ground, and the negative -6V below ground. I pointed out his confusion between voltage and charge, the former being the difference in energy stored per unit charge. Voltage is not a measure of charge, etc. But I must admit that I couldn't pinpoint the exact failure of his argument. What is the exact physical basis of voltage that, say, means that measuring between the +ve of a battery and the -ve of a seperate battery will give a reading of 0 volts? And I did just say that the ground can be considered 0 volts, even though measuring between ground and the +ve will give no voltage reading.