State Coulomb’s law for electrostatic force.
The sphere and particle $P$ are both positively charged. State the direction of the force on particle $P$.
The sphere and particle $P$ are both positively charged. State the position of particle $P$ for the force to be at its maximum.
Determine the ratio \[ \frac{\text{force on particle } P \text{ at } x = r}{\text{force on particle } P \text{ at } x = 4r} \].
When the charge on the sphere is $6.0 \times 10^{-7}\,\text{C}$, the electric field strength at the surface of the sphere is $1.5 \times 10^{6}\,\text{V m}^{-1}$. Electrical breakdown (a spark) occurs when the electric field strength at the surface of the sphere exceeds $2.0 \times 10^{6}\,\text{V m}^{-1}$. Determine the extra charge that can be added to the sphere before breakdown occurs.