What do they mean when they refer to it? The PE of a system, I get, and the potential of a point is the work per unit charge needed to bring a charge there, assuming the charge has no effect on the field (right?). A small conducting sphere originally has a charge +q. The sphere is lowered into a conducting can. Which of the following quantities are fixed as the sphere is lowered? A. Potential of the can B. Potential of the sphere C. Charge on the sphere D. Net charge on sphere and can When they say 'potential of the can', do they mean the amount of work per unit charge needed to bring charge to its surface? If so, is that ignoring the contribution to the E field of the sphere? Is this potential relative to something? Relative to the sphere? Do they mean the potential energy in the sphere-can system? Thanks for helping me sort this out!