When you rub a glass rod with silk, the rod becomes positively charged; when you rub a plastic rod with fur, the rod becomes negatively charged. Suppose you have a charged object but do not know whether it carries a positive or a negative charge. Explain how you could use a glass rod and a piece of silk to determine the sign of the charge on the unknown object.

Question

anonymous · Answer

Place the glass rod near the object, and see if it repels or attracts (glass rods have postive charge)

tcy · Answer

^
but rub that glass rod first :)

anonymous · Answer

^Yeah :D

theeric · Answer

It seems like you guys have this covered, but I'll recap.

You have a glass rod and silk, and an object with unknown charge.
The problem says that you can rub the glass rod and silk together so that the glass rod has a positive charge. (Side note: that means its electrons went to the silk!)

So, when you rub the glass with the silk, you do know that the glass is positively charged.

With electrostatic force, like charges repel. So, if the mystery object is repelled by a positive glass rod, this object is positively charged. To the contrary, if the mystery object is attracted, it is negatively charged.

anonymous · Answer

so what's the use of this line "when you rub a plastic rod with fur, the rod becomes negatively charged" on the question? o_0? lol

thanks guys :)

theeric · Answer

Haha, background detail. I really don't know.