A thin metallic spherical shell contains a charge Q on it. A point charge q is placed at the centre of the shell and another charge q1 is placed outside it. All three charges are positive. What is the force on the charge at the centre?

The charge of q1 outside the shell is positive and the metal is observed to be in a spherical shape. A negative charge is produced as a result of the induced positive charge from outside. Always an electric field will be zero inside the shell and electrons will move if and only of the electric force is applied. The movement of electrons gets stopped when the electric field comes in from different directions. Thus the electric field becomes zero. So there will not any force on q.