Question:
You and a friend have been given the task of designing a display for the science department that will demonstrate the strength of the electric force. Your friend comes
up with an idea that sounds neat theoretically, but you are not sure it is practical. They suggest you use an electric force to hold a glass marble in place on a
sloped plywood ramp. They would get the electric force by attaching uniformly charged spheres in a semicircle near the bottom of the ramp (see diagram below). Your
friend claims that if the charges on the marble and spheres and the slope of the ramp are chosen properly, the marble would be balanced at the centre of the
semicircle. To test this idea, you examine a system with a ramp at some angle and five spheres with some charge arranged uniformly in a semicircle of some radius. The
marble would sit in a slot of some length cut lengthwise into the center of the ramp. You measure the mass of the lightest marble you can find. What charge would you
need to put on the marble?