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for example, I rub a ballon on carpet. What is an experiment I can do to tell which object is net positive charged and which is negative?

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Compare With Known Positive/Negative Charge

Find some material which you know takes up or gives away electrons. Bring it close to the object in question. If they are of like charge, they will come together, otherwise they will repel.

The Lorentz Force

Take advantage of the Lorentz Force and move the object in question near a strong magnet. Use the Lorentz to figure out which way the particle ought to move, and observe which direction the object actually does move. This would be easiest, according to my opinion, if you ran it next to a wire which produces a magnetic field.

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  • $\begingroup$ And of course, to determine which is the north of the your magnet, you look at the Lorentz force it applies on a known positive/negative charge. $\endgroup$ – JiK Mar 9 '17 at 19:00
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The earliest definition seems to be: http://en.wikipedia.org/wiki/Electric_charge

In 1839, Michael Faraday showed that the apparent division between static electricity, current electricity, and bioelectricity was incorrect, and all were a consequence of the behavior of a single kind of electricity appearing in opposite polarities. It is arbitrary which polarity is called positive and which is called negative. Positive charge can be defined as the charge left on a glass rod after being rubbed with silk.[2]

So use this functional definition to create positive and negative charged bodies and see which one attracts the balloon...

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