a point charge q is placed in a cavity in a metal block. if a charge q is brought outside the metal , will the charge q feel any electric force ?
Yes, it will feel the force of attraction to the metal block. When a body, which consists from conducting material (e.g. metal) is placed in electric field, the phenomena of electric induction occurs. Electric field regroups electrons in conducting body by such way, that body gets polarized – side which is turned to the source of electric field (to the charge q) gets an opposite charge (-q’), magnitude of which depends from the size of initial charge q and from the distance between this charge and the body surface. Opposite side of the body (directed from initial charge q) gets charged with a charge of the same sign as the initial charge q. As you know, opposite charges are attracting each other. That’s why metal block will attract the point charge q.