physics--electron charge transfer

[kokenwa]

A metallic sphere has a charge of +4.2 nC. A negatively charged rod has a charge of -6.0 nC. When the rod touches the sphere, 7.3E9 electrons are transferred. What are the charges of the sphere and the rod now?


i dont even know where to begin. can i get some help?

[Doc Al]

When the rod touches the sphere, 7.3E9 electrons are transferred.
How much charge is that in terms of nC? What's the charge on each electron?

Realize that the net charge (the total charge on both objects) cannot change.

[kokenwa]

the charge on each electron is 1.6E-19C or 1.6E-10nC.

[Doc Al]

the charge on each electron is 1.6E-19.
Right: That's the charge of each electron in C. So when you add 7.3E9 electrons to the sphere, by how much does the charge on the sphere change?

And the charge on the rod? It just lost that number of electrons, so how did its charge change?

[kokenwa]

i got 3.066nC for the sphere by multiplying the transfer charge by the nC charge for the sphere but i cant find it for the rod. what do i do next?

[Doc Al]

I assume you mean that you added the negative charge from the electrons to the original positive charge of the sphere to get the new charge of the sphere. (What was the total negative charge of the electrons that were transfered in nC?)

To find the new charge of the rod, do the opposite: subtract the negative charge. For example: If something has a charge of -5 nC and loses -1 nC (of electrons), its charge is now just -4 nC.