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TWO Types of Charge

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Benjamin Franklin, back in the mid 1700s, determined through lots of experimentation that there were only two types of charge – he named one “positive” and one “negative.” This is the basic idea of what he did. He gathered materials – on the left we see a pair of materials, a rubber rod and a piece of animal fur – on the right a glass rod and a piece of silk. Those were his original materials. And what he did was basically take the rubber rod and rub it on the piece of fur. That combination had characteristics that this thing would pick up a charge. And then bring it near some little object like this, which is a pith ball – just a really small, really light object that you can see electrical interactions quite easily. Bring it near that and you see that initially it attracts and then it repels and you see this thing now being repelled by this rod then took a positively charged rod and if you take this glass rod, rub it on the silk, and then bring it near, it is definitely attracted. And you did that with a lot of different materials. He took the rubber rod, it was repelled, took the glass rod, it’s attracted. Took all different materials and only saw repulsion and attraction and determined that there must be only two types of charge. Named one type of charge “negative” – and this plastic rod – which is what we use as a replacement for the rubber rod – when rubbed with fur has a negative charge. His definition – rubber rod, now plastic, when rubbed with fur picks up a negative charge or has a negative charge. This glass rod when rubbed with silk has a positive charge. Those are his original definitions. And because simpler is always better, the simplest explanation is usually the best explanation, he decided that there weren’t two completely different types of charge but there was one, and that a lack of this type of charge would be the gain of another, basically. So here we have this thing is positive, and what he said was that as this one is rubbed against the silk, the silk loses the positive charge and that positive charge goes onto the glass rod. It’s called his “one fluid model.” He preferred positive. And when this happens, this plastic rod is negative because it loses its positive charge to the fur. That’s his one fluid model of electrostatics. Ben Franklin, mid-1700s.

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