Here is a typical description of motional EMF: "The figure below shows a conducting rod of length L being moved with a velocity v in a uniform magnetic field B: The magnetic force acting on a free electron in the rod will be directed upwards. As a result, electrons will start to accumulate at the top of the rod. The charge distribution of the rod will therefore change, and the top of the rod will have an excess of electrons (negative charge) while the bottom of the rod will have a deficit of electrons (positive charge). This will result in a potential difference between the ends of the rod equal to LvB." My question is: How do we calculate the number of excess/deficit electrons for a given L, v, and B?