Here's the question you clicked on:
richyw
Two identical conducting spheres are places with there centers 0.3m apart. one is given a charge of 12nC and the other a charge of -18nC a)find the electric charge exerted by one sphere on the other b)what if the spheres are connected by a conducting wire. Find the electric force each exerts on the other after they have come to equilibrium.
so is this just a trick question? I mean if they were point charges I would just plug them straight into coulomb's law\[\vec{F_e}=k_e\frac{q_1q_2}{r^2}\hat{r}\]
would being a sphere make it any different? I mean could it get more complicated from the charges moving to one side of the sphere to the other and making r smaller? or is there something to say that the exact force is the same as if they were assumed to be point charges. (also assuming that these are completely isolated from other forces) and for b) can I simply "cancel out" the forces and get 6nC and then use the same formula to figure out that force?
or wait, would it be -3nC in each one? you'll have to excuse me, it's been a long time since I did intro physics and this is my first course on electricity.
This problem use sphere for reasons, i guess. For sphere, from outside of the sphere it can be considered as point charge located at the center. So, yes, you can use Coloumb's force law for (a). (See further: Gauss's law) As for (b): yes, charge will distribute itself uniformly leaving total charge -6nC at surfaces. -3 each.
i think for a, what do you mean by the electric charge exerted?
Hehe, i'm wondering about that too. May be they mean electric force?
yeah it must be electric force. thanks a lot!