The kinetic energy of a charged particle decreases by $10 \mathrm{J}$ as it moves from a point at potential $100 \mathrm{V}$ to a point at potential $200 \mathrm{V}.$ Find the charge on the particle (in $\times {10}^{-1}\mathrm{C}$).

Two small spheres are attached to the ends of a long light nonconducting rod at a distance $40 \mathrm{mm}$ from each other. A third, middle sphere can slide along the rod without friction as shown. All three spheres are nonconducting, have identical masses $m=1 \mathrm{g},$ and a positive charge $q=1 \mathrm{C}$ is distributed evenly on the surface of each sphere. The whole system is placed on a horizontal frictionless nonconducting surface. Initially, all three spheres are at rest and the middle sphere is located a distance $30 \mathrm{mm}$ from one of the ends of the rod and a distance $10 \mathrm{mm}$ from the other. Find the maximum speed $v$ (in $\mathrm{m} {\mathrm{s}}^{-1}$) of the middle sphere after the system is released.

The figure shows four situations in which charges as indicated $\left(q>0\right)$ are fixed on an axis. How many situations is there a point to the left of the charges where an electron would be in equilibrium?